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... adequate unselected control line, which is quite important for minimizing the effects of environmental changes from one selection generation to the next and for detecting an asymmetrical response (DeFries, 1967). These several shortcomings may be contrasted to the DeFries and Hegmann experiment, in which repeatability was assured by the adoption of a cross between genetically fixed inbred strains, a single response measure served as the selection criterion, inbreeding was minimized, and replicated control and selected lines were included. Other methods for estimating h 2 (see Roberts, 1967a; Falconer, 1960) have been employed with greater success. These studies are summarized in Table 3 together with estimates from two selective breeding studies. It is interesting that heritability measures show a smaller range (.2 to .5) than values of ω 2 in Table 1 (.1 to .95). It is also interesting that four experiments with shuttle avoidance learning using four highly dissimilar populations found h 2 values of about .5. Although the proper interpretation of these measures of ω 2 , C.G.D., and h 2 is not readily apparent, some limitations on their generality are obvious. The inherent genetic variation of a ...

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...n experiment which can be attributed to differences between strains. When highly inbred strains are employed, between-strain variation should reflect primarily genetic variation, while within-strain variation should represent differences in postfertilization environment as well as error in measuring the behavior itself. Several estimates of ω 2 for strain comparisons are presented in Table 1. The values were derived from the F ratio for between-strains differences and the degrees of freedom between (dfb) and within (dfw) strains. It can be shown that the expression for estimating ω 2 given by Hays (1963, p. 382) reduces to for a one-way design with equal numbers of subjects per cell. It should be noted that only two reports (Scott and Fuller, 1965; Wahlsten, 1971) actually presented values for ω 2 . The remainder were derived by the present author. The wide range of estimated ω 2 values indicates that no simple statement can be made. It should be noted, however, that many values greater than 30% were obtained, which signifies a very substantial effect as judged by results from other areas of behavioral research. Coefficient of Genetic Determination Oliverio, Castellano, and Messeri (1971) ha...

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...re believed to influence learning and performance. Since most of the studies reviewed herein were relatively modest in their use of independent variables, it is clear that most researchers were not interested in this particular question. The more extensive experiments generally did not test anything resembling a law of learning. Hence, judgment must be suspended for lack of evidence. Lest there be a sudden upsurge in behavior-genetic analyses of learning principles, researchers should be aware of the current state of flux in the study of learning by the more traditional methods of psychology. Seligman (1970) questioned the principle of equal associability of all stimuli and responses using any reinforcement. He suggested that the laws of learning apply only to those responses which organisms are prepared to make to certain stimuli in certain motive states. The preparedness of an animal presumably can differ across strains and species. Bolles (1970) demonstrated that experimental manipulations such as CS termination may have quite different effects for different response modes like running or bar pressing. He maintained that a set of responses, the species-specific defense reactions (SSDR), is emi...

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...be subjected to genetic analysis. Animals of different learning abilities are readily available that have never endured electrical devastation or psychopharmacological perdition. Genetic methods may also be applied to some of the major questions within the areas of learning and memory research. Controversy over the unitary or dual nature of certain processes is particularly susceptible to genetic clarification. For example, it is of interest to know whether classical and instrumental learning are two distinct processes or different reflections of the same basic learning process (Miller, 1969; Rescorla and Solomon, 1967). If a situation can be devised in which classical and avoidance training are administered with identical CS, US, and response mode to different members of parent and offspring generations, it would be possible to calculate the genetic correlation between learning under the two contingencies. A very high rA would indicate that they in fact depend upon the same process, while rA = 0 would suggest that they are essentially independent processes. Intermediate values of rA would mean that the processes share common elements but also have unique aspects. Similar experiments can be done to study the...

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...m red light is employed, albinism has no effect upon avoidance learning. Other interpretations of the causes of learning deficits resulting from homozygosity for the albino gene have not been convincing. Fuller (1967) proposed that, since albinism results in a deficiency in both tyrosinase and dopa oxidase, learning deficits might be attributable to an imbalance in brain catecholamines. However, it is known that norepinephrine and related compounds are derived from tyrosine, not via tyrosinase, but rather via the enzyme tyrosine hydroxylase, which functions primarily in nervous system tissue (Cooper, Bloom, and Roth, 1970). The gene short-ear (se) has been shown to raise the hearing intensity threshold (Bundy, 1951). Denenberg, Ross, and Blumenfield (1963) found no effects of se upon several behaviors, including shock-escape learning. Abeelen (1966) subsequently reported that shock escape learning during jump-up avoidance training was significantly retarded for se/se mice compared to normal (se/+) littermates; no difference was observed for avoidance learning itself. No reason for the difference was evident. The above studies indicate that rd and c effects upon learning are indeed trivial when unintended. They...

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...s occurred previously. A well-known effect of inbreeding is to eliminate heterozygosity (Falconer, 1960). Thus, these studies also point to the importance of dominance as a genetic mechanism which influences learning. One difficulty with this simple dominance explanation of hybrid vigor arises when parent and F1 variances are compared. Since F1 of a cross between two highly inbred strains has no genetic variance, the phenotypic variance should not differ significantly from that of the parent strains. If the variances differ, significant epistatic interaction between loci probably is involved (Mather, 1949). Although Winston (1964) found that F1 variances resembled those of their parents, Schlesinger and Wimer (1967) observed a substantial reduction in the variance of most F1 hybrids. The most extreme case was a cross of DBA/2J and C3H/HeJ; the standard deviations in trials to acquisition were 8.37 and 9.33 for DBA and C3H, respectively, and 1.4 for F1. The reduction in F1 variance was of a magnitude similar to several examples given by Falconer (1960, Table 15.2). Rose and Parsons (1970) noted reduced variability in a learning score for F1 hybrids only early in training. Another problem appears...

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...e value of learning, and hence its heritability, may also be related to the breadth of a species' ecological niche. High ability to store and retrieve information should be especially useful when an animal typically encounters a wide range of foodstuffs, competitors, and building materials. Animals which occupy a very narrow niche, on the other hand, may be able to solve most problems with stereotyped responses to a limited number of stimuli. The ecological niche may also influence the kinds of learning abilities which will be highly developed in a certain species (see excellent discussion by Eibl-Eibesfeldt, 1970, Chap. 13). Another attribute of traits with high adaptive value is that they tend to degenerate during inbreeding and show a great increase when inbred strains are crossed. Since natural selection acts to reduce additive genetic variation by eliminating the less fit genotypes, the only genetic variance remaining after many generations of selection for traits closely related to fitness should be attributable to heterozygote superiority. This means that components of fitness should exhibit overdominance as well as low heritability. This important principle allows one to distinguish between tra...

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...n or emotion differences. Unfortunately, the McGill strains have not been the subjects of extensive learning tests as were Tryon's. Memory The ability to retain as well as store information is obviously a prerequisite for successful retrieval of that information at some later time. An animal of a certain genotype which either fails to store information permanently or stores it in a manner that makes retrieval difficult would appear to be deficient in acquisition of any task. Evidence exists that the process of memory storage requires a certain amount of time before a permanent record is made (McGaugh, 1966); the memory becomes less susceptible to disruption by diverse insults as time progresses. Thus, a strain which has a slower rate of memory "consolidation" would appear to be retarded in acquisition of a task at a fixed intertrial interval, assuming the interval is considerably shorter than the time required for efficient storage. Likewise, a strain which could not enter information into long-term storage at all would appear to be grossly deficient with widely spaced trials. The work by McGaugh and his colleagues has shown that the Tryon S1 and S3 strains differ in the timedependent aspects of...

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...cated that S1 (Bright) were superior to S3 with spatial cues but not with visual cues (Markowitz and Sorrells, 1964; Markowitz and Becker, 1965). However, Fehmi and McGaugh (1961) reported that Si was superior on a more difficult horizontal-vertical discrimination, which certainly required the utilization of visual cues. Although sensory abilities and preferences are indicated, conclusive evidence of their relevance to maze learning differences between the two lines is lacking. Motivation The relation between motivation and learning has a long history of theoretical dispute (see discussion by Kimble, 1961, Chap. 13). One central issue concerns the necessity of proper motivation to assure learning at all. Unfortunately, demonstrations of latent learning, sensory preconditioning, and transfer between drive states have not been attempted with genetic experiments. Whereas diverse opinions exist concerning the need for motivation to assure the acquisition of information, most theorists recognize the importance of proper motivation in order to guarantee the reliable performance of a learned response (see Estes, 1969). Vast research indicates that simple, unitary responses are acquired more rapidly w...

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...ces can never be subjected to genetic analysis. Animals of different learning abilities are readily available that have never endured electrical devastation or psychopharmacological perdition. Genetic methods may also be applied to some of the major questions within the areas of learning and memory research. Controversy over the unitary or dual nature of certain processes is particularly susceptible to genetic clarification. For example, it is of interest to know whether classical and instrumental learning are two distinct processes or different reflections of the same basic learning process (Miller, 1969; Rescorla and Solomon, 1967). If a situation can be devised in which classical and avoidance training are administered with identical CS, US, and response mode to different members of parent and offspring generations, it would be possible to calculate the genetic correlation between learning under the two contingencies. A very high rA would indicate that they in fact depend upon the same process, while rA = 0 would suggest that they are essentially independent processes. Intermediate values of rA would mean that the processes share common elements but also have unique aspects. Similar experim...

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...e individual and for the population. GENOTYPE-ENVIRONMENT INTERACTION The phenotypic expression of a particular genotype is known to reflect the individual's postfertilization environment prior to the time of testing. The important question in this regard is whether genotypes which lead to superior learning in one environment will be similarly endowed across a wide range of living conditions. If genotypic and experiential components of learning ability are truly additive (P = G + E), then conclusions drawn from studies of limited scope may be expected to have broad validity. The experiment by Cooper and Zubek (1958) demonstrated that rearing Thompson's (1954) bright and dull rat strains in either an enriched or an impoverished environment eliminated the strain differences in learning that were originally produced by selection in a normal lab environment. Likewise, pretraining experiences have been shown to affect some standard strains more than others. The handling of infant rats did not change later shuttle avoidance learning of the Sprague-Dawley strain, whereas handling greatly improved subsequent avoidance of both the Harlan and Rockland Long-Evans strains (Levine and Wetzel, 1963); with infantile ha...

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...ing. Although the experiments did not prove that the original Tryon strains diverged in learning rate because selection produced memory differences, McGaugh's research leaves little doubt that the S1 and S3 strains differed in memory processes. The differences were of such a magnitude as to account for virtually all of the between-strain variation in acquisition rate. Perhaps the most important implication of this finding is the extent to which memory processes are determinants of learning ability. In fact, only recently have learning theorists given due consideration to memory processes (see Estes, 1970). Other research on genetic differences in memory is less convincing. Bovet, Bovet-Nitti, and Oliverio (1969) presented data which showed that retention of a single passive-avoidance experience was good 10 sec after training but poor 24 hr later for C3H/HeJ mice; the reverse was obtained for DBA/2J mice. In addition, short intertrial intervals in shuttle avoidance led to good learning within a session for C3H mice, but retention was poor 24 hr later. On the other hand, DBA mice showed less change within a session but excellent retention the next day. The various experimental results led Bovet ...

