Object and spatial visual working memory activate separate neural systems in human cortex

Storage and executive processes in the frontal lobes

The human frontal cortex helps mediate working memory, a system that is used for temporary storage and manipulation of information and that is involved in many higher cognitive functions. Working memory includes two components: short-term storage (on the order of seconds) and executive processes that operate on the contents of storage. Recently, these two components have been investigated in functional neuroimaging studies. Studies of storage indicate that different frontal regions are activated for different kinds of information: storage for verbal materials activates Broca's area and left-hemisphere supplementary and premotor areas; storage of spatial information activates the right-hemisphere premotor cortex; and storage of object information activates other areas of the prefrontal cortex. Two of the fundamental executive processes are selective attention and task management. Both processes activate the anterior cingulate and dorsolateral prefrontal cortex.

Effect of familiarity on the processing of human faces

Most brain imaging studies on face perception have investigated the processing of unknown faces and addressed mainly the question of specific face processing in the human brain. The goal of this study was to highlight the effects of familiarity on the visual processing of faces. Using [15O]water 3D Positron Emission Tomography, regional cerebral blood flow distribution was measured in 11 human subjects performing an identical task (gender categorization) on both unknown and known faces. Subjects also performed two control tasks (a face recognition task and a visual pattern discrimination task). They were scanned after a training phase using videotapes during which they had been familiarized with and learned to recognize a set of faces. Two major results were obtained. On the one hand, we found bilateral activations of the fusiform gyri in the three face conditions, including the so-called fusiform-face area, a region in the right fusiform gyrus specifically devoted to face processing. This common activation suggests that different cognitive tasks performed on known and unknown faces require the involvement of this fusiform region. On the other hand, specific regional cerebral blood flow changes were related to the processing of known and unknown faces. The left amygdala, a structure involved in implicit learning of visual representations, was activated by the categorization task on unknown faces. The same task on known faces induced a relative decrease of activity in early visual areas. These differences between the two categorization tasks reveal that the human brain processes known and unknown faces differently.

Redefining the functional organization of working memory processes within human lateral prefrontal cortex

It is widely held that the frontal cortex plays a critical part in certain aspects of spatial and non-spatial working memory. One unresolved issue is whether there are functionally distinct subdivisions of the lateral frontal cortex that subserve different aspects of working memory. The present study used positron emission tomography (PET) to demonstrate that working memory processes within the human mid-dorsolateral and mid-ventrolateral frontal regions are organized according to the type of processing required rather than according to the nature (i.e. spatial or non-spatial), of the information being processed, as has been widely assumed. Two spatial working memory tasks were used which varied in the extent to which they required different executive processes. During a 'spatial span' task that required the subject to hold a sequence of five previously remembered locations in working memory a significant change in blood-flow was observed in the right mid-ventrolateral frontal cortex, but not in the anatomically and cytoarchitectonically distinct mid-dorsolateral frontal-lobe region. By contrast, during a '2-back' task that required the subject to continually update and manipulate an ongoing sequence of locations within working memory, significant blood flow increases were observed in both mid-ventrolateral and mid-dorsolateral frontal regions. When the two working memory tasks were compared directly, the one that emphasized manipulation of information within working memory yielded significantly greater activity in the right mid-dorsolateral frontal cortex only. This dissociation provides unambiguous evidence that the mid-dorsolateral and mid-ventrolateral frontal cortical areas make distinct functional contributions to spatial working memory and corresponds with a fractionation of working memory processes in psychological terms.

Cognitive and motivational operations in primate prefrontal neurons

To investigate neuronal mechanisms of higher cognitive activity, single neuronal activity has been recorded from the prefrontal cortex of monkeys trained in cognitive tasks. It has been well documented that there are prefrontal neurons which are involved in cognitive operations such as coding the meaning of a stimulus or retaining information in working memory. On the other hand, there are also prefrontal neurons which show reward-related activity changes. Two kinds of reward-related activities are found in the primate prefrontal cortex; one kind is concerned with processing post-trial events such as coding the reinforcement and/or error, and the other is concerned with the expectancy of the specific reward. These reward-related activities, which appear to be involved in motivational operations, are related also to cognitive operations such as coding the context in which the reward is given or omitted, or monitoring the context concerning the task situation with which the animal is faced. It seems that the prefrontal cortex is involved in the integration of cognitive and motivational information for goal-directed behavior. While the function of the prefrontal cortex is often associated with working memory, I introduce the idea that the prefrontal cortex plays important roles for coding, monitoring and providing the context for goal-directed behavior.

Temporal isolation of the neural correlates of spatial mnemonic processing with fMRI

The use of cognitive subtraction to study the neural substrates of the maintenance component of spatial working memory in humans relies upon the assumptions of the pure insertion of cognitive processes and a linear transform of neural activity to neuroimaging signal. Here, functional changes attributable to the memory requiring phase (referred to as the retention delay) of a spatial working memory task were temporally discriminated from those attributable to other behavioral subcomponents within trials using an experimental design that is argued to obviate these assumptions, as well as permit a joint test of their validity. The hypothesis that the assumptions of cognitive subtraction (as applied to neuroimaging) hold in general was not supported. Functional changes attributable to the retention delay were detected in the dorsolateral prefrontal cortex as well as in other cortical regions in a subset of the subjects, and in the right frontal eye field and right superior parietal lobule of all subjects (n=5). These results support models in which these regions are involved in maintaining spatial representations in humans. In addition, nearly all regions that evidenced such functional changes during the retention delay also evidenced functional changes during behaviors that did not require spatial working memory. This result tends to dispute models which posit the existence of gross neuroanatomical regions involved in solely mnemonic function.

