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

Correlation between evening and morning waking EEG and spatial orientation

The prefrontal, frontal, and parietal EEG of 16 healthy young adults (seven men, nine women; age=22.57+/-4.2) was recorded during the waking state (eyes closed) in the evening before and the morning following a second consecutive night spent in a sleep laboratory. Following the morning EEG recording session, participants were tested in a human-size maze upon five learning trials of a four-intersection route. Results on the fifth trial served as the learning index. We found a significant positive correlation between time taken to carry out the route and prefrontal, frontal EEG alpha-2 (10.0-12.75 Hz), and sigma (11.5-14.5 Hz) frequency bands. We also found that prefrontal and frontal theta activity correlated negatively with number of errors. No correlation was found between performance and neither alpha-1 (8.0-9.75 Hz) nor parietal EEG activity. These results confirm the involvement of the prefrontal and frontal cortices in the mechanisms responsible for modulating spatial orientation.

Do separate processes identify objects as exemplars versus members of basic-level categories? Evidence from hemispheric specialization

When an object is identified as a specific exemplar, is it analyzed differently than when it is identified at the basic level? On the basis of a previous theory, we predicted that the left hemisphere (LH) is specialized for classifying objects at the basic level and the right hemisphere (RH) is specialized for classifying objects as specific exemplars. To test this prediction, participants were asked to view lateralized pictures of animals, artifacts, and faces of famous people; immediately after each picture was presented, a label was read aloud by the computer, and the participants decided whether the label was correct for that picture. A label could name the object at either the basic level (e.g., bird) or as an exemplar (e.g., robin). As predicted, we found that basic-level labels were matched faster when pictures were presented in the right visual field (and hence encoded initially in the LH), whereas exemplar labels were matched faster when pictures were presented in the left visual field (and hence encoded initially in the RH).

Involvement of multiple functionally distinct cerebellar regions in visual discrimination: a human functional imaging study

We investigated the contribution of the human cerebellum to cerebral function during visual discrimination using PET and fMRI. The cognitive task was a successive discrimination of shades of brown with a parametric variation of the stimulus presentation rate and a constant task difficulty. The successive color discrimination task was contrasted to a dimming detection control task, with identical retinal input but with double the number of motor responses. Three sets of activated cerebellar and cerebral regions were observed: rate-dependent and rate-independent color discrimination networks and a motor-and-detection network. The rate-dependent color discrimination network included both an anterior and a posterior activation site in lobule-VI of the two lateral cerebellar hemispheres, whereas the rate-independent network involved a bilateral activation site in lateral Crus-I. Cerebellar sites of the motor-and-detection network were located in medial lobule-V bilaterally, in the vermis, and in posterior left Crus-I and right Crus-II. An additional fMRI study was performed to control for differences in motor output and response timing between the tasks. In this control study, the cerebellar activation sites of the rate-dependent and rate-independent color discrimination networks remained unaltered. The motor-and-detection network included cerebellar activations in posterior left Crus-I and right Crus-II, but none in lobule-V or the vermis. Thus, cerebellar activation sites of the motor-and-detection network could be subdivided into those related to a motor network and those belonging to a dimming detection network. We conclude that successive color discrimination activates multiple, functionally distinct cerebellar regions.

Dissociable effects of dopaminergic therapy on spatial versus non-spatial working memory in Parkinson's disease

There is now evidence for definite and early cognitive deficits in Parkinson's disease (PD), involving, in particular, executive functions and working memory. However, the distinction between visuo-spatial and non-spatial working memory deficits and the impact of dopamine on these deficits are still open to debate. The aim of this study was therefore to investigate cognitive and motor performance in PD patients in two conditional associative learning tasks requiring either spatial or non-spatial visual working memory. The subject had to point to visual targets according to the visual characteristics of memorised visual cues (colour, position and form). To assess the effect of L-dopa therapy, PD patients were studied over two consecutive days: one ON/OFF group of nine PD patients with treatment (ON condition) on the first day and without treatment (OFF condition) on the second day; and another OFF/ON group of nine PD patients tested on reverse. The PD groups were compared to a control group of nine age-matched healthy subjects. Our main data demonstrate that: (1) in PD patients with OFF treatment, the response time of manual pointing is increased mainly in the non-spatial working memory task; and (2) in PD patients with ON treatment, either the response time is normal (on the first day) or is increased in both visuo-spatial and non-spatial tasks. We suggest that this dissociation between spatial versus non-spatial working memory deficits in non-medicated PD might be related to compensatory mechanisms that occur following fronto-striatal dysfunction.