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...enetic Determination Oliverio, Castellano, and Messeri (1971) have presented the only calculations of C.G.D. for a learning phenotype. They found C.G.D. for percent correct in 500 trials of shuttle avoidance learning to be .64 for a cross of inbred mouse strains SEC/1 ReJ and C57BL/6J and .84 for the cross of DBA/2J and C57BL/6J. Corresponding values for total errors in 15 trials of a Lashley III maze were .50 (S × C) and .39 (D × C). Heritability Of the several methods available for calculating heritability (h 2 ), realized response to selection for learning appears to be the most efficient (Hill, 1971). It is unfortunate that the various selection studies mentioned above were improperly designed to allow estimation of realized heritability. Some of these difficulties are evident in Table 2, which lists several pertinent aspects of the experiments. A proper selection experiment by DeFries and Hegmann (1970) involving open field activity in mice is included in the table for purposes of comparison. No researcher can obtain today a population known to have the same genetic properties as any of those previous ones, because the breeding schemes employed by most animal suppliers are generally haph...

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...essary for research of a magnitude similar to that of McGaugh's to be undertaken. The importance of memory processes will be underscored if, for example, Bignami's RHA and RLA strains show time-dependent differences as well. Emotionality Animals which are otherwise quite capable of efficient learning may perform very poorly if a particular training situation evokes strong competing responses. In an avoidance learning task, freezing may appear to be a concommitant of great "fear" or "emotionality," or it may be learned because of unforeseen reinforcement contingencies which encourage freezing (McAllister and McAllister, 1971). Wilcock and Broadhurst (1967) obtained measures of defecation and ambulation in an open field, a presumed test of emotionality, in five inbred rat strains and then trained them in shuttle avoidance. The Pearson correlation between mean open-field defecations and mean number of avoidances for each strain was +.06, which hardly supported any interpretation of the emotionality hypothesis. Reynierse (1970) has performed several experiments which suggest that rats of the Sasco strain are more emotional and extinguish avoidance responding more quickly than Holtzman rats under certain conditions. H...

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... the chemistry of mouse brain has been reported (Sudak and Maas, 1964; Schlesinger and Griek, 1970), but observed differences have not been related to learning ability. Structural and organizational attributes of the brain have received scant attention in the genetic context. Wimer et al. (1969) found that inbred mouse strains which had a neocortex of relatively large volume tended to have a hippocampus of relatively small volume (Spearman r = -.83). They did not attempt to relate their data to learning ability. Visual pathways have been found to differ dramatically in albino and hooded rats (Lund, 1965). The organizational differences related to patterns of interocular transfer (Sheridan, 1965) and visual evoked potentials (Creel, Dustman, and Beck, 1970). Their relevance to normal learning differences has not been established. Neither have they proved that the differences are caused by the gene c in random bred populations. Given the large number of mutant genes which are known to affect brain organization (Sidman, Green, and Appel, 1965), it is likely that alleles more within the normal range of variation have similar effects upon organization. Future studies which examine detailed organiz...

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...rent, the similar ordering of strains suggested that the genetic differences affected learning at a quite general level. On the other hand, Fuller (1970) tested four inbred mouse strains on either active or passive shuttle avoidance with a procedure that used no discriminative CS. Strain, rank orders were completely inverted for the two procedures. Pharmacological manipulations suggested that activity or "kinetic drive" differences were more important than any differences in general learning ability. Resolution of these seemingly divergent findings has been made possible by the recent work of Oliverio et al. (1971) mentioned above. They calculated genetic correlations between shuttle avoidance learning, Lashley III maze learning, and wheel running activity. The rA between shuttle and maze learning was about .73 ± .12, indicating that common abilities are required for both tasks but that unique aspects exist as well. One of these "unique aspects" for shuttle avoidance was wheel-running activity, for rA between these two was about -.71 ± .12, which implies that high "kinetic drive" may interfere with discrete-trial avoidance learning. Wheel running was not related to maze learning. One feature of the lite...

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...greatly improved subsequent avoidance of both the Harlan and Rockland Long-Evans strains (Levine and Wetzel, 1963); with infantile handling SpragueDawley and Rockland were equivalent, while Sprague-Dawley was superior under the unhandled control condition. Infantile trauma (loud noises) increased the number of errors on later learning of a four-unit T maze equally for the three strains of mice tested by Winston (1963). Lindzey and Winston (1962) reported that gentle stroking before a trial improved learning of a six-unit T maze for the C57/B1/1 strain but did not change the scores for C3H/Bi. Freedman (1958) reported that either indulging or disciplining puppies of four strains of dogs had very temporary differential effects upon later inhibition training. Thus, early experience has highly variable effects on the learning abilities of different genotypes. Experiences prior to training may also affect the expression of hybrid vigor. Winston (1964) observed that infantile trauma, a loud noise, increased the number of errors in a water-escape maze for inbred mice but had minimal effects upon the F1 hybrids. One consequence of this operation was that all hybrids were superior to HP in the trauma cond...

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...gh and low error scores on a 17-unit maze of known, high reliability (.95); he also reduced inbreeding by using only 50% full-sib matings. He later added high fertility, good health, and coat color to the selection criteria. His results, which are widely known among psychologists, showed clear divergence of the two lines, such that very little overlap existed by the last generation of selection, F22 (Tryon, 1940). The maze bright and dull strains, termed S1 and S3, have been maintained since then by random breeding and are still available today. A very similar selection study was conducted by Heron (1935) using an automatic Minnesota 12-unit maze, and very similar results were obtained. Whereas the parent population averaged about 85 errors on trials 3 through 17, by F16 there was almost no overlap, the mean errors being 46.9 for the brights and 116.0 for the dulls (Heron, 1941). For some reason the brights were very superior even on the first trial, while the rate of error reduction was about the same for the two strains. Finally, Thompson (1954) selected for "intelligence" by administering 24 different problems on the Hebb-Williams maze; he also used full-sib matings exclusively until F6. Th...

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...or the cross of DBA/2J and C57BL/6J. Corresponding values for total errors in 15 trials of a Lashley III maze were .50 (S × C) and .39 (D × C). Heritability Of the several methods available for calculating heritability (h 2 ), realized response to selection for learning appears to be the most efficient (Hill, 1971). It is unfortunate that the various selection studies mentioned above were improperly designed to allow estimation of realized heritability. Some of these difficulties are evident in Table 2, which lists several pertinent aspects of the experiments. A proper selection experiment by DeFries and Hegmann (1970) involving open field activity in mice is included in the table for purposes of comparison. No researcher can obtain today a population known to have the same genetic properties as any of those previous ones, because the breeding schemes employed by most animal suppliers are generally haphazard and are certainly not uniform for different suppliers of the same outbred strains. Also, in all studies, except those of Schaefer (1968) and Bovet et al. (1969), the selection criterion was a composite of the learning score of primary interest and some other trait such as running time or fertility. This...

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....5. Although the proper interpretation of these measures of ω 2 , C.G.D., and h 2 is not readily apparent, some limitations on their generality are obvious. The inherent genetic variation of a population influences greatly the results, since reduction of VG through inbreeding or of VA through selection would lead to the observation of low h 2 . Similarly, environmental attributes can influence the VE component. Intuitively, rearing under uniform conditions is expected to yield the largest possible proportion of genetic variance, because VE should be small. However, recent evidence reported by Henderson (1970) clearly demonstrates that the typical restrictive laboratory environment may actually suppress the manifestations of genetic variation and thereby yield a lower heritability score than would otherwise be obtained if the animals were raised in an enriched environment. Thus, the magnitude of the heritability coefficient is affected by the environment of the subjects as well as their actual genetic variation and, as a result, cannot be relied upon to be invariant in other worlds. Another factor must be the reliability of the learning measure itself. If the environmental component, "E," is partit...

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...en strains. When placed upon a random, segregating genetic background, albinism led to reduced levels of active avoidance learning (Winston and Lindzey, 1964; Winston, Lindzey, and Conner, 1967), water maze learning with either visual or spatial cues (Werboff, Anderson, and Ross, 1967), and straight alley running for food (Tyler, 1970). Albino mice were superior, however, at inhibitory avoidance learning (Winston, Lindzey, and Conner, 1967). Albinism on the isogenic C57BL/6J background was shown to reduce learning of a black-white water maze discrimination (Fuller, 1967) and jumpup avoidance (Henry and Schlesinger, 1967). Wilcock (1969) recently reviewed these various experiments and concluded that effects of albinism upon behavior are instances of trivial pleiotropy, because lack of eye pigment leads to suppression of nearly any active behavior under bright lights. Several studies have shown that behavioral differences between albino and pigmented mice are greatly reduced when a very dim light is employed over the test area (McReynolds, Weir, and DeFries, 1967; Thiessen, Lindzey, and Owen, 1970). In all of the above studies of albinism and learning which reported illumination conditions, the lights were quit...

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...ning phenotype in a population can be attributed to genetic differences among individuals? In the case of strain comparisons with a one-way analysis of variance design, this question can be answered by calculating the strength of effect (ω 2 ). In experiments involving breeding, it is customary to posit a linear model for genetic effects and then partition variances appropriately. If an individual's score or phenotype (P) is partitioned into components of genetic (G) and environmental (E) origin, and if G and E do not interact, then P = G + E, and the variances are such that Vp = VG + VE (see Roberts, 1967a, for a more complete presentation). The relative contribution of genetic differences is VG/Vp; this ratio is sometimes termed the coefficient of genetic determination (C.G.D.). A valid measure of this coefficient necessitates that the effects attributable to G and E be clearly distinguishable. For a multitude of reasons, direct measures of this ratio are not easily obtained. However, a related measure, heritability, has similar properties and can be estimated accurately. Heritability (h 2 ) is the ratio of additive genetic variance to total phenotypic variance. Additive variance (VA) is a ma...

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...ilure to modify open-field defecation by selection for shuttle avoidance learning (Broadhurst and Bignami, 1965) contrasts with the significant differences in shuttle avoidance obtained after selection for open-field defecation (Broadhurst and Levine, 1963). These difficulties may be attributable in part to previous measures of emotionality. Low activity, as indicated by few square crossings in an open field, has generally been held to indicate freezing or immobility, but direct observation of postures of several inbred and selected mouse strains has revealed freezing to be a very rare event (Streng, 1971); mice with low activity scores tend to spend more time "air sniffing" or "object sniffing." The open-field defecation measure seems to be related more to social dominance or territorial marking than to fear in some situations (Bruell, 1969; Brain and Nowell, 1969). Other evidence suggests that rate of responding in avoidance training may be more clearly related to "kinetic drive" than to fear or emotionality (Fuller, 1970). Thus, further research on genetic variation in emotionality and avoidance learning must await the development of more meaningful operational definitions of emotion or fear...