Neurophysiological indices of strategy development and skill acquisition

In order to examine neurophysiological changes associated with the development of cognitive and visuomotor strategies and skills, spectral features of the EEG were measured as participants learned to perform new tasks. In one experiment eight individuals practiced working memory tasks that required development of either spatial or verbal rehearsal and updating strategies. In a second experiment six individuals practiced a video game with a difficult visuomotor tracking component. The alpha rhythm, which is attenuated by functional cortical activation, was affected by task practice. In both experiments, a lower-frequency, centrally distributed alpha component increased between practice sessions in a task-independent fashion, reflecting an overall decrease in the extent of cortical activation after practice. A second, higher-frequency, posterior component of the alpha rhythm displayed task-specific practice effects. Practice in the verbal working memory task resulted in an increase of this signal over right posterior regions, an effect not seen after practice with the spatial working memory task or with the video game. This between-task difference presumably reflects a continued involvement of the posterior region of the right hemisphere in tasks that invoke visuospatial processes. This finding thus provides neurophysiological evidence for the formation of a task-specific neurocognitive strategy. In the second experiment a third component of the alpha rhythm, localized over somatomotor cortex, was enhanced in conjunction with acquisition of tracking skill. These alpha band results suggest that cortical regions not necessary for task performance become less active as skills develop. In both experiments the frontal midline (Fm) theta rhythm also displayed increases over the course of test sessions. This signal is associated with states of focused concentration, and its enhancement might reflect the conscious control over attention associated with maintenance of a task-appropriate mental set. Overall, the results suggest that the EEG can be used to monitor practice-related changes in the patterns of cortical activity that are associated with task processing. Additionally, these results highlight the importance of ensuring that subjects have developed stable strategies for performance before drawing inferences about the functional architecture underlying specific cognitive processes.

Sustained activity in the medial wall during working memory delays

We have taken advantage of the temporal resolution afforded by functional magnetic resonance imaging (fMRI) to investigate the role played by medial wall areas in humans during working memory tasks. We demarcated the medial motor areas activated during simple manual movement, namely the supplementary motor area (SMA) and the cingulate motor area (CMA), and those activated during visually guided saccadic eye movements, namely the supplementary eye field (SEF). We determined the location of sustained activity over working memory delays in the medial wall in relation to these functional landmarks during both spatial and face working memory tasks. We identified two distinct areas, namely the pre-SMA and the caudal part of the anterior cingulate cortex (caudal-AC), that showed similar sustained activity during both spatial and face working memory delays. These areas were distinct from and anterior to the SMA, CMA, and SEF. Both the pre-SMA and caudal-AC activation were identified by a contrast between sustained activity during working memory delays as compared with sustained activity during control delays in which subjects were waiting for a cue to make a simple manual motor response. Thus, the present findings suggest that sustained activity during working memory delays in both the pre-SMA and caudal-AC does not reflect simple motor preparation but rather a state of preparedness for selecting a motor response based on the information held on-line.

The role of prefrontal cortex in working memory: examining the contents of consciousness

Working memory enables us to hold in our 'mind's eye' the contents of our conscious awareness, even in the absence of sensory input, by maintaining an active representation of information for a brief period of time. In this review we consider the functional organization of the prefrontal cortex and its role in this cognitive process. First, we present evidence from brain-imaging studies that prefrontal cortex shows sustained activity during the delay period of visual working memory tasks, indicating that this cortex maintains on-line representations of stimuli after they are removed from view. We then present evidence for domain specificity within frontal cortex based on the type of information, with object working memory mediated by more ventral frontal regions and spatial working memory mediated by more dorsal frontal regions. We also propose that a second dimension for domain specificity within prefrontal cortex might exist for object working memory on the basis of the type of representation, with analytic representations maintained preferentially in the left hemisphere and image-based representations maintained preferentially in the right hemisphere. Furthermore, we discuss the possibility that there are prefrontal areas brought into play during the monitoring and manipulation of information in working memory in addition to those engaged during the maintenance of this information. Finally, we consider the relationship of prefrontal areas important for working memory, both to posterior visual processing areas and to prefrontal areas associated with long-term memory.

Age-related changes in regional cerebral blood flow during working memory for faces

Young and old adults underwent positron emission tomography during the performance of a working memory task for faces (delayed match-to-sample), in which the delay between the sample and choice faces was varied from 1 to 21 s. Reaction time was slower and accuracy lower in the old group, but not markedly so. Values of regional cerebral blood flow (rCBF) were analyzed for sustained activity across delay conditions, as well as for changes as delay increased. Many brain regions showed similar activity during these tasks in both young and old adults, including left anterior prefrontal cortex, which had increased rCBF with delay, and ventral extrastriate cortex, which showed decreased rCBF with delay. However, old adults had less activation overall and less modulation of rCBF across delay in right ventrolateral prefrontal cortex than did the young adults. Old adults also showed greater rCBF activation in left dorsolateral prefrontal cortex across all WM delays and increased rCBF at short delays in left occipitoparietal cortex compared to young adults. Activity in many of these regions was differentially related to performance in that it was associated with decreasing response times in the young group and increasing response times in the older individuals. Thus despite the finding that performance on these memory tasks and associated activity in a number of brain areas are relatively preserved in old adults, differences elsewhere in the brain suggest that different strategies or cognitive processes are used by the elderly to maintain memory representations over short periods of time.

Cortical fMRI activation produced by attentive tracking of moving targets

Attention can be used to keep track of moving items, particularly when there are multiple targets of interest that cannot all be followed with eye movements. Functional magnetic resonance imaging (fMRI) was used to investigate cortical regions involved in attentive tracking. Cortical flattening techniques facilitated within-subject comparisons of activation produced by attentive tracking, visual motion, discrete attention shifts, and eye movements. In the main task, subjects viewed a display of nine green "bouncing balls" and used attention to mentally track a subset of them while fixating. At the start of each attentive-tracking condition, several target balls (e.g., 3/9) turned red for 2 s and then reverted to green. Subjects then used attention to keep track of the previously indicated targets, which were otherwise indistinguishable from the nontargets. Attentive-tracking conditions alternated with passive viewing of the same display when no targets had been indicated. Subjects were pretested with an eye-movement monitor to ensure they could perform the task accurately while fixating. For seven subjects, functional activation was superimposed on each individual's cortically unfolded surface. Comparisons between attentive tracking and passive viewing revealed bilateral activation in parietal cortex (intraparietal sulcus, postcentral sulcus, superior parietal lobule, and precuneus), frontal cortex (frontal eye fields and precentral sulcus), and the MT complex (including motion-selective areas MT and MST). Attentional enhancement was absent in early visual areas and weak in the MT complex. However, in parietal and frontal areas, the signal change produced by the moving stimuli was more than doubled when items were tracked attentively. Comparisons between attentive tracking and attention shifting revealed essentially identical activation patterns that differed only in the magnitude of activation. This suggests that parietal cortex is involved not only in discrete shifts of attention between objects at different spatial locations but also in continuous "attentional pursuit" of moving objects. Attentive-tracking activation patterns were also similar, though not identical, to those produced by eye movements. Taken together, these results suggest that attentive tracking is mediated by a network of areas that includes parietal and frontal regions responsible for attention shifts and eye movements and the MT complex, thought to be responsible for motion perception. These results are consistent with theoretical models of attentive tracking as an attentional process that assigns spatial tags to targets and registers changes in their position, generating a high-level percept of apparent motion.