Reasoning and working memory: common and distinct neuronal processes

The neuronal processes underlying reasoning and the related working memory subsystems were examined with functional magnetic resonance imaging (fMRI). Twelve volunteers solved relational reasoning problems which either supported a single (determinate) or several alternative solutions (indeterminate). In a second condition, participants maintained the identical premises of these problems in working memory without making inferences. Although problems were presented in auditory format, activity was detected for both reasoning and maintenance in a network comprising bilaterally the secondary visual cortex, the posterior cingulate cortex, and the medial anterior frontal cortex. In direct comparisons, reasoning was associated with stronger dorsolateral and medial prefrontal activation than maintenance, whereas maintenance led to stronger lateral parietal activation than reasoning. Participants' visuo-spatial abilities ("Block Design" score) covaried positively with behavioral performance and negatively with activity of the precuneus for reasoning, but not for maintenance. These results support the notion that relational reasoning is based on visuo-spatial mental models, and they help to distinguish the neuronal processes related to reasoning itself versus to the maintenance of problem information in working memory.

Lateralized cognitive processes and lateralized task control in the human brain

The principles underlying human hemispheric specialization are poorly understood. We used functional magnetic resonance imaging of letter and visuospatial decision tasks with identical word stimuli to address two unresolved problems. First, hemispheric specialization depended on the nature of the task rather than on the nature of the stimulus. Second, analysis of frontal candidate regions for cognitive control showed increased coupling between left anterior cingulate cortex (ACC) and left inferior frontal gyrus during letter decisions, whereas right ACC showed enhanced coupling with right parietal areas during visuospatial decisions. Cognitive control is thus localized in the same hemisphere as task execution.

Effect of parietal lobe lesions on saccade targeting and spatial memory in a naturalistic visual search task

The eye movements of two patients with parietal lobe lesions and four normal observers were measured while they performed a visual search task with naturalistic objects. Patients were slower to perform the task than the normal observers, and the patients had more fixations per trial, longer latencies for the first saccade during the visual search, and less accurate first and second saccades to the target locations during the visual search. The increases in response times for the patients compared to the normal observers were best predicted by increases in the number of fixations. In order to investigate the effects of spatial memory on search performance, in some trials observers saw a preview of the search display. The patients appeared to have difficulty using previously viewed information, unlike normal observers who benefit from the preview. This suggests a spatial memory deficit. The patients' deficits are consistent with the hypothesis that the parietal cortex has a role in the selection of targets for saccades, in memory for target location.

Identification of cerebral networks by classification of the shape of BOLD responses

Changes in regional blood oxygen level dependent (BOLD) signals in response to brief visual stimuli can exhibit a variety of time-courses. To demonstrate the anatomical distribution of BOLD response shapes during a match to sample task, a formal analysis of their time-courses is presented. An event-related design was used to estimate regional BOLD responses evoked by a cue word, which instructed the subject to attend to the motion or color of an upcoming target, and those evoked by a briefly presented moving target consisting of colored dots. Regional BOLD time-courses were adequately represented by the linear combination of three orthogonal waveforms. BOLD response shapes were then classified using a fuzzy clustering scheme. Three classes (sustained, phasic, and negative) best characterized cue responses. Four classes (sustained, sustained-phasic, phasic, and bi-phasic) best characterized target responses. In certain regions, the shape of the BOLD responses was modulated by the instruction to attend to the target's motion or color. A left frontal and a posterior parietal region showed sustained activity when motion was cued and transient activity when color was cued. A right thalamic and a left lateral occipital region showed sustained activity when color was cued and transient activity when motion was cued. Following the target several regions showed more sustained activity during motion than color trials. In summary, the effect of the task variable was focal following the cue and widespread following the target. We conclude that the temporal patterns of neural activity affected the shape of the BOLD signal.

The functional neuroanatomy of human working memory revisited. Evidence from 3-T fMRI studies using classical domain-specific interference tasks

In the present event-related functional magnetic resonance imaging study, the neural implementation of human working memory was reinvestigated using a factorial design with verbal and visuospatial item-recognition tasks each performed under single-task conditions, under articulatory suppression, and under visuospatial suppression. This approach allowed to differentiate between brain systems subserving domain-specific working memory processes and possible neural correlates of more "central" executive or storage functions. The results of this study indicate (1) a domain-specific functional-neuroanatomical organization of verbal and visuospatial working memory, (2) a dual architecture of verbal working memory in contrast to a unitary macroscopic architecture of visuospatial working memory, (3) possible neural correlates for a domain-unspecific "episodic buffer" in contrast to a failure to find brain areas attributable to a "central executive," and (4) competition for neuronal processing resources as the causal principle for the occurrence of domain-specific interference in working memory.