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.... The experiment by Cooper and Zubek (1958) demonstrated that rearing Thompson's (1954) bright and dull rat strains in either an enriched or an impoverished environment eliminated the strain differences in learning that were originally produced by selection in a normal lab environment. Likewise, pretraining experiences have been shown to affect some standard strains more than others. The handling of infant rats did not change later shuttle avoidance learning of the Sprague-Dawley strain, whereas handling greatly improved subsequent avoidance of both the Harlan and Rockland Long-Evans strains (Levine and Wetzel, 1963); with infantile handling SpragueDawley and Rockland were equivalent, while Sprague-Dawley was superior under the unhandled control condition. Infantile trauma (loud noises) increased the number of errors on later learning of a four-unit T maze equally for the three strains of mice tested by Winston (1963). Lindzey and Winston (1962) reported that gentle stroking before a trial improved learning of a six-unit T maze for the C57/B1/1 strain but did not change the scores for C3H/Bi. Freedman (1958) reported that either indulging or disciplining puppies of four strains of dogs had very temporary ...

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...should reflect primarily genetic variation, while within-strain variation should represent differences in postfertilization environment as well as error in measuring the behavior itself. Several estimates of ω 2 for strain comparisons are presented in Table 1. The values were derived from the F ratio for between-strains differences and the degrees of freedom between (dfb) and within (dfw) strains. It can be shown that the expression for estimating ω 2 given by Hays (1963, p. 382) reduces to for a one-way design with equal numbers of subjects per cell. It should be noted that only two reports (Scott and Fuller, 1965; Wahlsten, 1971) actually presented values for ω 2 . The remainder were derived by the present author. The wide range of estimated ω 2 values indicates that no simple statement can be made. It should be noted, however, that many values greater than 30% were obtained, which signifies a very substantial effect as judged by results from other areas of behavioral research. Coefficient of Genetic Determination Oliverio, Castellano, and Messeri (1971) have presented the only calculations of C.G.D. for a learning phenotype. They found C.G.D. for percent correct in 500 trials of shuttle avoidance lea...

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...sten and Cole, 1971). The learning abilities on diverse tasks of strains selected for learning rate on a single task are also of interest. Schaefer (1968), who selected for response duration in lever pressing, found that the mice with shorter response durations did in fact learn a T maze faster than the more persevering strain. This supported Schaefer's contention that response duration was an important determinant of intelligence. More extensive tests have been performed with the descendants of Tryon's lines (Brights are S1, Dulls are S3). Certainly, the most eminent study among these was by Searle (1949), who measured each subject on numerous maze tasks and other behaviors in addition to the original Tryon maze. Appropriately enough, S1 was quite superior to S3 on the original Tryon maze, and it was better on a 14-unit elevated maze as well, although some overlap existed in the latter scores. However, the S3 were superior to S1 rats in the water-escape tank, while no difference was apparent in the 16-unit and 6-unit discrimination tasks. The pattern of scores led Searle to suggest that a motivational difference existed, the Si strain being more highly motivated by hunger and the S3 by water-e...

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...hough some researchers have claimed to study the adaptive value of learning, their exclusive utilization of laboratory populations precludes meaningful interpretation of their results. Several methodological shortcomings of various experiments are considered, and important areas for future research are suggested. Article: Learning is a phenotype which has engaged the interests of numerous researchers seeking genetic bases for behavioral differences. In fact, much of the earliest research identifiable as behavior genetics dealt with some aspect of learning in animals (Bagg, 1916; Yerkes, 1916; Tolman, 1924). Ensuing experimentation was performed primarily by psychologists using genetically ill-defined populations. The rather recent appearance of standardized inbred mouse strains with widespread availability has led to renewed interest in the genetic analysis of learning, as well as other phenotypes. Several sophisticated quantitative genetic tools are now readily available for the study of learning. Examples of the application to learning research of selective breeding, the classical cross, the diallel cross, sib analysis, parent-offspring regression, and single-gene analysis have appeared recen...

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...and F2 generations, the intrasubject reliability of the maze test was so close to zero that Tolman abandoned his effort. Determined to avoid some of Tolman's problems, Tryon (1929) selected for high and low error scores on a 17-unit maze of known, high reliability (.95); he also reduced inbreeding by using only 50% full-sib matings. He later added high fertility, good health, and coat color to the selection criteria. His results, which are widely known among psychologists, showed clear divergence of the two lines, such that very little overlap existed by the last generation of selection, F22 (Tryon, 1940). The maze bright and dull strains, termed S1 and S3, have been maintained since then by random breeding and are still available today. A very similar selection study was conducted by Heron (1935) using an automatic Minnesota 12-unit maze, and very similar results were obtained. Whereas the parent population averaged about 85 errors on trials 3 through 17, by F16 there was almost no overlap, the mean errors being 46.9 for the brights and 116.0 for the dulls (Heron, 1941). For some reason the brights were very superior even on the first trial, while the rate of error reduction was about the sam...

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...been related to learning ability. Structural and organizational attributes of the brain have received scant attention in the genetic context. Wimer et al. (1969) found that inbred mouse strains which had a neocortex of relatively large volume tended to have a hippocampus of relatively small volume (Spearman r = -.83). They did not attempt to relate their data to learning ability. Visual pathways have been found to differ dramatically in albino and hooded rats (Lund, 1965). The organizational differences related to patterns of interocular transfer (Sheridan, 1965) and visual evoked potentials (Creel, Dustman, and Beck, 1970). Their relevance to normal learning differences has not been established. Neither have they proved that the differences are caused by the gene c in random bred populations. Given the large number of mutant genes which are known to affect brain organization (Sidman, Green, and Appel, 1965), it is likely that alleles more within the normal range of variation have similar effects upon organization. Future studies which examine detailed organizational aspects of brain, instead of homogenizing these differences, may detect patterns which relate to learning ability. Discussion of Genetic Correlate...

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...nflict situation was shown to decrease the learning of shuttle avoidance by BALB/c mice but to have no effect upon relearning by C57BL/10 mice (King and Mavromatis, 1956); an increase in freezing, a presumed concommitant of high emotion, was reported for the BALB strain. Skin-resistance changes resulting from electric shocks, which were believed to indicate relative fearfulness (Carran et al., 1964), were used to explain why the more "fearful" (i.e., greater resistance decrease after shock) C3H mice were better at both active shuttle (Carran et al., 1964) and passive avoidance (Carran, 1967). Fuller (1966) trained three strains of mice on Sidman avoidance in a shuttle box after injection of several doses of the tranquilizer chlorpromazine. While the rate of responding decreased for all strains at higher doses, the effect was minimal for the RF strain but quite large for C3HeB and C57BL/6 animals. Since the RF strain had a much lower operant rate than the other two under the placebo condition and showed little drug effect, it may have had a lower level of fear or emotion. Thus, in two experiments highly emotional animals learned to avoid more quickly or proficiently, in one experiment the highly...

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... crossing inbred strains to obtain F1, F2, and backcross generations provides an abundance of information which cannot be obtained by any environmental manipulations of inbred strains alone. Paramount among these benefits is the possibility of examining correlations between several aspects of learning which were observed to covary among the parent strains. When the strains are crossed, the measures of learning or other behaviors in the F1 and F2 generations may continue or cease to exhibit phenotypic correlations, depending on whether they are genetically related or independent, respectively. James (1941) seems to have been the first to employ this technique to study correlations. He observed correlations between body type and learning of leg-flexion avoidance and Pavlovian salivation training. The outcome of crossing two breeds was clear: In the two polar types ... there seems to be a definite correlation between bodily form and behavior. There is a harmonious relationship among the genetic factors for physical form, glandular conditions, and behavior. When the two polar types are bred together, however, this relation breaks up. A dog may inherit the bodily form of the basset hound, yet behav...

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...tended. They leave entirely unexplored the extent of sensory differences between strains, both in terms of relative acuities within a sensory mode and in terms of preferences for one sensory mode over another. Of course, such tests of sensory acuity and preference are time-consuming and require sophisticated learning paradigms. Nonetheless, they could be edifying. Several reports have appeared of differences in sensory processes between the Tryon rats. Tryon (1940) carried out numerous experiments which showed that surgically disrupting the senses had little effect on the behavior of Brights. Krechevsky (1933) tested Bright, Dull, and unselected rats on his insoluble hypothesis apparatus and observed that the Brights preferred spatial hypotheses, the Dulls used visual hypotheses, and the unselected rats showed no preference. Since these were the only differences noted, Krechevsky attributed the Bright-Dull difference to a "specific response ability" difference. A similar conclusion was reached by Wherry (1941), who subjected various response measures on the Tryon maze to factor analysis; the scores of Brights and Dulls on his three factors, forward going, food pointing, and goal gradient, suggested...

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...ackcrossed to CBA/J. Retinal degenerate offspring were not different from normals on jump-out avoidance, but they were greatly deficient at one-way avoidance (Wahlsten, 1972). Thus, many of the perplexing results of different experimenters (Bovet et al., 1969) may occur only when blind mice are required to run through a small hole in response to a visual stimulus. Although these results should surprise no one today, the presence of rd was certainly a source of much confusion in the past, and it impeded progress in the genetic analysis of learning. Albinism is no stranger to learning research. Lashley (1930) long ago demonstrated that the visual acuity of hooded rats exceeded that of albinos. More recent studies with mice have examined the effects of the c gene unconfounded with other genetic differences between strains. When placed upon a random, segregating genetic background, albinism led to reduced levels of active avoidance learning (Winston and Lindzey, 1964; Winston, Lindzey, and Conner, 1967), water maze learning with either visual or spatial cues (Werboff, Anderson, and Ross, 1967), and straight alley running for food (Tyler, 1970). Albino mice were superior, however, at inhibitory avoid...

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...urotransmitters does not correlate highly with maze learning (Rosenzweig, 1964). However, the relative concentration of ACh and AChE suggests that rats with higher ACh/AChE ratios are better able to solve mazes. As Rosenzweig himself pointed out, the data are not conclusive, and more research with other strains is needed. Nonetheless, the important idea that study of the joint functioning of many important neurochemicals is required to understand learning should be manifestly clear. Abundant research on genetic variation in the chemistry of mouse brain has been reported (Sudak and Maas, 1964; Schlesinger and Griek, 1970), but observed differences have not been related to learning ability. Structural and organizational attributes of the brain have received scant attention in the genetic context. Wimer et al. (1969) found that inbred mouse strains which had a neocortex of relatively large volume tended to have a hippocampus of relatively small volume (Spearman r = -.83). They did not attempt to relate their data to learning ability. Visual pathways have been found to differ dramatically in albino and hooded rats (Lund, 1965). The organizational differences related to patterns of interocular transfer (Sheridan, ...

