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A new theoretical framework, the EPIC (Executive-Process/Interactive-Control) architecture, provides the basis for accurate detailed computational models of human multipletask performance. Contrary to the traditional response-selection bottleneck hypothesis, EPIC's cognitive processor can select responses and do other procedural operations simultaneously for multiple concurrent tasks. Using this capacity together with flexible executive control of peripheral perceptual-motor components, EPIC computational models account well for various patterns of mean reaction times,. systematic individual differences in multiple-task performance, and influences of special training on people's task-coordination strategies. These diverse phenomena, and EPIC's success at modeling them, raise strong doubts about the existence of a pervasive immutable response-selection bottleneck in the human information-processing system. The present research therefore helps further characterize the nature of discrete versus continuous information processing.

Why is the human brain fundamentally limited when attempting to execute two tasks at the same time or in close succession? Two classical paradigms, psychological refractory period (PRP) and task switching, have independently approached this issue, making significant advances in our understanding of the architecture of cognition. Yet, there is an apparent contradiction between the conclusions derived from these two paradigms. The PRP paradigm, on the one hand, suggests that the simultaneous execution of two tasks is limited solely by a passive structural bottleneck in which the tasks are executed on a first-come, first-served basis. The task-switching paradigm, on the other hand, argues that switching back and forth between task configurations must be actively controlled by a central executive system (the system controlling voluntary, planned, and flexible action). Here we have explicitly designed an experiment mixing the essential ingredients of both paradigms: task uncertainty and task simultaneity. In addition to a central bottleneck, we obtain evidence for active processes of task setting (planning of the appropriate sequence of actions) and task disengaging (suppression of the plan set for the first task in order to proceed with the next one). Our results clarify the chronometric relations between these central components of dual-task processing, and in particular whether they operate serially or in parallel. On this basis, we propose a hierarchical model of cognitive architecture that provides a synthesis of task-switching and PRP paradigms.

The model of a single central bottleneck for human information processing is critically examined. Most evidence cited in support of the model has been observed within the overlapping tasks paradigm. It is shown here that most findings obtained within that paradigm and that were used to support the model are also consistent with a simple resource model. The most prominent findings are the millisecondfor-millisecond slope at the left of the RT2–SOA curve, the high RT1–RT2 correlation, the additivity of the effects on RT2 of SOA and of the difficulty of selecting R2, and the washout of the effect of S2 discriminability on RT2 in a dual-task condition. In addition, the asymmetry of the effects of the dual-task requirement on RT1 and RT2 can be accounted for by the resource model provided that it assumes uneven allocation of resources, which is quite reasonable in view of the task asymmetry inherent in the demand characteristics of the paradigm. The same is true for two other findings that appear to support the single-bottleneck model—that in the dual-task condition, the demand of the first task affects equally RT1 and RT2 and that its effect on RT1 is the same as the corresponding effect in the singletask

This article reports the replication of a finding first reported 30 years ago. The replication was motivated by Lien, Proctor, and Allen's (2002; LP&A hereafter) recent report of a nonreplication of Greenwald and Shulman's (1973; G&S hereafter) finding of perfect timesharing of two simultaneous two-choice tasks when both tasks were ideomotor (IM) compatible. IM compatibility (Greenwald, 1972) is a relationship between stimuli and responses that was suggested by ideomotor theory (Greenwald, 1970c; James, 1890; Knuf, Aschersleben, & Prinz, 2001)

this article should be addressed to M.-C. Lien, Mail Stop 262-4, NASA Ames Research Center, Moffett Field, CA 94035 or to R. W. Proctor, Department of Psychological Sciences, Purdue University, West Lafayette, IN 47907 (e-mail: mclien@ mail.arc.nasa.gov or proctor@psych.purdue.edu)

& In many situations, people can only compute one stimulusto-response mapping at a time, suggesting that response selection constitutes a ‘‘central processing bottleneck’ ’ in human information processing. Using fMRI, we tested whether common or distinct brain regions were involved in response selection across visual and auditory inputs, and across spatial and nonspatial mapping rules. We isolated brain regions involved in response selection by comparing two conditions that were identical in perceptual input and motor output, but differed in the complexity of the mapping rule. In the visual – manual task of Experiment 1, four vertical lines were positioned from left to right, and subjects pressed one of four keys to report which line was unique in length. In the auditory –manual task of Experiment 2, four tones were presented in succession, and subjects pressed one of four keys to report which tone was unique in duration. For both visual and auditory tasks, the mapping between target position and key position was either spatially compatible or incompatible. In the verbal task of Experiment 3, subjects used nonspatial mappings that were either compatible (‘‘same’ ’ if colors matched; ‘‘different’ ’ if they mismatched) or incompatible (the opposite). Extensive activation overlap was observed across all three experiments for incompatible versus compatible mapping in bilateral parietal and frontal regions. Our results indicate that common neural substrates are involved in response selection across input modalities and across spatial and nonspatial domains of stimulus-to-response mapping, consistent with behavioral evidence that response selection is a central process. &

This article reports three experiments whose results confirm the latter possibilities. We reach several related conclusions: (a) After only moderate practice, people can achieve virtually perfect time sharing between two basic choice reaction tasks; (b) when required, conservative executive control may postpone one such task while another is under way, yielding dual-task interference despite the potential for virtually perfect time sharing; and (c) personal preferences for cautious rather than daring task scheduling underlie individual differences in dual-task interference. Our experiments significantly extend previous reports of virtually perfect time sharing (e.g., Allport, Antonis, & Reynolds, 1972; Greenwald & Shulman, 1973; Shaffer, 1975), which have been discounted because they allegedly involved methodological artifacts (cf. McCann & Johnston, 1992; Pashler, 1994; Van Selst, Ruthruff, & Johnston, 1999)