Human brain regions required for the dividing and switching of attention between two features of a single object

This combined PET and ERP study was designed to identify the brain regions activated in switching and divided attention between different features of a single object using matched sensory stimuli and motor response. The ERP data have previously been reported in this journal [64]. We now present the corresponding PET data. We identified partially overlapping neural networks with paradigms requiring the switching or dividing of attention between the elements of complex visual stimuli. Regions of activation were found in the prefrontal and temporal cortices and cerebellum. Each task resulted in different prefrontal cortical regions of activation lending support to the functional subspecialisation of the prefrontal and temporal cortices being based on the cognitive operations required rather than the stimuli themselves.

Greater orbitofrontal activity predicts better memory for faces

Having demonstrated recently that the orbitofrontal cortex is selectively involved when novel abstract visual information is encoded, we investigated whether the same orbitofrontal area would be activated during the encoding of novel human faces. The present positron emission tomography study demonstrated that area 11 in the right orbitofrontal region, which is directly linked to the medial temporal lobe, is a critical frontal area selectively activated when normal human subjects are engaged in encoding visually presented faces. Furthermore, better face recognition performance correlated with greater cerebral blood flow specifically in this orbitofrontal area.

Reasoning, models, and images: behavioral measures and cortical activity

The goal of this study was to investigate the neurocognitive processes of mental imagery in deductive reasoning. Behavioral studies yielded four sorts of verbal relations: (1) visuospatial relations that are easy to envisage both visually and spatially; (2) visual relations that are easy to envisage visually but hard to envisage spatially; (3) spatial relations that are hard to envisage visually but easy to envisage spatially; and (4) control relations that are hard to envisage both visually and spatially. In three experiments, visual relations slowed the process of reasoning in comparison with control relations, whereas visuospatial and spatial relations yielded inferences comparable to those of control relations. An experiment using functional magnetic resonance imaging showed that in the absence of any correlated visual input (problems were presented acoustically via headphones), all types of reasoning problems evoked activity in the left middle temporal gyrus, in the right superior parietal cortex, and bilaterally in the precuneus. In the prefrontal cortex, increased activity was found in the middle and inferior frontal gyri. However, only the problems based on visual relations also activated areas of the visual association cortex corresponding to V2. The results indicate that cortical activity during reasoning depends on the nature of verbal relations. All relations elicit mental models that underlie reasoning, but visual relations in addition elicit visual images. This account resolves inconsistencies in the previous literature.

Functional topography of a distributed neural system for spatial and nonspatial information maintenance in working memory

We investigated the degree to which the distributed and overlapping patterns of activity for working memory (WM) maintenance of objects and spatial locations are functionally dissociable. Previous studies of the neural system responsible for maintenance of different types of information in WM have reported seemingly contradictory results concerning the degree to which spatial and nonspatial information maintenance leads to distinct patterns of activation in prefrontal cortex. These inconsistent results may be partly attributable to the fact that different types of objects were used for the "object WM task" across studies. In the current study, we directly compared the patterns of response during WM tasks for face identity, house identity, and spatial location using functional magnetic resonance imaging (fMRI). Furthermore, independence of the neural resources available for spatial and object WM was tested behaviorally using a dual-task paradigm. Together, these results suggest that the mechanisms for the maintenance of house identity information are distributed and overlapping with those that maintain spatial location information, while the mechanisms for maintenance of face identity information are relatively more independent. There is, however, a consistent functional topography that results in superior prefrontal cortex producing the greatest response during spatial WM tasks, and middle and inferior prefrontal cortices producing their greatest responses during object WM tasks, independent of the object type. These results argue for a dorsal-ventral functional organization for spatial and nonspatial information. However, objects may contain both spatial and nonspatial information and, thus, have a distributed but not equipotent representation across both dorsal and ventral prefrontal cortex.

Principles of cross-modal competition: evidence from deficits of attention

How does the attentional system coordinate the processing of stimuli presented simultaneously to different sensory modalities? We investigated this question with individuals with neurological damage who suffered from deficits of attention. In these individuals, we examined how the processing of tactile stimuli is affected by the simultaneous presentation of visual or auditory stimuli. The investigation demonstrated that two stimuli from different modalities are in competition when attention is directed to the perceptual attributes of both, but not when attention is directed to the perceptual attributes of one and the semantic attributes of the other. These findings reveal a differentiated attentional system in which competition is modulated by the level of stimulus representation to which attention is directed.

Distributed neural systems for temporal production: a functional MRI study

Using functional magnetic resonance imaging (fMRI), we investigated the neural substrates for computing time intervals. Five right-handed males were asked to judge if a digit probe belonged to a string of digits presented immediately before but to provide their response only after 1.5s had elapsed. This time estimation condition, compared with control working memory and motor tasks, was associated with increased activity in the middle occipital gyri, in the right inferior parietal lobe, and bilaterally in the prefrontal cortex. We argue that activity elicited in the occipital lobe provides duration information about visual stimuli that can be quantified at the level of the inferior parietal lobe. Comparison with time reference information depends on the bilateral prefrontal cortex.

