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erations, respectively; and (iii) left posterior/ventral (Broca's area) and bilateral posterior/dorsal areas were more activated during WM than during ER, possibly reflecting phonological and generic WM operations, respectively. Second, hippocampal and parahippocampal regions were activated not only for ER but also for WM. This result suggests that indexing operations mediated by the medial temporal lobes apply to both long-term and short-term memory traces. Overall, our results show that direct cross-function comparisons are critical to understand the role of different brain regions in various cognitive functions. 2002 Elsevier Science (USA) INTRODUCTION During the past decade, numerous positron emission tomography (PET) and functional MRI (fMRI) studies have investigated the neural correlates of different cognitive functions (for a review, see Cabeza and Nyberg, 2000). Although most studies have focused on a single function (see however, LaBar et al., 1999; Braver et al., 2001; Ny

The dorsolateral prefrontal cortex (DPFC) and the posterior parietal cortex (PPC) are anatomically and functionally interconnected, and have been implicated in working memory and the preparation for behavioral action. To substantiate those functions at the neuronal level, we designed a visuomotor task that dissociated the perceptual and executive aspects of the perception–action cycle in both space and time. In that task, the trial-initiating cue (a color) indicated with different degrees of certainty the direction of the correct manual response 12 s later. We recorded extracellular activity from 258 prefrontal and 223 parietal units in two monkeys performing the task. In the DPFC, some units (memory cells) were attuned to the color of the cue, independent of the response-direction it connoted. Their discharge tended to diminish in the course of the delay between cue and response. In contrast, few color-related units were found in PPC, and these did not show decreasing patterns of delay activity. Other units in both cortices (set cells) were attuned to response-direction and tended to accelerate their firing in anticipation of the response and in proportion to the predictability of its direction. A third group of units was related to the determinacy of the act; their firing was attuned to the certainty with which the animal could predict the correct response, whatever its direction. Cells of the three types were found closely intermingled histologically. These findings further support and define the role of DPFC in executive functions and in the temporal closure of the perception– action cycle. The findings also agree with the involvement of PPC in spatial aspects of visuomotor behavior, and add a temporal integrative dimension to that involvement. Together, the results provide physiological evidence for the role of a prefrontal–parietal network in the integration of perception with action across time.

The dominant cognitive theory of working memory (WM) postulates a strict architectural segregation between components responsible for the short-term active maintenance of information and those responsible for the control and coordination of that information. Cognitive neuroscience research has provided strong evidence that the prefrontal cortex (PFC) serves as an important neural substrate of WM. However, the literature is mixed as to whether PFC should be considered a storage or control component. A theory is presented that attempts to resolve this conflict by postulating that PFC represents and actively maintains context information. These maintained representations provide a mechanism of control by serving as a top-down bias on the local competitive interactions that occur during processing. As such, it is suggested that storage and control functions are integrated within PFC. This theory is implemented as connectionist computational model. Simulation studies are described which demonstrate that the model can account for a wide range of behavioral data associated with performance of a simple task paradigm that probes both the storage and control functions of WM. Two neuroimaging studies are then presented which directly test the predictions of the model regarding the role of PFC in context processing. Taken together, the results provide new insights into the relationship between storage and control in WM, and the role of PFC in subserving these functions. i Working Memory and Prefrontal Cortex

The purpose of this study was to identify the brain regions invoked when subjects attempt to learn verbal materials for a subsequent memory test. Twelve healthy subjects undertook two different tasks: reading and encoding of word pairs, while they were being scanned using [150]H20 positron emission tomography (PET). As expected, the encoding pairs were remembered much better (recall 39% vs. 8%; P < 0.001) than reading pairs in a subsequent memory test. The encoding scans, as compared to reading scans, showed activation of the left prefrontal cortex, the anterior cingulate cortex and the left medial temporal cortex. The left prefrontal activations were in two discrete regions: (i) a left anterior and inferior left prefrontal (Brodmann's areas 45, 46) which we attribute to semantic processing; and (ii) a left posterior mid-frontal region (BA 6, 44) which may reflect rote rehearsal. We interpret the data to suggest that when subjects use cognitive strategies of semantic processing and rote-rehearsal to learn words, they invoke discrete regions of the left prefrontal cortex. And this activation of the left prefrontal cortex along with the medial temporal region leads to a neurophysiological memory trace which can be used to guide subsequent memory retrieval.