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...ns, termed S1 and S3, have been maintained since then by random breeding and are still available today. A very similar selection study was conducted by Heron (1935) using an automatic Minnesota 12-unit maze, and very similar results were obtained. Whereas the parent population averaged about 85 errors on trials 3 through 17, by F16 there was almost no overlap, the mean errors being 46.9 for the brights and 116.0 for the dulls (Heron, 1941). For some reason the brights were very superior even on the first trial, while the rate of error reduction was about the same for the two strains. Finally, Thompson (1954) selected for "intelligence" by administering 24 different problems on the Hebb-Williams maze; he also used full-sib matings exclusively until F6. The error scores of the high and low lines diverged significantly, but by F6 so many matings were infertile that inbreeding had to be abandoned. Since these early efforts, psychologists have become aware that the two goals of selection, high- and lowscoring genotypes and genetic fixation by inbreeding, are diametrically opposed. Selection operates on genetic variance, which is progressively reduced by inbreeding. This is not to say that no response ...

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...d to maze learning. One feature of the literature on strain variation in avoidance learning appeared to argue against any significant general learning ability. The problem was that some investigators observed certain strains, e.g., C3H or CBA, to learn very slowly, if at all (Bovet et al., 1968; Bovet-Nitti, 1969), while others found the same strains to be among the best learners (Stasik, 1970; Collins, 1964). Wahlsten (1971) obtained this result within one experiment; the CBA/J strain learned jump-out avoidance most quickly but was very poor at one-way avoidance. Subsequent genetic analyses (Wahlsten, 1972) demonstrated that the interaction was caused by the gene retinal degeneration (rd). When effects of rd and albinism (c) were eliminated, strain ranks were similar with the two procedures. Although the above experiments with inbred mice indicate the importance of general learning ability, research with other species has frequently revealed substantial strain-by-training procedure interactions. Harrington (1968) reported that certain rat strains were much better on certain problems of the Hebb-Williams maze but were inferior on other problems. Pryor and Otis (1970) found that rats of the Buffal...

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... the two measures was different; only learning rate exhibited significant heterosis. Since the frequency of jumping may not be a perfect indicant of motivation during shock, motivational differences cannot be ruled out entirely. Selection for learning has produced motivational differences in two instances. As mentioned above for the Tryon strains, the Brights appeared to be more highly motivated by hunger, while the Dulls had greater aversion to water (Searle, 1949). Variable results obtained with shock motivation. Heron's (1935) rats were selected on a maze task very similar to Tryon's. When Heron and Skinner (1940) extinguished bar-pressing for food reward, they found that more rapid extinction for the maze dull strain could be attributed to its lower rate of pressing at the onset of extinction; they suggested that the brights were more hungry. Harris (1940) reanalyzed the original Heron maze data and discovered that the ratio of running time to mean errors on a trial was generally smaller for the brights, which was held to be indicative of a weaker drive state in the dulls. Kruse (1941) observed that the brights ate more food under the usual deprivation condition and that they seemed to be more emotion...

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...own to affect some standard strains more than others. The handling of infant rats did not change later shuttle avoidance learning of the Sprague-Dawley strain, whereas handling greatly improved subsequent avoidance of both the Harlan and Rockland Long-Evans strains (Levine and Wetzel, 1963); with infantile handling SpragueDawley and Rockland were equivalent, while Sprague-Dawley was superior under the unhandled control condition. Infantile trauma (loud noises) increased the number of errors on later learning of a four-unit T maze equally for the three strains of mice tested by Winston (1963). Lindzey and Winston (1962) reported that gentle stroking before a trial improved learning of a six-unit T maze for the C57/B1/1 strain but did not change the scores for C3H/Bi. Freedman (1958) reported that either indulging or disciplining puppies of four strains of dogs had very temporary differential effects upon later inhibition training. Thus, early experience has highly variable effects on the learning abilities of different genotypes. Experiences prior to training may also affect the expression of hybrid vigor. Winston (1964) observed that infantile trauma, a loud noise, increased the number of errors in a water-...

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...dance. Subsequent genetic analyses (Wahlsten, 1972) demonstrated that the interaction was caused by the gene retinal degeneration (rd). When effects of rd and albinism (c) were eliminated, strain ranks were similar with the two procedures. Although the above experiments with inbred mice indicate the importance of general learning ability, research with other species has frequently revealed substantial strain-by-training procedure interactions. Harrington (1968) reported that certain rat strains were much better on certain problems of the Hebb-Williams maze but were inferior on other problems. Pryor and Otis (1970) found that rats of the Buffalo strain achieved criterion more quickly than Fischer rats for successive brightness discrimination in an underwater T maze but that the Fischer strain was superior at pole-displacement avoidance learning. James (1941) subjected basset, German shepherd, and saluki dogs, which were rated as lethargic, active, and very active, respectively, to restraint in a conditioned reflex stand and then to leg flexion avoidance training. The lethargic bassets submitted easily to restraint, required intense shock to elicit a leg flexion, and never performed the avoidance consist...

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...iek, 1970), but observed differences have not been related to learning ability. Structural and organizational attributes of the brain have received scant attention in the genetic context. Wimer et al. (1969) found that inbred mouse strains which had a neocortex of relatively large volume tended to have a hippocampus of relatively small volume (Spearman r = -.83). They did not attempt to relate their data to learning ability. Visual pathways have been found to differ dramatically in albino and hooded rats (Lund, 1965). The organizational differences related to patterns of interocular transfer (Sheridan, 1965) and visual evoked potentials (Creel, Dustman, and Beck, 1970). Their relevance to normal learning differences has not been established. Neither have they proved that the differences are caused by the gene c in random bred populations. Given the large number of mutant genes which are known to affect brain organization (Sidman, Green, and Appel, 1965), it is likely that alleles more within the normal range of variation have similar effects upon organization. Future studies which examine detailed organizational aspects of brain, instead of homogenizing these differences, may detect patterns whic...

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...olidation was normally faster for S1 but could be accelerated in S3 by administering stimulant drugs. This notion was strengthened by a study of the time-dependent effects of posttrial ECS using a Lashley III maze and one trial per day (Thomson et al., 1961); if no ECS was given, errors by S1 and S3 were equal, but ECS 45 sec after a trial increased error scores more for S3 than S1 and ECS at 75 sec increased errors above control levels only for S3. Similar facilitation of learning by posttrial injection of physostigmine for S3 but not for S1 on a Lashley III maze was reported by Stratton and Petrinovich (1963), but they observed a large difference in favor of S1 in the control group at one trial per day, which contradicted the finding of Thomson et al. (1961). Perhaps this can be attributed to their learning measure, trials to criterion, which differed from the usual procedure of giving a fixed amount of training. Although the experiments did not prove that the original Tryon strains diverged in learning rate because selection produced memory differences, McGaugh's research leaves little doubt that the S1 and S3 strains differed in memory processes. The differences were of such a magnitude as to ac...

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...(Henderson, 1968a; Wahlsten, 1971) reported lower values of h 2 (.2) and ω 2 (.1). Estimation of rtt will aid the interpretation of h 2 in the future. The magnitude of ω 2 and h 2 may also be influenced by the difficulty of the task employed. Wahlsten (1971) found that requiring mice to either run (one-way) or jump (jump-out) led to ω 2 values of .34 and .18, respectively, but that a smaller ω 2 of .11 resulted when each subject could either run or jump (optional) to escape or avoid shock (see Table 1). Other simple tasks such as CER conditioning (Henderson, 1968a) and straight-alley running (Tyler and McClearn, 1970) show low heritabilities (.2 to .3), while the more difficult shuttle avoidance yields C.G.D. of over .6 and h 2 of about .5. Thus, genotypes which are all sufficient for learning simple tasks may not be equally effective when the demands for processing information are increased. Since the above studies provide only indirect evidence, this idea should be subjected to direct testing in the future. It will be necessary to devise a battery of tests in which only task difficulty is varied without changing the source of motivation, the relevant sensory modality, or the motor response requirements. ...

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... population, the average degree of dominance may be determined by comparing the F1 mean score to the mean of the two parent strains, the midparent score (MP), or to the highest scoring parent (HP). All instances where F1 is greater than MP are characterized by hybrid vigor or heterosis. The results of several such genetic studies of learning are summarized in Table 4. In most studies employing inbred mouse strains as parents, significant directional dominance was observed. The F1 hybrids were generally superior to the average of their parents for learning of a two-choice maze for food reward (Vicari, 1929), lever pressing for food reward (Smart, 1970), water-escape learning (Winston, 1964; Winston and Lindzey, 1964), shock-avoidance learning (Collins, 1964; Schlesinger and Wimer, 1967; Abeelen, 1966; Rose and Parsons, 1970; Wahlsten, 1971; Oliverio et al., 1971), and CER conditioning (Henderson, 1968a). Many instances of overdominance were also reported. Several experiments with selected strains, summarized in Table 4, have been reported. Neither Tryon (1940) nor McGaugh, Westbrook, and Burt (1961) found heterosis in a cross of the Tryon bright (S1) and dull (S3) strains. In both studies the F1...

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... genetic variation, while within-strain variation should represent differences in postfertilization environment as well as error in measuring the behavior itself. Several estimates of ω 2 for strain comparisons are presented in Table 1. The values were derived from the F ratio for between-strains differences and the degrees of freedom between (dfb) and within (dfw) strains. It can be shown that the expression for estimating ω 2 given by Hays (1963, p. 382) reduces to for a one-way design with equal numbers of subjects per cell. It should be noted that only two reports (Scott and Fuller, 1965; Wahlsten, 1971) actually presented values for ω 2 . The remainder were derived by the present author. The wide range of estimated ω 2 values indicates that no simple statement can be made. It should be noted, however, that many values greater than 30% were obtained, which signifies a very substantial effect as judged by results from other areas of behavioral research. Coefficient of Genetic Determination Oliverio, Castellano, and Messeri (1971) have presented the only calculations of C.G.D. for a learning phenotype. They found C.G.D. for percent correct in 500 trials of shuttle avoidance learning to be .64 f...

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...interest and some other trait such as running time or fertility. This means that the response to selection no longer has a simple relation (i.e., heritability) to cumulated selection differential; it is instead dependent upon the heritability of the composite and the genetic correlation between learning and the other components of the selection criterion. Finally, none of the experiments utilized an adequate unselected control line, which is quite important for minimizing the effects of environmental changes from one selection generation to the next and for detecting an asymmetrical response (DeFries, 1967). These several shortcomings may be contrasted to the DeFries and Hegmann experiment, in which repeatability was assured by the adoption of a cross between genetically fixed inbred strains, a single response measure served as the selection criterion, inbreeding was minimized, and replicated control and selected lines were included. Other methods for estimating h 2 (see Roberts, 1967a; Falconer, 1960) have been employed with greater success. These studies are summarized in Table 3 together with estimates from two selective breeding studies. It is interesting that heritability measures show a sm...