Reproducibility of fMRI results across four institutions using a spatial working memory task

Four U.S. sites formed a consortium to conduct a multisite study of fMRI methods. The primary purpose of this consortium was to examine the reliability and reproducibility of fMRI results. FMRI data were collected on healthy adults during performance of a spatial working memory task at four different institutions. Two sets of data from each institution were made available. First, data from two subjects were made available from each site and were processed and analyzed as a pooled data set. Second, statistical maps from five to eight subjects per site were made available. These images were aligned in stereotactic space and common regions of activation were examined to address the reproducibility of fMRI results when both image acquisition and analysis vary as a function of site. Our grouped and individual data analyses showed reliable patterns of activation in dorsolateral prefrontal cortex and posterior parietal cortex during performance of the working memory task across all four sites. This multisite study, the first of its kind using fMRI data, demonstrates highly consistent findings across sites.

Investigation of facial recognition memory and happy and sad facial expression perception: an fMRI study

We investigated facial recognition memory (for previously unfamiliar faces) and facial expression perception with functional magnetic resonance imaging (fMRI). Eight healthy, right-handed volunteers participated. For the facial recognition task, subjects made a decision as to the familiarity of each of 50 faces (25 previously viewed; 25 novel). We detected signal increase in the right middle temporal gyrus and left prefrontal cortex during presentation of familiar faces, and in several brain regions, including bilateral posterior cingulate gyri, bilateral insulae and right middle occipital cortex during presentation of unfamiliar faces. Standard facial expressions of emotion were used as stimuli in two further tasks of facial expression perception. In the first task, subjects were presented with alternating happy and neutral faces; in the second task, subjects were presented with alternating sad and neutral faces. During presentation of happy facial expressions, we detected a signal increase predominantly in the left anterior cingulate gyrus, bilateral posterior cingulate gyri, medial frontal cortex and right supramarginal gyrus, brain regions previously implicated in visuospatial and emotion processing tasks. No brain regions showed increased signal intensity during presentation of sad facial expressions. These results provide evidence for a distinction between the neural correlates of facial recognition memory and perception of facial expression but, whilst highlighting the role of limbic structures in perception of happy facial expressions, do not allow the mapping of a distinct neural substrate for perception of sad facial expressions.

Grasping spatial relationships: failure to demonstrate allocentric visual coding in a patient with visual form agnosia

The cortical visual mechanisms involved in processing spatial relationships remain subject to debate. According to one current view, the "dorsal stream" of visual areas, emanating from primary visual cortex and culminating in the posterior parietal cortex, mediates this aspect of visual processing. More recently, others have argued that while the dorsal stream provides egocentric coding of visual location for motor control, the separate "ventral" stream is needed for allocentric spatial coding. We have assessed the visual form agnosic patient DF, whose lesion mainly affects the ventral stream, on a prehension task requiring allocentric spatial coding. She was presented with transparent circular disks. Each disk had circular holes cut in it. DF was asked to reach out and grasp the disk by placing her fingers through the holes. The disks either had three holes (for forefinger, middle finger, and thumb) or two holes (for forefinger and thumb). The distance between the forefinger and thumb holes, and the orientation of the line formed by them, were independently varied. DF was quite unable to adjust her grip aperture or her hand orientation in the three-hole task. Although she was able to orient her hand appropriately for the two-hole disks, she still remained unable to adjust her grip aperture to the distance between the holes. These findings are consistent with the idea that allocentric processing of spatial information requires a functioning ventral stream, even when the information is being used to guide a motor response.

Cortical afferents of the perirhinal, postrhinal, and entorhinal cortices of the rat

We have divided the cortical regions surrounding the rat hippocampus into three cytoarchitectonically discrete cortical regions, the perirhinal, the postrhinal, and the entorhinal cortices. These regions appear to be homologous to the monkey perirhinal, parahippocampal, and entorhinal cortices, respectively. The origin of cortical afferents to these regions is well-documented in the monkey but less is known about them in the rat. The present study investigated the origins of cortical input to the rat perirhinal (areas 35 and 36) and postrhinal cortices and the lateral and medial subdivisions of the entorhinal cortex (LEA and MEA) by placing injections of retrograde tracers at several locations within each region. For each experiment, the total numbers of retrogradely labeled cells (and cell densities) were estimated for 34 cortical regions. We found that the complement of cortical inputs differs for each of the five regions. Area 35 receives its heaviest input from entorhinal, piriform, and insular areas. Area 36 receives its heaviest projections from other temporal cortical regions such as ventral temporal association cortex. Area 36 also receives substantial input from insular and entorhinal areas. Whereas area 36 receives similar magnitudes of input from cortices subserving all sensory modalities, the heaviest projections to the postrhinal cortex originate in visual associational cortex and visuospatial areas such as the posterior parietal cortex. The cortical projections to the LEA are heavier than to the MEA and differ in origin. The LEA is primarily innervated by the perirhinal, insular, piriform, and postrhinal cortices. The MEA is primarily innervated by the piriform and postrhinal cortices, but also receives minor projections from retrosplenial, posterior parietal, and visual association areas.