Prefrontal-posterior parietal networks in schizophrenia: primary dysfunctions and secondary compensations

BACKGROUND

Working memory (WM) deficits are well known in schizophrenia and have been associated with abnormal activation patterns of the prefrontal cortex (PFC) during cognitive performance. The magnitude and particularly the direction of the PFC activation -- i.e., increased (hyperfrontality) or decreased (hypofrontality) -- in schizophrenia, as well as its pathophysiological implications, remain controversial. Working memory is supported by a distributed neural network, whose main components are the PFC and the posterior parietal (PPC) cortices. Monkey studies indicate that, during WM performance, PFC functional lesions may be compensated by the PPC if task demands center mainly on anticipating responses, but not if they center on remembering cues. We hypothesized that a primarily dysfunctional PFC in schizophrenia might show hypofrontality or hyperfrontality as a result, respectively, of efficient or inefficient PPC compensation, as dictated by task demands. To test our proposition, we biased the demands of WM tasks toward anticipating responses or remembering cues and measured its impact on the PFC-PPC functional balance in a group of schizophrenic patients and one of normal control subjects.

METHODS

We used functional magnetic resonance imaging to measure correlates of neuronal activity in the PFC and PPC of schizophrenic patients and control subjects performing WM tasks that either demanded information retention or allowed for response anticipation.

RESULTS

When compared to control subjects, schizophrenic patients exhibited decreased PFC activation and increased PPC activation during anticipatory WM performance, and increased PFC activation during mnemonic WM performance.

CONCLUSIONS

In schizophrenia, a PFC dysfunction results in hypo- or hyperfrontality as a function of whether other alternate areas of a PFC-PPC network for WM are available and efficacious in supporting specific task demands.

Tracking the time-course of attentional involvement in spatial working memory: an event-related potential investigation

Spatial working memory is a cognitive brain mechanism that enables the temporary maintenance and manipulation of spatial information. Recent neuroimaging and behavioral studies have led to the proposal that directed spatial attention is the mechanism by which location information is maintained in spatial working memory. Yet it is unclear whether attentional involvement is required throughout the period of active maintenance or is only invoked during discrete task-phases such as mnemonic encoding. In the current study, we aimed to track the time-course of attentional involvement during spatial working memory by recording event-related brain potentials (ERPs) from healthy volunteers. In Experiment 1, subjects performed a delayed-recognition task. Each trial began with the presentation of a brief stimulus (S1) that indicated the relevant location that subjects were to maintain in working memory. A 4.8-5.3 sec delay interval followed during which a single task-irrelevant probe was presented. The delay interval concluded with a test item (S2) to which subjects made a response indicating whether the S2-location was the same as the S1-memory location. To determine if attention was differentially engaged during discrete phases of the trial, task-irrelevant probes were presented early (400-800 msec following S1-offset) or late (2600-3000 msec following S1-offset) during the delay interval. Sensory-evoked ERPs (P1 and N1) elicited by these irrelevant probes showed attention-like modulations with greater amplitude responses for probes occurring at the S1-memory locations in comparison to probes presented at other locations. This pattern was obtained for both early- and late-delay probes. Probe-evoked activity during delayed-recognition trials was similar to activity observed when spatial attention was explicitly focused on a location in visual space (Experiment 2). These results are consistent with a model of spatial working memory in which perceptual level selective attention is utilized throughout the entire period of active maintenance to keep relevant spatial information in mind.

The role of prefrontal cortex in working-memory capacity, executive attention, and general fluid intelligence: an individual-differences perspective

We provide an "executive-attention" framework for organizing the cognitive neuroscience research on the constructs of working-memory capacity (WMC), general fluid intelligence, and prefrontal cortex (PFC) function. Rather than provide a novel theory of PFC function, we synthesize a wealth of single-cell, brain-imaging, and neuropsychological research through the lens of our theory of normal individual differences in WMC and attention control (Engle, Kane, & Tuholski, 1999; Engle, Tuholski, Laughlin, & Conway, 1999). Our critical review confirms the prevalent view that dorsolateral PFC circuitry is critical to executive-attention functions. Moreover, although the dorsolateral PFC is but one critical structure in a network of anterior and posterior "attention control" areas, it does have a unique executive-attention role in actively maintaining access to stimulus representations and goals in interference-rich contexts. Our review suggests the utility of an executive-attention framework for guiding future research on both PFC function and cognitive control.