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...oor STM and good LTM. A most unfortunate aspect of their work was their concentration on two inbred strains, C3H/HeJ and DBA/2J, which differ in numerous ways other than learning ability. Recent evidence has demonstrated that strain differences are quite small when tasks are employed that do not require the utilization of visual cues by C3H mice with retinal degeneration. The strains C3H/HeJ, CBA/J, and DBA/2J all show good short-term retention of a simple active avoidance task (Wahlsten, 1971, 1972). Both C3H and DBA also appear to have intact long-term retention for several avoidance tasks (Duncan, Grossen, and Hunt, 1971; Wahlsten and Weening, unpublished data). Results of other researchers reporting memory differences in mice (Wimer et al., 1968; Randt et al., 1971) must also be viewed with skepticism because they showed that DBA/2J exhibits poor long-term retention, which is contrary to the data of many others. Other studies which involved tests of long-term retention in many strains found no significant strain differences (Henderson, 1968a; Stasik, 1970). Thus, certain inbred strains of mice may have impaired long-term retention or retarded consolidation rates, but their identities are currently unknown. I...

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...otivation and learning has a long history of theoretical dispute (see discussion by Kimble, 1961, Chap. 13). One central issue concerns the necessity of proper motivation to assure learning at all. Unfortunately, demonstrations of latent learning, sensory preconditioning, and transfer between drive states have not been attempted with genetic experiments. Whereas diverse opinions exist concerning the need for motivation to assure the acquisition of information, most theorists recognize the importance of proper motivation in order to guarantee the reliable performance of a learned response (see Estes, 1969). Vast research indicates that simple, unitary responses are acquired more rapidly when the animal is more highly motivated by either food or water deprivation or electric shock (Bitterman and Schoel, 1970). Hence it would surprise no one if strains found to learn at different rates also were differentially motivated by identical operations or if motivation changed as a correlated response to selection for learning rate. Of course, neither would it be surprising if motivational differences accounted for only part of the variation in learning rates. Pure associative learning ability might vary ...

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...as better on a 14-unit elevated maze as well, although some overlap existed in the latter scores. However, the S3 were superior to S1 rats in the water-escape tank, while no difference was apparent in the 16-unit and 6-unit discrimination tasks. The pattern of scores led Searle to suggest that a motivational difference existed, the Si strain being more highly motivated by hunger and the S3 by water-escape. Rosenzweig, Krech, Bennett, and Longweil (1958) tested S1 and S3 on the Hebb-Williams, Dashiell, and Lashley III mazes using food reward and found the S1 strain to be superior on all three. Fehmi and McGaugh (1961) found that S1 learned a horizontal-vertical discrimination faster than S3, but they found no difference in black-white discrimination learning. Their result was extended when Wolfer (1963) observed that S1 exhibited fewer errors on a Lashley III maze than S3 at each of three different deprivation levels but that the two always had similar running times. In several recent studies avoidance learning has been tested as well. The S3 rats were better at avoidance learning in an ATLAS maze with visual cues (Markowitz and Sorrells, 1964) but not with spatial cues (Markowitz and Becker, 1965), while ...

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...with other genetic differences between strains. When placed upon a random, segregating genetic background, albinism led to reduced levels of active avoidance learning (Winston and Lindzey, 1964; Winston, Lindzey, and Conner, 1967), water maze learning with either visual or spatial cues (Werboff, Anderson, and Ross, 1967), and straight alley running for food (Tyler, 1970). Albino mice were superior, however, at inhibitory avoidance learning (Winston, Lindzey, and Conner, 1967). Albinism on the isogenic C57BL/6J background was shown to reduce learning of a black-white water maze discrimination (Fuller, 1967) and jumpup avoidance (Henry and Schlesinger, 1967). Wilcock (1969) recently reviewed these various experiments and concluded that effects of albinism upon behavior are instances of trivial pleiotropy, because lack of eye pigment leads to suppression of nearly any active behavior under bright lights. Several studies have shown that behavioral differences between albino and pigmented mice are greatly reduced when a very dim light is employed over the test area (McReynolds, Weir, and DeFries, 1967; Thiessen, Lindzey, and Owen, 1970). In all of the above studies of albinism and learning which rep...

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...neral learning ability in animals may be analogous to the concept of intelligence (g) in humans and in this respect is a measure which should transcend the specific requirements of any one task. Bovet et al. (1969) reported that the rank ordering of nine mouse strains on a shuttle-avoidance task was very consistent with the relative abilities of the strains in Lashley III maze learning (Spearman r = .92). Since the two training procedures were vastly different, the similar ordering of strains suggested that the genetic differences affected learning at a quite general level. On the other hand, Fuller (1970) tested four inbred mouse strains on either active or passive shuttle avoidance with a procedure that used no discriminative CS. Strain, rank orders were completely inverted for the two procedures. Pharmacological manipulations suggested that activity or "kinetic drive" differences were more important than any differences in general learning ability. Resolution of these seemingly divergent findings has been made possible by the recent work of Oliverio et al. (1971) mentioned above. They calculated genetic correlations between shuttle avoidance learning, Lashley III maze learning, and wheel run...

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...nsory input obviously occupies a position of primacy. Information must enter the brain before it can be evaluated and stored. Genotypes which lead to differential abilities to gather sensory data should differ in learning rates as a result. Research with strains homozygous for retinal degeneration (rd) has revealed that visual input is necessary for solving certain tasks but not for others. Strains such as C3H and CBA that have rodless retinas did very poorly on black-white discrimination (Wimer and Weller, 1969), pattern discrimination (Bovet-Nitti, 1969), and barpressing to turn on a light (Goodrick, 1967), but they could learn a position discrimination quite well (Alpern and Marriott, 1972). Although C3H mice performed very poorly when a light stimulus was employed (Bovet et al., 1968), Duncan, Grossen, and Hunt (1971) have shown that good avoidance learning may occur when the light is replaced by a buzzer stimulus (see also Oliverio, 1967). The CBA/J strain was able to learn rapidly to avoid when the task required jumping onto a large platform but encountered great difficulty when the task required running through a small hole (Wahlsten, 1971). However, the CBA/CaJ subline, which has normal v...

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.... Wahlsten (1971) obtained this result within one experiment; the CBA/J strain learned jump-out avoidance most quickly but was very poor at one-way avoidance. Subsequent genetic analyses (Wahlsten, 1972) demonstrated that the interaction was caused by the gene retinal degeneration (rd). When effects of rd and albinism (c) were eliminated, strain ranks were similar with the two procedures. Although the above experiments with inbred mice indicate the importance of general learning ability, research with other species has frequently revealed substantial strain-by-training procedure interactions. Harrington (1968) reported that certain rat strains were much better on certain problems of the Hebb-Williams maze but were inferior on other problems. Pryor and Otis (1970) found that rats of the Buffalo strain achieved criterion more quickly than Fischer rats for successive brightness discrimination in an underwater T maze but that the Fischer strain was superior at pole-displacement avoidance learning. James (1941) subjected basset, German shepherd, and saluki dogs, which were rated as lethargic, active, and very active, respectively, to restraint in a conditioned reflex stand and then to leg flexion avoida...

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...duced motivational differences in two instances. As mentioned above for the Tryon strains, the Brights appeared to be more highly motivated by hunger, while the Dulls had greater aversion to water (Searle, 1949). Variable results obtained with shock motivation. Heron's (1935) rats were selected on a maze task very similar to Tryon's. When Heron and Skinner (1940) extinguished bar-pressing for food reward, they found that more rapid extinction for the maze dull strain could be attributed to its lower rate of pressing at the onset of extinction; they suggested that the brights were more hungry. Harris (1940) reanalyzed the original Heron maze data and discovered that the ratio of running time to mean errors on a trial was generally smaller for the brights, which was held to be indicative of a weaker drive state in the dulls. Kruse (1941) observed that the brights ate more food under the usual deprivation condition and that they seemed to be more emotional, too. In these respects, Heron's rats resembled those of Tryon, for mild motivational differences were noted in both groups. In neither case were the motivational differences proved to be genetically related to learning differences. The McGill b...

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...ue to pretesting environment and e from noise in the measuring instrument, it follows that Vp = VG + VE + Ve. Ve will be small for tests with high test-retest reliability (rtt) or when many repeated measures on the same animals are administered. The data presented by Bovet, Bovet-Nitti, and Oliverio (1969, p. 140) show that individual scores in shuttle avoidance are very stable from day to day when 100 trials are administered; in turn they find large strain differences (ω 2 = .95, Table 1). On the other hand, experiments which examined relatively short learning sequences of only a few trials (Henderson, 1968a; Wahlsten, 1971) reported lower values of h 2 (.2) and ω 2 (.1). Estimation of rtt will aid the interpretation of h 2 in the future. The magnitude of ω 2 and h 2 may also be influenced by the difficulty of the task employed. Wahlsten (1971) found that requiring mice to either run (one-way) or jump (jump-out) led to ω 2 values of .34 and .18, respectively, but that a smaller ω 2 of .11 resulted when each subject could either run or jump (optional) to escape or avoid shock (see Table 1). Other simple tasks such as CER conditioning (Henderson, 1968a) and straight-alley running (Tyler and McClea...

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...ue to pretesting environment and e from noise in the measuring instrument, it follows that Vp = VG + VE + Ve. Ve will be small for tests with high test-retest reliability (rtt) or when many repeated measures on the same animals are administered. The data presented by Bovet, Bovet-Nitti, and Oliverio (1969, p. 140) show that individual scores in shuttle avoidance are very stable from day to day when 100 trials are administered; in turn they find large strain differences (ω 2 = .95, Table 1). On the other hand, experiments which examined relatively short learning sequences of only a few trials (Henderson, 1968a; Wahlsten, 1971) reported lower values of h 2 (.2) and ω 2 (.1). Estimation of rtt will aid the interpretation of h 2 in the future. The magnitude of ω 2 and h 2 may also be influenced by the difficulty of the task employed. Wahlsten (1971) found that requiring mice to either run (one-way) or jump (jump-out) led to ω 2 values of .34 and .18, respectively, but that a smaller ω 2 of .11 resulted when each subject could either run or jump (optional) to escape or avoid shock (see Table 1). Other simple tasks such as CER conditioning (Henderson, 1968a) and straight-alley running (Tyler and McClea...

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...s, showed clear divergence of the two lines, such that very little overlap existed by the last generation of selection, F22 (Tryon, 1940). The maze bright and dull strains, termed S1 and S3, have been maintained since then by random breeding and are still available today. A very similar selection study was conducted by Heron (1935) using an automatic Minnesota 12-unit maze, and very similar results were obtained. Whereas the parent population averaged about 85 errors on trials 3 through 17, by F16 there was almost no overlap, the mean errors being 46.9 for the brights and 116.0 for the dulls (Heron, 1941). For some reason the brights were very superior even on the first trial, while the rate of error reduction was about the same for the two strains. Finally, Thompson (1954) selected for "intelligence" by administering 24 different problems on the Hebb-Williams maze; he also used full-sib matings exclusively until F6. The error scores of the high and low lines diverged significantly, but by F6 so many matings were infertile that inbreeding had to be abandoned. Since these early efforts, psychologists have become aware that the two goals of selection, high- and lowscoring genotypes and genetic f...