Differential activation of right superior parietal cortex and intraparietal sulcus by spatial and nonspatial attention

Neuropsychological and functional neuroimaging studies have implicated the right posterior parietal cortex (PPC) in human spatial attention. We tested the hypothesis that this area is also involved in nonspatial aspects of attention and working memory using positron emission tomography in healthy volunteers. In an initial experiment, digits were presented in pseudo-random spatial locations, and subjects attended either to locations or digits in order to detect single targets (attention condition) or to sequences of stimuli (working memory (WM) condition). Right superior parietal cortex (BA7) and intraparietal sulcus (IPS) were active during both spatial (locations) and nonspatial (digits) tasks compared to rest, although more so for the former. Additionally, right PPC was activated to an even greater extent during tests of WM than of attention, especially for tests of spatial WM. There were no differences in activation of dorsolateral prefrontal cortex in the spatial versus nonspatial versions of the task, contrary to many previous studies. A follow-up experiment which presented abstract objects in a fixed, central location confirmed that right IPS was active during tests of nonspatial attention and also that this activation is not due to incidental spatial representation of digit stimuli. However, BA7 was not activated by this nonspatial, nondigit attentional task. Overall, these data suggest first that right IPS is recruited for both nonspatial and spatial attention and WM. Second, right BA7 is recruited specifically for spatial (both direct and indirect) forms of attentional processing. Finally, PPC activations in spatial WM tasks are likely to be due to a combination of spatial perception, attention, and WM, rather than to any of these individually.

Reopening the mental imagery debate: lessons from functional anatomy

Over the past few years, the neural bases of mental imagery have been both a topic of intense debate and a domain of extensive investigations using either PET or fMRI that have provided new insights into the cortical anatomy of this cognitive function. Several studies have in fact demonstrated that there exist types of mental imagery that do not rely on primary/early visual areas, whereas a consensus now exists on the validity of the dorsal/ventral-route model in the imagery domain. More importantly, these studies have provided evidence that, in addition to higher order visual areas, mental imagery shares common brain areas with other major cognitive functions, such as language, memory, and movement, depending on the nature of the imagery task. This body of recent results indicates that there is no unique mental imagery cortical network; rather, it reflects the high degree of interaction between mental imagery and other cognitive functions.

Functional MRI studies of spatial and nonspatial working memory

Single-unit recordings in monkeys have revealed neurons in the lateral prefrontal cortex that increase their firing during a delay between the presentation of information and its later use in behavior. Based on monkey lesion and neurophysiology studies, it has been proposed that a dorsal region of lateral prefrontal cortex is necessary for temporary storage of spatial information whereas a more ventral region is necessary for the maintenance of nonspatial information. Functional neuroimaging studies, however, have not clearly demonstrated such a division in humans. We present here an analysis of all reported human functional neuroimaging studies plotted onto a standardized brain. This analysis did not find evidence for a dorsal/ventral subdivision of prefrontal cortex depending on the type of material held in working memory, but a hemispheric organization was suggested (i.e., left-nonspatial; right-spatial). We also performed functional MRI studies in 16 normal subjects during two tasks designed to probe either nonspatial or spatial working memory, respectively. A group and subgroup analysis revealed similarly located activation in right middle frontal gyrus (Brodmann's area 46) in both spatial and nonspatial [working memory-control] subtractions. Based on another model of prefrontal organization [M. Petrides, Frontal lobes and behavior, Cur. Opin. Neurobiol., 4 (1994) 207-211], a reconsideration of the previous imaging literature data suggested that a dorsal/ventral subdivision of prefrontal cortex may depend upon the type of processing performed upon the information held in working memory.

Functional organization of spatial and nonspatial working memory processing within the human lateral frontal cortex

The present study used functional magnetic resonance imaging to demonstrate that performance of visual spatial and visual nonspatial working memory tasks involve the same regions of the lateral prefrontal cortex when all factors unrelated to the type of stimulus material are appropriately controlled. These results provide evidence that spatial and nonspatial working memory may not be mediated, respectively, by mid-dorsolateral and mid-ventrolateral regions of the frontal lobe, as widely assumed, and support the alternative notion that specific regions of the lateral prefrontal cortex make identical executive functional contributions to both spatial and nonspatial working memory.

Induced gamma-band activity during the delay of a visual short-term memory task in humans

It has been hypothesized that visual objects could be represented in the brain by a distributed cell assembly synchronized on an oscillatory mode in the gamma-band (20-80 Hz). If this hypothesis is correct, then oscillatory gamma-band activity should appear in any task requiring the activation of an object representation, and in particular when an object representation is held active in short-term memory: sustained gamma-band activity is thus expected during the delay of a delayed-matching-to-sample task. EEG was recorded while subjects performed such a task. Induced (e.g., appearing with a jitter in latency from one trial to the next) gamma-band activity was observed during the delay. In a control task, in which no memorization was required, this activity disappeared. Furthermore, this gamma-band activity during the rehearsal of the first stimulus representation in short-term memory peaked at both occipitotemporal and frontal electrodes. This topography fits with the idea of a synchronized cortical network centered on prefrontal and ventral visual areas. Activities in the alpha band, in the 15-20 Hz band, and in the averaged evoked potential were also analyzed. The gamma-band activity during the delay can be distinguished from all of these other components of the response, on the basis of either its variations or its topography. It thus seems to be a specific functional component of the response that could correspond to the rehearsal of an object representation in short-term memory.

A neurological dissociation between preserved visual and impaired spatial processing in mental imagery

Studies on primates have shown that visual and spatial perceptual analysis depends on two separate neural pathways, associated with the processing of "what" and "where" an object is (visual characteristics and spatial coordinates, respectively). Similar dichotomies have been proposed by cognitive psychologists (e.g., the contrast between visual and spatial processing in working memory) and by neuropsychologists (e.g., the distinction between topographic agnosia and amnesia). In this paper we report the case of a patient with a severe spatial disorientation whose perceptual processing of visual and spatial information was normal, but in imagery tasks she had a dissociation between preserved visual and impaired spatial processing. While her ability to represent objects visually was intact, she failed in any task requiring mental rotation, recall of spatial position or execution of spatially based imagery operations. The case clearly demonstrates that visual and spatial imagery are functionally independent processes which must rely on different underlying neural systems. This pattern of impairment also explains the associated topographical amnesia as an inability to integrate spatial information in a mental map.