Investigating the generators of the scalp recorded visuo-verbal P300 using cortically constrained source localization

Considerable ambiguity exists about the generators of the scalp recorded P300, despite a vast body of research employing a diverse range of methodologies. Previous investigations employing source localization techniques have been limited largely to equivalent current dipole models, with most studies identifying medial temporal and/or hippocampal sources, but providing little information about the contribution of other cortical regions to the generation of the scalp recorded P3. Event-related potentials (ERPs) were recorded from 5 subjects using a 124-channel sensor array during the performance of a visuo-verbal Oddball task. Cortically constrained, MRI-guided boundary element modeling was used to identify the cortical generators of this target P3 in individual subjects. Cortical generators of the P3 were localized principally to the intraparietal sulcus (IPS) and surrounding superior parietal lobes (SPL) bilaterally in all subjects, though with some variability across subjects. Two subjects also showed activity in the lingual/inferior occipital gyrus and mid-fusiform gyrus. A group cortical surface was calculated by non-linear warping of each subject's segmented cortex followed by averaging and creation of a group mesh. Source activity identified across the group reflected the individual subject activations in the IPS and SPL bilaterally and in the lingual/inferior occipital gyrus primarily on the left. Activation of IPS and SPL is interpreted to reflect the role of this region in working memory and related attention processes and visuo-motor integration. The activity in left lingual/inferior occipital gyrus is taken to reflect activation of regions associated with modality-specific analysis of visual word forms.

The neural basis for categorization in semantic memory

We asked young adults to categorize written object descriptions into one of two categories, based on a rule or on overall similarity, while we monitored regional brain activity with functional magnetic resonance imaging (fMRI). We found significantly greater recruitment of left dorsolateral prefrontal cortex for rule-based categorization in direct comparison with similarity-based categorization. Recruitment of right ventral frontal cortex and thalamus was uniquely associated with rule-based categorization as well. These observations lend support to the claim that executive functions such as working memory, inhibitory control, and selective attention contribute to rule-based categorization. Right inferior parietal activation was uniquely associated with similarity-based categorization. This region may play an important role in overall feature configuration that is important for this form of categorization. We found other brain regions recruited for both rule-based and similarity-based categorization: Anterior cingulate cortex may support the implementation of executive functions during situations with competing response alternatives; and left inferior parietal cortex may be related to the integration of feature knowledge about objects represented in modality-specific association cortices. We also administered a degraded-similarity condition where the task of categorizing a written object description was made more difficult by perceptually degrading the stimulus materials. The degraded condition and the rule-based condition, but not the similarity-based condition, were associated with caudate activation. The caudate may support resource demands that are not specific for a particular categorization process. These findings associate partially distinct large-scale neural networks with different forms of categorization in semantic memory.

Orbitofrontal cortex and memory formation

Which one of the many regions of the anatomically heterogeneous prefrontal cortex is part of the critical core of the neural circuit for encoding? This positron emission tomography (PET) experiment measured changes in cerebral blood flow (CBF) in normal human participants during the presentation of abstract visual information in four conditions that varied in their encoding demands. As encoding increased across the different conditions, there was an increase in activity in the right orbitofrontal cortex and the right parahippocampal region. No significant activation peaks were present in any other region of the frontal or temporal lobe. These findings indicate that the orbitofrontal cortex, which is massively connected to the medial temporal cortex, is a critical frontal region for memory formation.

Word retrieval learning modulates right frontal cortex in patients with left frontal damage

Previous studies have suggested that recovery or compensation of language function after a lesion in the left hemisphere may depend on mechanisms in the right hemisphere. However, a direct relationship between performance and right hemisphere activity has not been established. Here, we show that patients with left frontal lesions and partially recovered aphasia learn, at a normal rate, a novel word retrieval task that requires the damaged cortex. Verbal learning is accompanied by specific response decrements in right frontal and right occipital cortex, strongly supporting the compensatory role of the right hemisphere. Furthermore, responses in left occipital cortex are abnormal and not modulated by practice. These findings indicate that frontal cortex is a source of top-down signals during learning.

Neural correlates of memory for object identity and object location: effects of aging