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...acement avoidance learning. James (1941) subjected basset, German shepherd, and saluki dogs, which were rated as lethargic, active, and very active, respectively, to restraint in a conditioned reflex stand and then to leg flexion avoidance training. The lethargic bassets submitted easily to restraint, required intense shock to elicit a leg flexion, and never performed the avoidance consistently, whereas the German shepherds struggled violently when restrained but learned to avoid very rapidly. He later trained similar groups of dogs on conditioned salivation and then on leg-flexion avoidance (James, 1953). The active dogs gave poor conditioned salivary responses but were good at avoidance, although some struggled to a degree which made reliable measurement of any learning quite impossible. The lethargic types had good salivation responses early in training, but they tended to fall asleep later; they seldom learned to avoid. However, dogs of medium activity demonstrated both good salivation and proficient avoidance. Dykman, Murphree, and Peters (1969) also observed interactions with their bold and friendly (A) and timid (E) strains of pointer dogs. When operant bar-press training for food rewar...

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...s for each strain was +.06, which hardly supported any interpretation of the emotionality hypothesis. Reynierse (1970) has performed several experiments which suggest that rats of the Sasco strain are more emotional and extinguish avoidance responding more quickly than Holtzman rats under certain conditions. However, in no experiment was a strain difference in rate of initial acquisition observed under any duration of safe compartment confinement. A conflict situation was shown to decrease the learning of shuttle avoidance by BALB/c mice but to have no effect upon relearning by C57BL/10 mice (King and Mavromatis, 1956); an increase in freezing, a presumed concommitant of high emotion, was reported for the BALB strain. Skin-resistance changes resulting from electric shocks, which were believed to indicate relative fearfulness (Carran et al., 1964), were used to explain why the more "fearful" (i.e., greater resistance decrease after shock) C3H mice were better at both active shuttle (Carran et al., 1964) and passive avoidance (Carran, 1967). Fuller (1966) trained three strains of mice on Sidman avoidance in a shuttle box after injection of several doses of the tranquilizer chlorpromazine. While the rate of re...

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...ned with shock motivation. Heron's (1935) rats were selected on a maze task very similar to Tryon's. When Heron and Skinner (1940) extinguished bar-pressing for food reward, they found that more rapid extinction for the maze dull strain could be attributed to its lower rate of pressing at the onset of extinction; they suggested that the brights were more hungry. Harris (1940) reanalyzed the original Heron maze data and discovered that the ratio of running time to mean errors on a trial was generally smaller for the brights, which was held to be indicative of a weaker drive state in the dulls. Kruse (1941) observed that the brights ate more food under the usual deprivation condition and that they seemed to be more emotional, too. In these respects, Heron's rats resembled those of Tryon, for mild motivational differences were noted in both groups. In neither case were the motivational differences proved to be genetically related to learning differences. The McGill bright and dull rats selected on the Hebb-Williams maze (Thompson, 1954) have also received some attention. It is interesting to note that a prime reason for using the Hebb-Williams battery of problems was to select for a more general ...

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...all three. Fehmi and McGaugh (1961) found that S1 learned a horizontal-vertical discrimination faster than S3, but they found no difference in black-white discrimination learning. Their result was extended when Wolfer (1963) observed that S1 exhibited fewer errors on a Lashley III maze than S3 at each of three different deprivation levels but that the two always had similar running times. In several recent studies avoidance learning has been tested as well. The S3 rats were better at avoidance learning in an ATLAS maze with visual cues (Markowitz and Sorrells, 1964) but not with spatial cues (Markowitz and Becker, 1965), while the S1 strain seemed to be superior in wheel-turn avoidance (Zerbolio et al., 1965) but inferior in jump-out avoidance (Powell and Leach, 1967). Thus, research with the Tryon strains has confirmed the findings of the many strain comparisons in that reversals in learning rates may occur when strains are tested on tasks having many differences. The existence of such interactions makes it imperative that the degree of genetic correlation between tasks be quantified as was done by Oliverio et al. (1971). The wisdom of extending these methods to a larger number of strains and tasks in futur...

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...ng food reward and found the S1 strain to be superior on all three. Fehmi and McGaugh (1961) found that S1 learned a horizontal-vertical discrimination faster than S3, but they found no difference in black-white discrimination learning. Their result was extended when Wolfer (1963) observed that S1 exhibited fewer errors on a Lashley III maze than S3 at each of three different deprivation levels but that the two always had similar running times. In several recent studies avoidance learning has been tested as well. The S3 rats were better at avoidance learning in an ATLAS maze with visual cues (Markowitz and Sorrells, 1964) but not with spatial cues (Markowitz and Becker, 1965), while the S1 strain seemed to be superior in wheel-turn avoidance (Zerbolio et al., 1965) but inferior in jump-out avoidance (Powell and Leach, 1967). Thus, research with the Tryon strains has confirmed the findings of the many strain comparisons in that reversals in learning rates may occur when strains are tested on tasks having many differences. The existence of such interactions makes it imperative that the degree of genetic correlation between tasks be quantified as was done by Oliverio et al. (1971). The wisdom of extending these m...

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... for efficient storage. Likewise, a strain which could not enter information into long-term storage at all would appear to be grossly deficient with widely spaced trials. The work by McGaugh and his colleagues has shown that the Tryon S1 and S3 strains differ in the timedependent aspects of memory storage but that both strains are able to enter information properly into long-term storage. The spacing of trials on a Lashley III maze was important, for the superiority of S1 at short intervals (ITI 30 sec) vanished at an ITI of 5 min or more (McGaugh, Jennings, and Thomson, 1962). A later study (McGaugh and Cole, 1965) found that ITI interacted with age, for in young rats S1 was superior only at a long ITI (30 min). The difference between S1 and S3 with massed trials was eliminated by pretrial injection of the drug 1757 I.S., which improved learning only for S3 (McGaugh, Westbrook, and Burt, 1961). Spaced trials (one per day) gave equivalent performance for S1 and S3 on a 14-unit T maze, and posttrial injection of picrotoxin greatly facilitated learning by S3 only (Breen and McGaugh, 1961). These studies supported the hypothesis that the rate of consolidation was normally faster for S1 but could be accelera...

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...nterval is considerably shorter than the time required for efficient storage. Likewise, a strain which could not enter information into long-term storage at all would appear to be grossly deficient with widely spaced trials. The work by McGaugh and his colleagues has shown that the Tryon S1 and S3 strains differ in the timedependent aspects of memory storage but that both strains are able to enter information properly into long-term storage. The spacing of trials on a Lashley III maze was important, for the superiority of S1 at short intervals (ITI 30 sec) vanished at an ITI of 5 min or more (McGaugh, Jennings, and Thomson, 1962). A later study (McGaugh and Cole, 1965) found that ITI interacted with age, for in young rats S1 was superior only at a long ITI (30 min). The difference between S1 and S3 with massed trials was eliminated by pretrial injection of the drug 1757 I.S., which improved learning only for S3 (McGaugh, Westbrook, and Burt, 1961). Spaced trials (one per day) gave equivalent performance for S1 and S3 on a 14-unit T maze, and posttrial injection of picrotoxin greatly facilitated learning by S3 only (Breen and McGaugh, 1961). These studies supported the hypothesis that the rate of consolidation was nor...

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...ependent aspects of memory storage but that both strains are able to enter information properly into long-term storage. The spacing of trials on a Lashley III maze was important, for the superiority of S1 at short intervals (ITI 30 sec) vanished at an ITI of 5 min or more (McGaugh, Jennings, and Thomson, 1962). A later study (McGaugh and Cole, 1965) found that ITI interacted with age, for in young rats S1 was superior only at a long ITI (30 min). The difference between S1 and S3 with massed trials was eliminated by pretrial injection of the drug 1757 I.S., which improved learning only for S3 (McGaugh, Westbrook, and Burt, 1961). Spaced trials (one per day) gave equivalent performance for S1 and S3 on a 14-unit T maze, and posttrial injection of picrotoxin greatly facilitated learning by S3 only (Breen and McGaugh, 1961). These studies supported the hypothesis that the rate of consolidation was normally faster for S1 but could be accelerated in S3 by administering stimulant drugs. This notion was strengthened by a study of the time-dependent effects of posttrial ECS using a Lashley III maze and one trial per day (Thomson et al., 1961); if no ECS was given, errors by S1 and S3 were equal, but ECS 45 sec after a trial...

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...7). Albinism on the isogenic C57BL/6J background was shown to reduce learning of a black-white water maze discrimination (Fuller, 1967) and jumpup avoidance (Henry and Schlesinger, 1967). Wilcock (1969) recently reviewed these various experiments and concluded that effects of albinism upon behavior are instances of trivial pleiotropy, because lack of eye pigment leads to suppression of nearly any active behavior under bright lights. Several studies have shown that behavioral differences between albino and pigmented mice are greatly reduced when a very dim light is employed over the test area (McReynolds, Weir, and DeFries, 1967; Thiessen, Lindzey, and Owen, 1970). In all of the above studies of albinism and learning which reported illumination conditions, the lights were quite bright, although precise values were never given by the experimenters. Wilcock estimated that they ranged from 50 to 180 ft-c, which is far in excess of levels found to suppress activity in an open-field (McReynolds et al., 1967). Therefore, the albinism effect may have nothing to do with central nervous system differences. Wilcock's interpretation is supported by a recent experiment by Wahlsten (1972). The albino strain A/J was observed to le...

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...ssary for solving certain tasks but not for others. Strains such as C3H and CBA that have rodless retinas did very poorly on black-white discrimination (Wimer and Weller, 1969), pattern discrimination (Bovet-Nitti, 1969), and barpressing to turn on a light (Goodrick, 1967), but they could learn a position discrimination quite well (Alpern and Marriott, 1972). Although C3H mice performed very poorly when a light stimulus was employed (Bovet et al., 1968), Duncan, Grossen, and Hunt (1971) have shown that good avoidance learning may occur when the light is replaced by a buzzer stimulus (see also Oliverio, 1967). The CBA/J strain was able to learn rapidly to avoid when the task required jumping onto a large platform but encountered great difficulty when the task required running through a small hole (Wahlsten, 1971). However, the CBA/CaJ subline, which has normal vision, was able to learn both tasks as well as other strains with normal vision (Wahlsten, 1972). That this difference between CBA/J and CBA/CaJ was a result of rd became clear when F1 mice of a CBA/J by C57BL/6J cross were backcrossed to CBA/J. Retinal degenerate offspring were not different from normals on jump-out avoidance, but they wer...

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...n in F1 variance was of a magnitude similar to several examples given by Falconer (1960, Table 15.2). Rose and Parsons (1970) noted reduced variability in a learning score for F1 hybrids only early in training. Another problem appears in studies of dominance variance in heterogeneous populations. Significant dominance variance will lead to an intraclass correlation between full-sibs which is more than twice that between half-sibs in sib analysis (see Falconer, 1960). However, applications of sib analysis to learning (Table 3) have found no evidence of dominance variance (Willham et al., 1963; Oliverio, 1971; Oliverio et al., 1971). This was somewhat unexpected in the experiment of Oliverio et al. (1971), since substantial dominance was indicated in the crosses of inbred strains from which the randomly bred populations were derived. These results also suggest that epistasis may be important. Thus, neither of the criteria for inheritance of fitness characters, low heritability and heterozygote superiority, are unequivocally met by current data on the learning phenotype. Another problem for the study of the adaptive value of learning is that genetic research has been conducted in the lab with domes...