Positron emission tomography was used to investigate cerebral blood flow (CBF) changes associated with the processing of speech. In a first experiment normal right-handed volunteers were scanned under two conditions that required phonetic processing (discrimination of final consonants and phoneme monitoring), and one baseline condition of passive listening. Analysis was carried out by paired-image subtraction, with MRI overlay for anatomical localization. Comparison of each phonetic condition with the baseline condition revealed increased CBF in the left frontal lobe, close to the border between Broca's area and the motor cortex, and in a left parietal region. A second experiment showed that this area was not activated by a semantic judgment task. Reanalysis of data from an earlier study, in which various baseline conditions were used, confirmed that this region of left frontal cortex is consistently involved in phonetic tasks. The findings support a model whereby articulatory processes involving a portion of

activations can be handled. Optimization: Number of voxels typically much larger than the number of experiment and the number of point smaller than the number of experiments { Compute the distance in a subspace with the size of the experiment. If only points are considered: Compute a distance between the points. 4 Figure 2: Query volume formed from (Sergent et al., 1992) | a letter and object (visual) processing paper with the 5 original points and an isosurface in the volume. Figure 3: Result list: All vision experiments. 5 Figure 4: Result page with automatically generated corner cube visualization (Corbetta et al., 1993). Two cluster of activations. 6 References

of the thorniest problems faced by cognitive scientists (Allport, 1989; Kahneman, 1973; Posner and Boies, 1971; Schneider and Shiffrin, 1977; Wickens, 1980). When considering attention at the cognitive level, we must consider the following functional issues. First, an individual operating in an environment is bombarded by a vast array of perceptual inputs simultaneously and must, in order to function effectively, somehow select certain things for enhanced processing while ignoring others (Allport, 1989; Posner, 1991). (Selective attention may, of course, be further subdivided into operations such as disengagement from a current focus, engagement of a new focus, and sustained focal attention over time (Posner and Peterson, 1990; Posner, 1991).) Second, there appear to be limits on the number of things that can be processed simultaneously; that is, a bottleneck or capacity limitation exists on the ability to divide attention between multiple stimuli or mental events. Much early laborato

Previous research has identified the prefrontal cortex (PFC) as a brain region that is critical for cognitive control. Currently, theorists remain divided about whether to view the PFC as primarily a coordinative, mnemonic, or inhibitory structure. A theory is presented that attempts to resolve some of the apparent conflicts between the predominant views on PFC control functions. In this theory, PFC is proposed to actively maintain representations of context information. These maintained representations provide a mechanism of control by serving as a top-down bias on the local competitive interactions that occur during processing. As such, it is suggested that PFC performs both mnemonic and inhibitory functions in the service of control, and that each is preferentially observable under different task situations. A series of behavioral, computational, and neuroimaging studies are presented that demonstrates how this theory can account for a wide range of data associated with performance ...

prefrontal cortex; ERP 5 event-related potential; fMRI 5 functional MRI; rCBF 5 regional cerebral blood flow Introduction Human memory consists of multiple forms of learning which differ in the component processes and neural networks that mediate their acquisition and retrieval (e.g. Cohen and Squire, 1980; Graf and Schacter, 1985; Roediger, 1990; Tulving and Schacter, 1990; Squire, 1992; Gabrieli, 1998). One important form of learning is episodic memory, which refers to memory for experiences that are associated with a specific spatial and temporal learning context (Tulving, 1972, 1983). Retrieval from episodic memory is thought to entail the conscious recollection of aspects of the past, and consists of multiple component processes including the representation of retrieval Oxford University Press 1998 context-dependent by varying the context in which retrieval was performed; this was done by changing test instructions. Importantly, study and test stimuli were held constant, with