Matching patterns of activity in primate prefrontal area 8a and parietal area 7ip neurons during a spatial working memory task

Single-unit recording studies of posterior parietal neurons have indicated a similarity of neuronal activation to that observed in the dorsolateral prefrontal cortex in relation to performance of delayed saccade tasks. A key issue addressed in the present study is whether the different classes of neuronal activity observed in these tasks are encountered more frequently in one or the other area or otherwise exhibit region-specific properties. The present study is the first to directly compare these patterns of neuronal activity by alternately recording from parietal area 7ip and prefrontal area 8a, under the identical behavioral conditions, within the same hemisphere of two monkeys performing an oculomotor delayed response task. The firing rate of 222 posterior parietal and 235 prefrontal neurons significantly changed during the cue, delay, and/or saccade periods of the task. Neuronal responses in the two areas could be distinguished only by subtle differences in their incidence and timing. Thus neurons responding to the cue appeared earliest and were more frequent among the task-related neurons within parietal cortex, whereas neurons exhibiting delay-period activity accounted for a larger proportion of task-related neurons in prefrontal cortex. Otherwise, the task-related neuronal activities were remarkably similar. Cue period activity in prefrontal and parietal cortex exhibited comparable spatial tuning and temporal duration characteristics, taking the form of phasic, tonic, or combined phasic/tonic excitation in both cortical populations. Neurons in both cortical areas exhibited sustained activity during the delay period with nearly identical spatial tuning. The various patterns of delay-period activity-tonic, increasing or decreasing, alone or in combination with greater activation during cue and/or saccade periods-likewise were distributed to both cortical areas. Finally, similarities in the two populations extended to the proportion and spatial tuning of presaccadic and postsaccadic neuronal activity occurring in relation to the memory-guided saccade. The present findings support and extend evidence for a faithful duplication of receptive field properties and virtually every other dimension of task-related activity observed when parietal and prefrontal cortex are recruited to a common task. This striking similarity attests to the principal that information shared by a prefrontal region and a sensory association area with which it is connected is domain specific and not subject to hierarchical elaboration, as is evident at earlier stages of visuospatial processing.

Transient activation of inferior prefrontal cortex during cognitive set shifting

The Wisconsin Card Sorting Test, which probes the ability to shift attention from one category of stimulus attributes to another (shifting cognitive sets), is the most common paradigm used to detect human frontal lobe pathology. However, the exact relationship of this card test to prefrontal function and the precise anatomical localization of the cognitive shifts involved are controversial. By isolating shift-related signals using the temporal resolution of functional magnetic resonance imaging, we reproducibly found transient activation of the posterior part of the bilateral inferior frontal sulci. This activation was larger as the number of dimensions (relevant stimulus attributes that had to be recognized) were increased. These results suggest that the inferior frontal areas play an essential role in the flexible shifting of cognitive sets.

Human brain regions involved in direction discrimination

To obtain further evidence for the functional specialization and task-dependent processing in the human visual system, we used positron emission tomography to compare regional cerebral blood flow in two direction discrimination tasks and four control tasks. The stimulus configuration, which was identical in all tasks, included the motion of a random dot pattern, dimming of a fixation point, and a tone burst. The discrimination tasks comprised the identification of motion direction and successive direction discrimination. The control tasks were motion detection, dimming detection, tone detection, and passive viewing. There was little difference in the activation patterns evoked by the three detection tasks except for decreased activity in the parietal cortex during the detection of a tone. Thus attention to a nonvisual stimulus modulated different visual cortical regions nonuniformly. Comparison of successive discrimination with motion detection yielded significant activation in the right fusiform gyrus, right lingual gyrus, right frontal operculum, left inferior frontal gyrus, and right thalamus. The fusiform and opercular activation sites persisted even after subtracting direction identification from successive discrimination, indicating their involvement in temporal comparison. Functional magnetic resonance imaging (fMRI) experiments confirmed the weak nature of the activation of human MT/V5 by successive direction discrimination but also indicated the involvement of an inferior satellite of human MT/V5. The fMRI experiments moreover confirmed the involvement of human V3A, lingual, and parietal regions in successive discrimination. Our results provide further evidence for the functional specialization of the human visual system because the cortical regions involved in direction discrimination partially differ from those involved in orientation discrimination. They also support the principle of task-dependent visual processing and indicate that the right fusiform gyrus participates in temporal comparison, irrespective of the stimulus attribute.

An area specialized for spatial working memory in human frontal cortex

Working memory is the process of maintaining an active representation of information so that it is available for use. In monkeys, a prefrontal cortical region important for spatial working memory lies in and around the principal sulcus, but in humans the location, and even the existence, of a region for spatial working memory is in dispute. By using functional magnetic resonance imaging in humans, an area in the superior frontal sulcus was identified that is specialized for spatial working memory. This area is located more superiorly and posteriorly in the human than in the monkey brain, which may explain why it was not recognized previously.

A neural system for human visual working memory

Working memory is the process of actively maintaining a representation of information for a brief period of time so that it is available for use. In monkeys, visual working memory involves the concerted activity of a distributed neural system, including posterior areas in visual cortex and anterior areas in prefrontal cortex. Within visual cortex, ventral stream areas are selectively involved in object vision, whereas dorsal stream areas are selectively involved in spatial vision. This domain specificity appears to extend forward into prefrontal cortex, with ventrolateral areas involved mainly in working memory for objects and dorsolateral areas involved mainly in working memory for spatial locations. The organization of this distributed neural system for working memory in monkeys appears to be conserved in humans, though some differences between the two species exist. In humans, as compared with monkeys, areas specialized for object vision in the ventral stream have a more inferior location in temporal cortex, whereas areas specialized for spatial vision in the dorsal stream have a more superior location in parietal cortex. Displacement of both sets of visual areas away from the posterior perisylvian cortex may be related to the emergence of language over the course of brain evolution. Whereas areas specialized for object working memory in humans and monkeys are similarly located in ventrolateral prefrontal cortex, those specialized for spatial working memory occupy a more superior and posterior location within dorsal prefrontal cortex in humans than in monkeys. As in posterior cortex, this displacement in frontal cortex also may be related to the emergence of new areas to serve distinctively human cognitive abilities.