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...discrimination learning. Their result was extended when Wolfer (1963) observed that S1 exhibited fewer errors on a Lashley III maze than S3 at each of three different deprivation levels but that the two always had similar running times. In several recent studies avoidance learning has been tested as well. The S3 rats were better at avoidance learning in an ATLAS maze with visual cues (Markowitz and Sorrells, 1964) but not with spatial cues (Markowitz and Becker, 1965), while the S1 strain seemed to be superior in wheel-turn avoidance (Zerbolio et al., 1965) but inferior in jump-out avoidance (Powell and Leach, 1967). Thus, research with the Tryon strains has confirmed the findings of the many strain comparisons in that reversals in learning rates may occur when strains are tested on tasks having many differences. The existence of such interactions makes it imperative that the degree of genetic correlation between tasks be quantified as was done by Oliverio et al. (1971). The wisdom of extending these methods to a larger number of strains and tasks in future research needs no emphasis. Of course, learning rate is one thing, but a full-blown law of learning is quite something else. Strains could differ wid...

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...her than learning ability. Recent evidence has demonstrated that strain differences are quite small when tasks are employed that do not require the utilization of visual cues by C3H mice with retinal degeneration. The strains C3H/HeJ, CBA/J, and DBA/2J all show good short-term retention of a simple active avoidance task (Wahlsten, 1971, 1972). Both C3H and DBA also appear to have intact long-term retention for several avoidance tasks (Duncan, Grossen, and Hunt, 1971; Wahlsten and Weening, unpublished data). Results of other researchers reporting memory differences in mice (Wimer et al., 1968; Randt et al., 1971) must also be viewed with skepticism because they showed that DBA/2J exhibits poor long-term retention, which is contrary to the data of many others. Other studies which involved tests of long-term retention in many strains found no significant strain differences (Henderson, 1968a; Stasik, 1970). Thus, certain inbred strains of mice may have impaired long-term retention or retarded consolidation rates, but their identities are currently unknown. If memory variations underly differences in learning rate for a wide range of strains besides S1 and S3, it will be necessary for research of a magnit...

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...sk, freezing may appear to be a concommitant of great "fear" or "emotionality," or it may be learned because of unforeseen reinforcement contingencies which encourage freezing (McAllister and McAllister, 1971). Wilcock and Broadhurst (1967) obtained measures of defecation and ambulation in an open field, a presumed test of emotionality, in five inbred rat strains and then trained them in shuttle avoidance. The Pearson correlation between mean open-field defecations and mean number of avoidances for each strain was +.06, which hardly supported any interpretation of the emotionality hypothesis. Reynierse (1970) has performed several experiments which suggest that rats of the Sasco strain are more emotional and extinguish avoidance responding more quickly than Holtzman rats under certain conditions. However, in no experiment was a strain difference in rate of initial acquisition observed under any duration of safe compartment confinement. A conflict situation was shown to decrease the learning of shuttle avoidance by BALB/c mice but to have no effect upon relearning by C57BL/10 mice (King and Mavromatis, 1956); an increase in freezing, a presumed concommitant of high emotion, was reported for the BAL...

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...ning phenotype in a population can be attributed to genetic differences among individuals? In the case of strain comparisons with a one-way analysis of variance design, this question can be answered by calculating the strength of effect (ω 2 ). In experiments involving breeding, it is customary to posit a linear model for genetic effects and then partition variances appropriately. If an individual's score or phenotype (P) is partitioned into components of genetic (G) and environmental (E) origin, and if G and E do not interact, then P = G + E, and the variances are such that Vp = VG + VE (see Roberts, 1967a, for a more complete presentation). The relative contribution of genetic differences is VG/Vp; this ratio is sometimes termed the coefficient of genetic determination (C.G.D.). A valid measure of this coefficient necessitates that the effects attributable to G and E be clearly distinguishable. For a multitude of reasons, direct measures of this ratio are not easily obtained. However, a related measure, heritability, has similar properties and can be estimated accurately. Heritability (h 2 ) is the ratio of additive genetic variance to total phenotypic variance. Additive variance (VA) is a ma...

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...where F1 is greater than MP are characterized by hybrid vigor or heterosis. The results of several such genetic studies of learning are summarized in Table 4. In most studies employing inbred mouse strains as parents, significant directional dominance was observed. The F1 hybrids were generally superior to the average of their parents for learning of a two-choice maze for food reward (Vicari, 1929), lever pressing for food reward (Smart, 1970), water-escape learning (Winston, 1964; Winston and Lindzey, 1964), shock-avoidance learning (Collins, 1964; Schlesinger and Wimer, 1967; Abeelen, 1966; Rose and Parsons, 1970; Wahlsten, 1971; Oliverio et al., 1971), and CER conditioning (Henderson, 1968a). Many instances of overdominance were also reported. Several experiments with selected strains, summarized in Table 4, have been reported. Neither Tryon (1940) nor McGaugh, Westbrook, and Burt (1961) found heterosis in a cross of the Tryon bright (S1) and dull (S3) strains. In both studies the F1 mean was very close to MP. Bignami (1965) obtained moderate heterosis in a cross of his high (RHA) and low (RLA) avoidance strains taken from the third generation of selective breeding. The mean numbers of avoidances in ...

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...ising approach would be to measure directly the competing responses by observation or photographic analysis. The Nervous System Since learning is presumably a manifestation of the functioning of the brain, strains whose brains differ radically should likewise differ in learning ability. The big question here, though, is which of the multitudinous aspects of the brain are related to learning. The weight of the brain appears to bear little or no relation to learning ability in rats and mice. Brain weightlearning correlations have been inconsistent over the years for the Tryon S1 and S3 strains (Rosenzweig, 1964). Furthermore, the brains of Heron's bright and dull strains did not differ in weight after 14 generations of selection (Silverman, Shapiro, and Heron, 1940). Wimer and Prater (1966) found that mice selected for high brain weight required fewer trials to learn a black-white discrimination than those selected for low brain weight. However, Collins (1970b) found that the largest difference in discrimination learning was not between high and low lines but instead was between the control line (more errors) and the selected lines. In addition, environmental enrichment or isolation had different eff...

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... a line selected with concurrent inbreeding. A large response to selection was observed in the very first selected generation, and even larger separation of lines was obtained by the fifth generation. The parent population averaged 104.9 avoidances in 250 trials, while by F5 the high line had a mean of 170.6 avoidances compared to 50.9 for the low line. No difficulties with sterility were reported for either of the lines. Bovet et al. (1969) also obtained a rapid response to selection for shuttle avoidance learning in mice, although they reported only a line selected for high scores. Finally, Schaefer (1968), believing that response duration was a determinant of intelligence, selected for the time required to perform 100 lever presses on an FR10 schedule for food reward in mice. He reported two generations of selection for long and short times with no sib matings. In both generations there was a significant difference (p < .01) between the two lines. It is evident that success in selectively breeding for high and low learning rates in laboratory rats and mice is commonplace. Taken together with the numerous strain comparisons mentioned above as well as more sophisticated genetic experiments to be...

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... highest scoring parent (HP). All instances where F1 is greater than MP are characterized by hybrid vigor or heterosis. The results of several such genetic studies of learning are summarized in Table 4. In most studies employing inbred mouse strains as parents, significant directional dominance was observed. The F1 hybrids were generally superior to the average of their parents for learning of a two-choice maze for food reward (Vicari, 1929), lever pressing for food reward (Smart, 1970), water-escape learning (Winston, 1964; Winston and Lindzey, 1964), shock-avoidance learning (Collins, 1964; Schlesinger and Wimer, 1967; Abeelen, 1966; Rose and Parsons, 1970; Wahlsten, 1971; Oliverio et al., 1971), and CER conditioning (Henderson, 1968a). Many instances of overdominance were also reported. Several experiments with selected strains, summarized in Table 4, have been reported. Neither Tryon (1940) nor McGaugh, Westbrook, and Burt (1961) found heterosis in a cross of the Tryon bright (S1) and dull (S3) strains. In both studies the F1 mean was very close to MP. Bignami (1965) obtained moderate heterosis in a cross of his high (RHA) and low (RLA) avoidance strains taken from the third generation of selective breed...

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...al question (see Bruell, 1967). The means by which these difficulties may be overcome are quite numerous. Study of learning ability of wild populations would be a good place to start. Although methodological problems are certain to be encountered in the study of truly wild animals, transporting them to seminatural habitats which allow controlled observation and stimulus presentation as well as individual identification might provide a good starting point. Commensal populations, which already live in close proximity to man, are especially good candidates for such experimentation (Bruell, 1970; Selander and Yang, 1970). It would be important to test the animals before too many generations had elapsed away from the original environment. Another strategy of immediate utility would be to release groups of lab animals of known gene frequencies and learning abilities into environments in which only the influx of migrants of the same species was controlled. Subsequent generations could be retrieved, "domesticated," and then tested for learning and so forth. Environments could be arranged with and without predators or with and without a limited food supply. This strategy would be especially interesting if strains ...

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...l volume (Spearman r = -.83). They did not attempt to relate their data to learning ability. Visual pathways have been found to differ dramatically in albino and hooded rats (Lund, 1965). The organizational differences related to patterns of interocular transfer (Sheridan, 1965) and visual evoked potentials (Creel, Dustman, and Beck, 1970). Their relevance to normal learning differences has not been established. Neither have they proved that the differences are caused by the gene c in random bred populations. Given the large number of mutant genes which are known to affect brain organization (Sidman, Green, and Appel, 1965), it is likely that alleles more within the normal range of variation have similar effects upon organization. Future studies which examine detailed organizational aspects of brain, instead of homogenizing these differences, may detect patterns which relate to learning ability. Discussion of Genetic Correlates The above studies of genetic correlates of learning emphasize several points mentioned earlier. 1. Presentation of mere correlations between learning ability and other attributes of inbred strains cannot establish a causal relationship. The strains must be crossbred, and the correlations...

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...ning is presumably a manifestation of the functioning of the brain, strains whose brains differ radically should likewise differ in learning ability. The big question here, though, is which of the multitudinous aspects of the brain are related to learning. The weight of the brain appears to bear little or no relation to learning ability in rats and mice. Brain weightlearning correlations have been inconsistent over the years for the Tryon S1 and S3 strains (Rosenzweig, 1964). Furthermore, the brains of Heron's bright and dull strains did not differ in weight after 14 generations of selection (Silverman, Shapiro, and Heron, 1940). Wimer and Prater (1966) found that mice selected for high brain weight required fewer trials to learn a black-white discrimination than those selected for low brain weight. However, Collins (1970b) found that the largest difference in discrimination learning was not between high and low lines but instead was between the control line (more errors) and the selected lines. In addition, environmental enrichment or isolation had different effects on brain weight and learning ability of the selected lines (Collins, 1970b). Although brain weight-learning correlations have not been reported for inb...