The purpose of this study was to investigate the effects of aging on memory for object identity and object location to determine whether aging affects both posterior neocortical areas that are domain-specific and other brain regions, such as pre-frontal cortex, that are involved in encoding and retrieval regardless of the information that is processed (domain-general). We used positron emission tomography (PET) to measure changes in regional cerebral blood flow (rCBF) in younger and older participants while they were engaged in encoding and retrieving information about object identity and object location. Compared to young adults, older adults showed decreased activation in domain-specific regions of inferior parietal and inferior temporal cortex while engaged in processing (encoding and retrieving) information about object location and object identity, respectively. This decreased specificity in the older adults was accompanied by greater domain-general activation in right prefrontal and premotor cortex during perceptual encoding than during retrieval. Conversely, the younger participants showed greater domain-general activation in right extrastriate cortex (Brodmann area (BA) 18) during retrieval. Moreover, we found that medial temporal and frontal lobes were synergistically activated in younger adults but not in older adults. The pattern of decreased specificity of activation in posterior neocortex with greater activation in anterior neocortex suggests that, with age, compensatory domain-general mechanisms in anterior neocortex are recruited to mitigate altered domain-specific processes. Thus, the results of the present study indicate that the relation between the presumed integrity of various structures, such as the hippocampus, prefrontal cortex, and posterior neocortex, and their pattern of activation, is a complex one that is influenced by age, by the perceptual and cognitive demands of the task and their interaction.

Neural correlates of visual working memory: fMRI amplitude predicts task performance

We used fMRI to investigate how moment-to-moment neural activity contributes to success or failure on individual trials of a visual working memory (WM) task. We found that different nodes of a distributed cortical network were activated to a greater extent for correct compared to incorrect trials during stimulus encoding, memory maintenance during delays, and at test. A logistic regression analysis revealed that the fMRI signal amplitude during the delay interval in a network of frontoparietal regions predicted successful performance on a trial-by-trial basis. Differential delay activity occurred even for only those trials in which BOLD activity during encoding was strong, demonstrating that it was not a simple consequence of effective versus ineffective encoding. Our results indicate that accurate memory depends on strong sustained signals that span the delay interval of WM tasks.

Contributions of subregions of the prefrontal cortex to working memory: evidence from brain lesions in humans

We investigated working memory in patients with focal brain damage involving subregions of the prefrontal cortex (PFC). Lesions in the dorsal portion of lateral PFC or the ventromedial portion of orbital PFC did not impair performance in tasks that required maintenance and monitoring of object or spatial information. Larger lesions involving both ventral and dorsal parts of the lateral PFC impaired maintenance and monitoring of spatial and object information, with more severe deficits observed in the spatial tasks. The results support a distributed localization of function in lateral PFC during working memory.

Comparative cytoarchitectonic analysis of the human and the macaque ventrolateral prefrontal cortex and corticocortical connection patterns in the monkey

A comparison of the cytoarchitecture of the human and the macaque monkey ventrolateral prefrontal cortex demonstrated a region in the monkey that exhibits the architectonic characteristic of area 45 in the human brain. This region occupies the dorsal part of the ventrolateral prefrontal convexity just below area 9/46v. Rostroventral to area 45 in the human brain lies a large cortical region labelled as area 47 by Brodmann. The ventrolateral component of this region extending as far as the lateral orbital sulcus has architectonic characteristics similar to those of the ventrolateral prefrontal region labelled by Walker as area 12 in the macaque monkey. We designated this region in both the human and the monkey ventrolateral prefrontal cortex as area 47/12. Thus, area 47/12 designates the specific part of the zone previously labelled as area 47 in the human brain that has the same overall architectonic pattern as that of Walker's area 12 in the macaque monkey brain. The cortical connections of these two areas were examined in the monkey by injecting fluorescent retrograde tracers. Although both area 45 and area 47/12 as defined here had complex multimodal input, they could be differentiated in terms of some of their inputs. Retrograde tracers restricted to area 47/12 resulted in heavy labelling of neurons in the rostral inferotemporal visual association cortex and in temporal limbic areas (i.e. perirhinal and parahippocampal cortex). In contrast, injections of tracers into dorsally adjacent area 45 demonstrated strong labelling in the superior temporal gyrus (i.e. the auditory association cortex) and the multimodal cortex in the upper bank of the superior temporal sulcus.

Whose arm is it anyway? An fMRI case study of supernumerary phantom limb

Under normal circumstances, information from a number of sources is combined to compute a unitary percept of the body. However, after pathology these influences may be perceived simultaneously, resulting in multiple dissociated conscious representations. In a recent paper, we described subject E.P., a right-handed female stroke patient with a right frontomesial lesion who sporadically experiences a supernumerary 'ghost' left arm that occupies the previous position of the real left arm after a delay of 60-90 s. We used a delayed response paradigm with functional MRI to examine the haemodynamic correlates of E.P.'s illusion. Comparison of periods of time during scanning when the ghost arm was present against when it was not revealed a single cluster (9 voxels, t = 5.11, P < 0.012 corrected for multiple comparisons) located on the right medial wall in the supplementary motor area ('SMA proper'). Our results suggest that areas traditionally classified as part of the motor system can influence the conscious perception of the body. We propose that, as a consequence of her injury, E.P. is aware of the position of the phantom limb in this 'action space' while also continuing to be aware of the true position of her real limb on the basis of afferent somatosensory information.