Recognition memory in rats--I. Concepts and classification

Recognition is the process by which a subject is aware that a stimulus has been previously experienced. It requires that the characteristics of events are perceived, discriminated, identified and then compared (matched) against a memory of the characteristics of previously experienced events. Understanding recognition memory, its underlying neuronal mechanisms, its dysfunction and alleviation of the latter by putative cognition enhancing drugs is a major research target and has triggered a wealth of animal studies. One of the most widely used animals for this purpose is the rat, and it is the rat's recognition memory which is the focus of this review. In this first part, concepts of recognition memory, stages of mnemonic processing and paradigms for the measurement of the rat's recognition memory will be discussed. In two subsequent articles (parts II and III) we will focus on the neuronal mechanisms underlying recognition memory in rats. Three major points arise from the comparison of paradigms that have in the past been used to assess recognition memory in rats. First, it should be realized that some tasks which, at face value, can all be considered to measure recognition memory in rats, may not assess recognition memory at all but may, for example, be based on recall rather than recognition. Second, it is evident that different types of recognition memory can be distinguished and that tasks differ in the type of recognition memory taxed. Some paradigms, for example, measure familiarity, whereas others assess recency. Furthermore, paradigms differ as to whether spatial stimuli or items are employed. Third, different processes, ranging from stimulus-response learning to the formation of concepts, may be involved to varying extent in different tasks. These are important considerations and question the predictive validity of the results obtained from studies examining, for example, the effects of putative cognition enhancing drugs.

Networks of domain-specific and general regions involved in episodic memory for spatial location and object identity

Positron emission tomography (PET) was used to investigate human episodic memory for spatial location and object identity. We measured regional cerebral bloodflow (rCBF) while subjects engaged in perceptual matching of the location or the identity of line drawings of objects. Perceptual matching also involved incidental encoding of the presented information. Subsequently, rCBF was measured when subjects retrieved the location or the identity of these objects from memory. Using the multivariate partial least squares image analysis, we identified three patterns of activity across the brain that allowed us to distinguish structures that are differentially involved in processing spatial location and object identity from structures that are differentially involved in encoding and retrieval but operate across both domains. Domain-specificity was evident by increased rCBF during the processing of spatial location in the right middle occipital gyrus, supramarginal gyrus, and superior temporal sulcus, and by increased rCBF during the processing of object identity in portions of bilateral lingual and fusiform gyri. There was a nearly complete overlap between domain-specific dorsal and ventral extrastriate cortex activations during perceptual matching and memory retrieval. Evidence of domain-specificity was also found in the prefrontal cortex and the left hippocampus, but the effect interacted with encoding and retrieval. Domain-general structures included bilateral superior temporal cortex regions, which were preferentially activated during encoding, and portions of bilateral middle and inferior frontal gyri, which were preferentially activated during retrieval. Together, our data suggest that encoding and retrieval in episodic memory depend on the interplay between domain-specific structures, most of which are involved in memory as well as perception, and domain-general structures, some of which operate more at encoding and others more at retrieval.

Dissociation Of working memory from decision making within the human prefrontal cortex

We tested the hypothesis that cognitive functions related to working memory (assessed with delay tasks) are distinct from those related to decision making (assessed with a gambling task), and that working memory and decision making depend in part on separate anatomical substrates. Normal controls (n = 21), subjects with lesions in the ventromedial (VM) (n = 9) or dorsolateral/high mesial (DL/M) prefrontal cortices (n = 10), performed on (1) modified delay tasks that assess working memory and (2) a gambling task designed to measure decision making. VM subjects with more anterior lesions (n = 4) performed defectively on the gambling but not the delay task. VM subjects with more posterior lesions (n = 5) were impaired on both tasks. Right DL/M subjects were impaired on the delay task but not the gambling task. Left DL/M subjects were not impaired on either task. The findings reveal a cognitive and anatomic double dissociation between deficits in decision making (anterior VM) and working memory (right DL/M). This presents the first direct evidence of such effects in humans using the lesion method and underscores the special importance of the VM prefrontal region in decision making, independent of a direct role in working memory.

Coexistence of attention-based facilitation and inhibition in the human cortex

A key function of attention is to select an appropriate subset of available information by facilitation of attended processes and/or inhibition of irrelevant processing. Functional imaging studies, using positron emission tomography, have during different experimental tasks revealed decreased neuronal activity in areas that process input from unattended sensory modalities. It has been hypothesized that these decreases reflect a selective inhibitory modulation of nonrelevant cortical processing. In this study we addressed this question using a continuous arithmetical task with and without concomitant disturbing auditory input (task-irrelevant speech). During the arithmetical task, irrelevant speech did not affect task-performance but yielded decreased activity in the auditory and midcingulate cortices and increased activity in the left posterior parietal cortex. This pattern of modulation is consistent with a top down inhibitory modulation of a nonattended input to the auditory cortex and a coexisting, attention-based facilitation of task-relevant processing in higher order cortices. These findings suggest that task-related decreases in cortical activity may be of functional importance in the understanding of both attentional mechanisms and task-related information processing.

Dissociable forms of inhibitory control within prefrontal cortex with an analog of the Wisconsin Card Sort Test: restriction to novel situations and independence from "on-line" processing

Attentional set-shifting and discrimination reversal are sensitive to prefrontal damage in the marmoset in a manner qualitatively similar to that seen in man and Old World monkeys, respectively (Dias et al., 1996b). Preliminary findings have demonstrated that although lateral but not orbital prefrontal cortex is the critical locus in shifting an attentional set between perceptual dimensions, orbital but not lateral prefrontal cortex is the critical locus in reversing a stimulus-reward association within a particular perceptual dimension (Dias et al., 1996a). The present study presents this analysis in full and extends the results in three main ways by demonstrating that (1) mechanisms of inhibitory control and "on-line" processing are independent within the prefrontal cortex, (2) impairments in inhibitory control induced by prefrontal damage are restricted to novel situations, and (3) those prefrontal areas involved in the suppression of previously established response sets are not involved in the acquisition of such response sets. These findings suggest that inhibitory control is a general process that operates across functionally distinct regions within the prefrontal cortex. Although damage to lateral prefrontal cortex causes a loss of inhibitory control in attentional selection, damage to orbitofrontal cortex causes a loss of inhibitory control in affective processing. These findings provide an explanation for the apparent discrepancy between human and nonhuman primate studies in which disinhibition as measured on the Wisconsin Card Sort Test is associated with dorsolateral prefrontal damage, whereas disinhibition as measured on discrimination reversal is associated with orbitofrontal damage.