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...y be determined by comparing the F1 mean score to the mean of the two parent strains, the midparent score (MP), or to the highest scoring parent (HP). All instances where F1 is greater than MP are characterized by hybrid vigor or heterosis. The results of several such genetic studies of learning are summarized in Table 4. In most studies employing inbred mouse strains as parents, significant directional dominance was observed. The F1 hybrids were generally superior to the average of their parents for learning of a two-choice maze for food reward (Vicari, 1929), lever pressing for food reward (Smart, 1970), water-escape learning (Winston, 1964; Winston and Lindzey, 1964), shock-avoidance learning (Collins, 1964; Schlesinger and Wimer, 1967; Abeelen, 1966; Rose and Parsons, 1970; Wahlsten, 1971; Oliverio et al., 1971), and CER conditioning (Henderson, 1968a). Many instances of overdominance were also reported. Several experiments with selected strains, summarized in Table 4, have been reported. Neither Tryon (1940) nor McGaugh, Westbrook, and Burt (1961) found heterosis in a cross of the Tryon bright (S1) and dull (S3) strains. In both studies the F1 mean was very close to MP. Bignami (1965) obt...

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... shuttle avoidance was wheel-running activity, for rA between these two was about -.71 ± .12, which implies that high "kinetic drive" may interfere with discrete-trial avoidance learning. Wheel running was not related to maze learning. One feature of the literature on strain variation in avoidance learning appeared to argue against any significant general learning ability. The problem was that some investigators observed certain strains, e.g., C3H or CBA, to learn very slowly, if at all (Bovet et al., 1968; Bovet-Nitti, 1969), while others found the same strains to be among the best learners (Stasik, 1970; Collins, 1964). Wahlsten (1971) obtained this result within one experiment; the CBA/J strain learned jump-out avoidance most quickly but was very poor at one-way avoidance. Subsequent genetic analyses (Wahlsten, 1972) demonstrated that the interaction was caused by the gene retinal degeneration (rd). When effects of rd and albinism (c) were eliminated, strain ranks were similar with the two procedures. Although the above experiments with inbred mice indicate the importance of general learning ability, research with other species has frequently revealed substantial strain-by-training procedur...

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...entration of single neurotransmitters does not correlate highly with maze learning (Rosenzweig, 1964). However, the relative concentration of ACh and AChE suggests that rats with higher ACh/AChE ratios are better able to solve mazes. As Rosenzweig himself pointed out, the data are not conclusive, and more research with other strains is needed. Nonetheless, the important idea that study of the joint functioning of many important neurochemicals is required to understand learning should be manifestly clear. Abundant research on genetic variation in the chemistry of mouse brain has been reported (Sudak and Maas, 1964; Schlesinger and Griek, 1970), but observed differences have not been related to learning ability. Structural and organizational attributes of the brain have received scant attention in the genetic context. Wimer et al. (1969) found that inbred mouse strains which had a neocortex of relatively large volume tended to have a hippocampus of relatively small volume (Spearman r = -.83). They did not attempt to relate their data to learning ability. Visual pathways have been found to differ dramatically in albino and hooded rats (Lund, 1965). The organizational differences related to patterns of in...

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... background was shown to reduce learning of a black-white water maze discrimination (Fuller, 1967) and jumpup avoidance (Henry and Schlesinger, 1967). Wilcock (1969) recently reviewed these various experiments and concluded that effects of albinism upon behavior are instances of trivial pleiotropy, because lack of eye pigment leads to suppression of nearly any active behavior under bright lights. Several studies have shown that behavioral differences between albino and pigmented mice are greatly reduced when a very dim light is employed over the test area (McReynolds, Weir, and DeFries, 1967; Thiessen, Lindzey, and Owen, 1970). In all of the above studies of albinism and learning which reported illumination conditions, the lights were quite bright, although precise values were never given by the experimenters. Wilcock estimated that they ranged from 50 to 180 ft-c, which is far in excess of levels found to suppress activity in an open-field (McReynolds et al., 1967). Therefore, the albinism effect may have nothing to do with central nervous system differences. Wilcock's interpretation is supported by a recent experiment by Wahlsten (1972). The albino strain A/J was observed to learn very slowly compared to pigment...

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...lated to learning differences. The McGill bright and dull rats selected on the Hebb-Williams maze (Thompson, 1954) have also received some attention. It is interesting to note that a prime reason for using the Hebb-Williams battery of problems was to select for a more general learning ability and thereby circumvent the "less interesting" motivational differences produced by Tryon and Heron. When Thompson and Bindra (1952) tested the F4 generation of selected rats for food eating, eating time, defecation, urination, and timidity, a significant strain difference was obtained only for urination. Thompson (1953) also tested exploratory activity under several deprivation levels, but again no strain differences were manifest. Thus, Thompson's original goal was met; learning differences existed without concommitant motivation or emotion differences. Unfortunately, the McGill strains have not been the subjects of extensive learning tests as were Tryon's. Memory The ability to retain as well as store information is obviously a prerequisite for successful retrieval of that information at some later time. An animal of a certain genotype which either fails to store information permanently or stores it in a m...

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...too. In these respects, Heron's rats resembled those of Tryon, for mild motivational differences were noted in both groups. In neither case were the motivational differences proved to be genetically related to learning differences. The McGill bright and dull rats selected on the Hebb-Williams maze (Thompson, 1954) have also received some attention. It is interesting to note that a prime reason for using the Hebb-Williams battery of problems was to select for a more general learning ability and thereby circumvent the "less interesting" motivational differences produced by Tryon and Heron. When Thompson and Bindra (1952) tested the F4 generation of selected rats for food eating, eating time, defecation, urination, and timidity, a significant strain difference was obtained only for urination. Thompson (1953) also tested exploratory activity under several deprivation levels, but again no strain differences were manifest. Thus, Thompson's original goal was met; learning differences existed without concommitant motivation or emotion differences. Unfortunately, the McGill strains have not been the subjects of extensive learning tests as were Tryon's. Memory The ability to retain as well as store information is obv...

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...y the process of inbreeding. The first goal was to show that genes affected learning ability, and the second was presumably to allow subsequent analyses of the genetic mechanisms involved. Tolman (1924) selected for high and low scores based upon a "rough pooling" of errors, running time, and number of perfect runs in a complex maze for two generations. Although the two selected lines were significantly different in both the F1 and F2 generations, the intrasubject reliability of the maze test was so close to zero that Tolman abandoned his effort. Determined to avoid some of Tolman's problems, Tryon (1929) selected for high and low error scores on a 17-unit maze of known, high reliability (.95); he also reduced inbreeding by using only 50% full-sib matings. He later added high fertility, good health, and coat color to the selection criteria. His results, which are widely known among psychologists, showed clear divergence of the two lines, such that very little overlap existed by the last generation of selection, F22 (Tryon, 1940). The maze bright and dull strains, termed S1 and S3, have been maintained since then by random breeding and are still available today. A very similar selection study w...

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...earning. Albinism is no stranger to learning research. Lashley (1930) long ago demonstrated that the visual acuity of hooded rats exceeded that of albinos. More recent studies with mice have examined the effects of the c gene unconfounded with other genetic differences between strains. When placed upon a random, segregating genetic background, albinism led to reduced levels of active avoidance learning (Winston and Lindzey, 1964; Winston, Lindzey, and Conner, 1967), water maze learning with either visual or spatial cues (Werboff, Anderson, and Ross, 1967), and straight alley running for food (Tyler, 1970). Albino mice were superior, however, at inhibitory avoidance learning (Winston, Lindzey, and Conner, 1967). Albinism on the isogenic C57BL/6J background was shown to reduce learning of a black-white water maze discrimination (Fuller, 1967) and jumpup avoidance (Henry and Schlesinger, 1967). Wilcock (1969) recently reviewed these various experiments and concluded that effects of albinism upon behavior are instances of trivial pleiotropy, because lack of eye pigment leads to suppression of nearly any active behavior under bright lights. Several studies have shown that behavioral differences bet...

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...nditioning, however, the E dogs achieved a significantly higher frequency of conditioned leg-flexion responses to a 500-Hz tone. In contrast to the skeletal motor CR measure, heart rate revealed a superior discrimination between positive and negative tones for the A dogs. Similar results were obtained for respiration rate. Thus, the measure of learning determined to a large extent which strain of dogs was judged to have superior learning ability. Strain interactions may also attenuate the generality of statements based upon group data when genetically variable dog populations are studied (see Wahlsten and Cole, 1971). The learning abilities on diverse tasks of strains selected for learning rate on a single task are also of interest. Schaefer (1968), who selected for response duration in lever pressing, found that the mice with shorter response durations did in fact learn a T maze faster than the more persevering strain. This supported Schaefer's contention that response duration was an important determinant of intelligence. More extensive tests have been performed with the descendants of Tryon's lines (Brights are S1, Dulls are S3). Certainly, the most eminent study among these was by Searle (1949), who m...

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...nfusion in the past, and it impeded progress in the genetic analysis of learning. Albinism is no stranger to learning research. Lashley (1930) long ago demonstrated that the visual acuity of hooded rats exceeded that of albinos. More recent studies with mice have examined the effects of the c gene unconfounded with other genetic differences between strains. When placed upon a random, segregating genetic background, albinism led to reduced levels of active avoidance learning (Winston and Lindzey, 1964; Winston, Lindzey, and Conner, 1967), water maze learning with either visual or spatial cues (Werboff, Anderson, and Ross, 1967), and straight alley running for food (Tyler, 1970). Albino mice were superior, however, at inhibitory avoidance learning (Winston, Lindzey, and Conner, 1967). Albinism on the isogenic C57BL/6J background was shown to reduce learning of a black-white water maze discrimination (Fuller, 1967) and jumpup avoidance (Henry and Schlesinger, 1967). Wilcock (1969) recently reviewed these various experiments and concluded that effects of albinism upon behavior are instances of trivial pleiotropy, because lack of eye pigment leads to suppression of nearly any active behavior under bright lights. Severa...

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...in sensory processes between the Tryon rats. Tryon (1940) carried out numerous experiments which showed that surgically disrupting the senses had little effect on the behavior of Brights. Krechevsky (1933) tested Bright, Dull, and unselected rats on his insoluble hypothesis apparatus and observed that the Brights preferred spatial hypotheses, the Dulls used visual hypotheses, and the unselected rats showed no preference. Since these were the only differences noted, Krechevsky attributed the Bright-Dull difference to a "specific response ability" difference. A similar conclusion was reached by Wherry (1941), who subjected various response measures on the Tryon maze to factor analysis; the scores of Brights and Dulls on his three factors, forward going, food pointing, and goal gradient, suggested that Brights showed spatial and Dulls visual orientations. Later work indicated that S1 (Bright) were superior to S3 with spatial cues but not with visual cues (Markowitz and Sorrells, 1964; Markowitz and Becker, 1965). However, Fehmi and McGaugh (1961) reported that Si was superior on a more difficult horizontal-vertical discrimination, which certainly required the utilization of visual cues. Although s...