Human brain regions involved in visual categorization

Categorization of dot patterns is a frequently used paradigm in the behavioral study of natural categorization. To determine the human brain regions involved in categorization, we used Positron Emission Tomography to compare regional Cerebral Blood Flow patterns in two tasks employing patterns that consisted of nine dots. In the categorization task, subjects categorized novel exemplars of two categories, generated by distorting two prototypes, and other random dot patterns. In the control task, subjects judged the position of similarly distorted patterns. Each task was presented at two matched levels of difficulty. Fixation of the fixation target served as baseline condition. The categorization task differentially activated the orbitofrontal cortex and two dorsolateral prefrontal regions. These three prefrontal regions were equally weakly active in the position discrimination task and the baseline condition. The intraparietal sulcus was activated in both tasks, albeit significantly less in the position discrimination than in the categorization task. A similar activation pattern was present in the neostriatum. Task difficulty had no effect. These functional imaging results show that the dot-pattern categorization task strongly engages prefrontal and parietal cortical areas. The activation of prefrontal cortex during visual categorization in humans agrees with the recent finding of category-related responses in macaque prefrontal neurons.

Painful stimuli evoke different stimulus-response functions in the amygdala, prefrontal, insula and somatosensory cortex: a single-trial fMRI study

Only recently have neuroimaging studies moved away from describing regions activated by noxious stimuli and started to disentangle subprocesses within the nociceptive system. One approach to characterizing the role of individual regions is to record brain responses evoked by different stimulus intensities. We used such a parametric single-trial functional MRI design in combination with a thulium:yttrium-aluminium-granate infrared laser and investigated pain, stimulus intensity and stimulus awareness (i.e. pain-unrelated) responses in nine healthy volunteers. Four stimulus intensities, ranging from warm to painful (300-600 mJ), were applied in a randomized order and rated by the subjects on a five-point scale (P0-4). Regions in the dorsolateral prefrontal cortex and the intraparietal sulcus differentiated between P0 (not perceived) and P1 but exhibited no further signal increase with P2, and were related to stimulus perception and subsequent cognitive processing. Signal changes in the primary somatosensory cortex discriminated between non-painful trials (P0 and P1), linking this region to basic sensory processing. Pain-related regions in the secondary somatosensory cortex and insular cortex showed a response that did not distinguish between innocuous trials (P0 and P1) but showed a positive linear relationship with signal changes for painful trials (P2-4). This was also true for the amygdala, with the exception that, in P0 trials in which the stimulus was not perceived (i.e. 'uncertain' trials), the evoked signal changes were as great as in P3 trials, indicating that the amygdala is involved in coding 'uncertainty', as has been suggested previously in relation to classical conditioning.

A matching advantage for dynamic human faces

In a series of three experiments, we used a sequential matching task to explore the impact of non-rigid facial motion on the perception of human faces. Dynamic prime images, in the form of short video sequences, facilitated matching responses relative to a single static prime image. This advantage was observed whenever the prime and target showed the same face but an identity match was required across expression (experiment 1) or view (experiment 2). No facilitation was observed for identical dynamic prime sequences when the matching dimension was shifted from identity to expression (experiment 3). We suggest that the observed dynamic advantage, the first reported for non-degraded facial images, arises because the matching task places more emphasis on visual working memory than typical face recognition tasks. More specifically, we believe that representational mechanisms optimised for the processing of motion and/or change-over-time are established and maintained in working memory and that such 'dynamic representations' (Freyd, 1987 Psychological Review 94 427-438) capitalise on the increased information content of the dynamic primes to enhance performance.

Prefrontal dopamine D1 receptors and working memory in schizophrenia

Studies in nonhuman primates documented that appropriate stimulation of dopamine (DA) D1 receptors in the dorsolateral prefrontal cortex (DLPFC) is critical for working memory processing. The defective ability of patients with schizophrenia at working memory tasks is a core feature of this illness. It has been postulated that this impairment relates to a deficiency in mesocortical DA function. In this study, D1 receptor availability was measured with positron emission tomography and the selective D1 receptor antagonist [11C]NNC 112 in 16 patients with schizophrenia (seven drug-naive and nine drug-free patients) and 16 matched healthy controls. [11C]NNC 112 binding potential (BP) was significantly elevated in the DLPFC of patients with schizophrenia (1.63 +/- 0.39 ml/gm) compared with control subjects (1.27 +/- 0.44 ml/gm; p = 0.02). In patients with schizophrenia, increased DLPFC [11C]NNC 112 BP was a strong predictor of poor performance at the n-back task, a test of working memory. These findings confirm that alteration of DLPFC D1 receptor transmission is involved in working memory deficits presented by patients with schizophrenia. Increased D1 receptor availability observed in patients with schizophrenia might represent a compensatory (but ineffective) upregulation secondary to sustained deficiency in mesocortical DA function.