Areal segregation of face-processing neurons in prefrontal cortex

A central issue in cognitive neuroscience concerns the functional architecture of the prefrontal cortex and the degree to which it is organized by sensory domain. To examine this issue, multiple areas of the macaque monkey prefrontal cortex were mapped for selective responses to visual stimuli that are prototypical of the brain's object vision pathway-pictorial representations of faces. Prefrontal neurons not only selectively process information related to the identity of faces but, importantly, such neurons are localized to a remarkably restricted area. These findings suggest that the prefrontal cortex is functionally compartmentalized with respect to the nature of its inputs.

Cognitive planning in humans: neuropsychological, neuroanatomical and neuropharmacological perspectives

In recent years, considerable progress has been made in understanding the cognitive and neuroanatomical basis of high-level planning behaviour through a combination of neuropsychological, neuropharmacological and functional neuroimaging approaches. In this article, early evidence suggesting a relationship between planning impairments and damage to the frontal lobe is reviewed and several contemporary studies of planning behaviour in patients with circumscribed frontal lobe excisions are described in detail. These neuropsychological investigations, together with recent functional neuroimaging studies of normal control subjects, have identified a specific area within the mid-dorsolateral frontal cortex of humans which appears to be critically involved in the cognitive processes that mediate efficient planning. The functions of this region, both in cognitive planning and in related functions such as working memory, are then discussed in the context of a general theoretical framework for understanding the functional organization of "executive" processes within the human lateral frontal cortex. In the final sections, the relationship between the planning deficits observed after intrinsic frontal lobe damage and those exhibited by patients with neuropathology of primarily sub-cortical origin, such as Parkinson's disease, is discussed. A central model for much of this work has been the concept of cortico-striatal circuitry which emphasizes the relationship between the neocortex and the striatum. The combined evidence from comparative studies in patients and from functional neuroimaging studies on Parkinson's disease suggests that altered cortico-striatal interactions may disrupt normal planning function at a number of levels, possibly consequent upon intrinsic striatal pathology on the one hand and the partial loss of (frontal) cortical input to the basal ganglia on the other.

Parietal and hippocampal contribution to topokinetic and topographic memory

This paper reviews the involvement of the parietal cortex and the hippocampus in three kinds of spatial memory tasks which all require a memory of a previously experienced movement in space. The first task compared, by means of positron emission tomography (PET) scan techniques, the production, in darkness, of self-paced saccades (SAC) with the reproduction, in darkness, of a previously learned sequence of saccades to visual targets (SEQ). The results show that a bilateral increase of activity was seen in the depth of the intraparietal sulcus and the medial superior parietal cortex (superior parietal gyrus and precuneus) together with the frontal sulcus but only in the SEQ task, which involved memory of the previously seen targets and possibly also motor memory. The second task is the vestibular memory contingent task, which requires that the subject makes, in darkness, a saccade to the remembered position of a visual target after a passively imposed whole-body rotation. Deficits in this task, which involves vestibular memory, were found predominantly in patients with focal vascular lesions in the parieto-insular (vestibular) cortex, the supplementary motor area-supplementary eye field area, and the prefrontal cortex. The third task requires mental navigation from the memory of a previously learned route in a real environment (the city of Orsay in France). A PET scan study has revealed that when subjects were asked to remember visual landmarks there was a bilateral activation of the middle hippocampal regions, left inferior temporal gyrus, left hippocampal regions, precentral gyrus and posterior cingulate gyrus. If the subjects were asked to remember the route, and their movements along this route, bilateral activation of the dorsolateral cortex, posterior hippocampal areas, posterior cingulate gyrus, supplementary motor areas, right middle hippocampal areas, left precuneus, middle occipital gyrus, fusiform gyrus and lateral premotor area was found. Subtraction between the two conditions reduced the activated areas to the left hippocampus, precuneus and insula. These data suggest that the hippocampus and parietal cortex are both involved in the dynamic aspects of spatial memory, for which the name 'topokinetic memory' is proposed. These dynamic aspects could both overlap and be different from those involved in the cartographic and static aspects of 'topographic' memory.

Dissociating working memory from task difficulty in human prefrontal cortex

A functional magnetic resonance imaging (fMRI) study was conducted to determine whether prefrontal cortex (PFC) increases activity in working memory (WM) tasks as a specific result of the demands placed on WM, or to other processes affected by the greater difficulty of such tasks. Increased activity in dorsolateral PFC (DLPFC) was observed during task conditions that placed demands on active maintenance (long retention interval) relative to control conditions matched for difficulty. Furthermore, the activity was sustained over the entire retention interval and did not increase when task difficulty was manipulated independently of WM requirements. This contrasted with the transient increases in activity observed in the anterior cingulate, and other regions of frontal cortex, in response to increased task difficulty but not WM demands. Thus, this study established a double-dissociation between regions responsive to WM versus task difficulty, indicating a specific involvement of DLPFC and related structures in WM function.

The time course of spatial and object learning in Parkinson's disease

Parkinson's disease (PD) is characterized by spatial memory dysfunction, but the selectivity of the deficit remains unclear. We addressed this issue by comparing performance on spatial and object variants of a conditional associative learning task, and by analysing the data with time series analytical techniques. The 11 PD subjects and 15 normal control subjects learned stimulus-stimulus pairings through trial-and-error learning. PD subjects were selectively impaired on the spatial condition: they required more trials to achieve criterion, learned at a slower rate and displayed a working memory deficit. The groups did not differ in the object condition. These results suggest a distinction between material-specific spatial and object visual memory systems. Further, they indicate that spatial learning and memory are selectively impaired in early PD, suggesting that interactions between the basal ganglia and prefrontal cortex are important for the mediation of high-level cognition.

Frontal and parietal networks for conditional motor learning: a positron emission tomography study

Studies on nonhuman primates show that the premotor (PM) and prefrontal (PF) areas are necessary for the arbitrary mapping of a set of stimuli onto a set of responses. However, positron emission tomography (PET) measurements of regional cerebral blood flow (rCBF) in human subjects have failed to reveal the predicted rCBF changes during such behavior. We therefore studied rCBF while subjects learned two arbitrary mapping tasks. In the conditional motor task, visual stimuli instructed which of four directions to move a joystick (with the right, dominant hand). In the evaluation task, subjects moved the joystick in a predetermined direction to report whether an arrow pointed in the direction associated with a given stimulus. For both tasks there were three rules: for the nonspatial rule, the pattern within each stimulus determined the correct direction; for the spatial rule, the location of the stimulus did so; and for the fixed-response rule, movement direction was constant regardless of the pattern or its location. For the nonspatial rule, performance of the evaluation task led to a learning-related increase in rCBF in a caudal and ventral part of the premotor cortex (PMvc, area 6), bilaterally, as well as in the putamen and a cingulate motor area (CM, area 24) of the left hemisphere. Decreases in rCBF were observed in several areas: the left ventro-orbital prefrontal cortex (PFv, area 47/12), the left lateral cerebellar hemisphere, and, in the right hemisphere, a dorsal and rostral aspect of PM (PMdr, area 6), dorsal PF (PFd, area 9), and the posterior parietal cortex (area 39/40). During performance of the conditional motor task, there was only a decrease in the parietal area. For the spatial rule, no rCBF change reached significance for the evaluation task, but in the conditional motor task, a ventral and rostral premotor region (PMvr, area 6), the dorsolateral prefrontal cortex (PFdl, area 46), and the posterior parietal cortex (area 39/40) showed decreasing rCBF during learning, all in the right hemisphere. These data confirm the predicted rCBF changes in premotor and prefrontal areas during arbitrary mapping tasks and suggest that a broad frontoparietal network may show decreased synaptic activity as arbitrary rules become more familiar.

What fMRI has taught us about human vision

The recent application of functional magnetic resonance imaging (fMRI) to visual studies has begun to elucidate how the human visual system is anatomically and functionally organized. Bottom-up hierarchical processing among visual cortical areas has been revealed in experiments that have correlated brain activations with human perceptual experience. Top-down modulation of activity within visual cortical areas has been demonstrated through studies of higher cognitive processes such as attention and memory.

Positron-emission tomography imaging of long-term shape recognition challenges

Long-term visual memory performance was impaired by two types of challenges: a diazepam challenge on acquisition and a sensory challenge on recognition. Using positron-emission tomography regional cerebral blood flow imaging, we studied the effect of these challenges on regional brain activation during the delayed recognition of abstract visual shapes as compared with a baseline fixation task. Both challenges induced a significant decrease in differential activation in the left fusiform gyrus, suggesting that this region is involved in the automatic or volitional comparison of incoming and stored stimuli. In contrast, thalamic differential activation increased in response to memory challenges. This increase might reflect enhanced retrieval attempts as a compensatory mechanism for restoring recognition performance.

The functional organization of working memory processes within human lateral frontal cortex: the contribution of functional neuroimaging

Recent functional neuroimaging studies have provided a wealth of new information about the likely organization of working memory processes within the human lateral frontal cortex. This article seeks to evaluate the results of these studies in the context of two contrasting theoretical models of lateral frontal-lobe function, developed through lesion and electrophysiological recording work in non-human primates (Goldman-Rakic, 1994, 1995; Petrides, 1994, 1995). Both models focus on a broadly similar distinction between anatomically and cytoarchitectonically distinct dorsolateral and ventrolateral frontal cortical areas, but differ in the precise functions ascribed to those regions. Following a review of the relevant anatomical data, the origins of these two theoretical positions are considered in some detail and the main predictions arising from each are identified. Recent functional neuroimaging studies of working memory processes are then critically reviewed in order to assess the extent to which they support either, or both, sets of predictions. The results of this meta-analysis suggest that lateral regions of the frontal lobe are not functionally organized according to stimulus modality, as has been widely assumed, but that specific regions within the dorsolateral or ventrolateral frontal cortex make identical functional contributions to both spatial and non-spatial working memory.

Ventral prefrontal cortex is not essential for working memory

It is widely held that the prefrontal cortex is important for working memory. It has been suggested that the inferior convexity (IC) may play a special role in working memory for form and color (). We have therefore assessed the ability of monkeys with IC lesions to perform visual pattern association tasks and color-matching tasks, both with and without delay. In experiment 1, six monkeys were trained on a visual association task with delays of up to 2 sec. Conservative IC lesions that removed lateral area 47/12 in three animals had no effect on the task. Further experiments showed that these lesions had no effect on the postoperative new learning of a color-matching task with delays of up to 2 sec or versions of the visual association task involving delays of up to 8 sec. In experiment 2, larger lesions of both areas 47/12 and 45A were made in the three control animals. This lesion caused a profound deficit in the ability to relearn simultaneous color matching, but subsequent matching with delays of up to 8 sec was clearly unimpaired. We suggest that the IC may be more important for stimulus selection and attention as opposed to working memory.

Differential activation of the caudate nucleus in primates performing spatial and nonspatial working memory tasks

The caudate nucleus is part of an anatomical network subserving functions associated with the dorsolateral prefrontal cortex (DLPFC). The aim of the present study was to investigate whether the metabolic activity in the striatum reflects specific changes in working memory tasks, which are known to be dependent on the DLPFC, and whether these changes reflect the topographic ordering of prefrontal connections within the striatum. Local cerebral glucose utilization (LCGU) rates were assessed in the striatum by the 14C-2-deoxyglucose method in monkeys that performed a spatial (delayed spatial alternation), a nonspatial (delayed object alternation) visual working memory task, or tasks that did not involve working memory, i.e., a visual pattern discrimination or sensorimotor paradigm. The results show a topographic segregation of activation related to spatial and nonspatial working memory, respectively. The delayed spatial alternation task increases LCGU rates bilaterally by 33-43% in the head of the caudate nucleus, where efferents from the dorsolateral prefrontal cortex project most densely. The delayed object alternation task enhances LCGU rates bilaterally by 32-37% in the body of the caudate nucleus, which is innervated by the temporal cortex. The visual pattern discrimination task similarly activated the body of the caudate, but in a smaller region and only in the right hemisphere. These findings provide the first evidence for metabolic activation of the caudate nuclei in working memory, supporting the role of this nucleus as a node in a neural network mediating DLPFC-dependent working memory processes. The double dissociation of activation observed suggests an anatomical and functional segregation of cortico-striatal circuits subserving spatial and nonspatial cognitive operations.