The quantitative nature of a visual task differentiates between ventral and dorsal stream

The aim of the present positron emission tomography (PET) study was to investigate how visual processing in dorsal and ventral streams depends on the quantitative nature of the task. In the same-different task, participants identified the presence of an orientation difference between two gratings, presented centrally in succession. In the quantification task, participants estimated the magnitude of the difference and compared it to a fixed standard. Detection of dimming of the fixation point was used as a control task. Visual input, motor responses, and performance were equated across tasks. Subtracting same-different from quantification yielded significant activation in the left superior parietal lobule and left ventral premotor cortex, consistent with results obtained in number-processing tasks. The reverse subtraction yielded activation in the right inferior temporal gyrus, in agreement with earlier results. These results demonstrate that a single attribute can be processed either in the ventral or dorsal stream, depending on the cognitive operations required by the tasks.

A prospective study of motor recovery following multiple subpial transections

A prospective study of motor recovery was undertaken in a patient scheduled to undergo multiple subpial transections (MST) of right sensorimotor cortex. Pre-transection, functional MRI (fMRI) and cortical stimulation mapping confirmed left hand motor control within right primary motor cortex. Immediately post-transection, behavioral testing demonstrated preserved strength bilaterally but decreased dexterity in the left hand. Seven weeks post-transection, dexterity returned to normal and left hand finger tapping corresponded with multiple bilateral foci of fMRI activation. At 16 weeks, fMRI activation returned to pre-transection levels. These data indicate that cortical injury due to MST resulted in the temporary recruitment of distant cortical sites which presumably subserved normal motor function during recovery.

Dissociation of working memory processing associated with native and second languages: PET investigation

Verbal working memory plays a significant role in language comprehension and problem-solving. The prefrontal cortex has been suggested as a critical area in working memory. Given that domain-specific dissociations of working memory may exist within the prefrontal cortex, it is possible that there may also be further functional divisions within the verbal working memory processing. While differences in the areas of the brain engaged in native and second languages have been demonstrated, little is known about the dissociation of verbal working memory associated with native and second languages. We have used H2(15)O positron emission tomography in 14 normal subjects in order to identify the neural correlates selectively involved in working memory of native (Korean) and second (English) languages. All subjects were highly proficient in the native language but poorly proficient in the second language. Cognitive tasks were a two-back task for three kinds of visually presented objects: simple pictures, English words, and Korean words. The anterior portion of the right dorsolateral prefrontal cortex and the left superior temporal gyrus were activated in working memory for the native language, whereas the posterior portion of the right dorsolateral prefrontal cortex and the left inferior temporal gyrus were activated in working memory for the second language. The results suggest that the right dorsolateral prefrontal cortex and left temporal lobe may be organized into two discrete, language-related functional systems. Internal phonological processing seems to play a predominant role in working memory processing for the native language with a high proficiency, whereas visual higher order control does so for the second language with a low proficiency.

Integration of emotion and cognition in the lateral prefrontal cortex

We used functional MRI to test the hypothesis that emotional states can selectively influence cognition-related neural activity in lateral prefrontal cortex (PFC), as evidence for an integration of emotion and cognition. Participants (n = 14) watched short videos intended to induce emotional states (pleasant/approach related, unpleasant/withdrawal related, or neutral). After each video, the participants were scanned while performing a 3-back working memory task having either words or faces as stimuli. Task-related neural activity in bilateral PFC showed a predicted pattern: an Emotion x Stimulus crossover interaction, with no main effects, with activity predicting task performance. This highly specific result indicates that emotion and higher cognition can be truly integrated, i.e., at some point of processing, functional specialization is lost, and emotion and cognition conjointly and equally contribute to the control of thought and behavior. Other regions in lateral PFC showed hemispheric specialization for emotion and for stimuli separately, consistent with a hierarchical and hemisphere-based mechanism of integration.

The effects of encoding task on age-related differences in the functional neuroanatomy of face memory

Age-related differences in brain activity mediating face recognition were examined using positron emission tomography. Participants encoded faces using a pleasant-unpleasant judgment, a right-left orientation task, and intentional learning. Scans also were obtained during recognition. Both young and old groups showed signficant effects of encoding task on recognition accuracy, but older adults showed reduced accuracy overall. Increased brain activity in older adults was similar to that seen in young adults during conditions associated with deeper processing, but was reduced during the shallow encoding and recognition conditions. Left prefrontal activity was less in older adults during encoding, but greater during recognition. Differential correlations of brain activity and behavior were found that suggest older adults use unique neural systems to facilitate face memory.

Sentence processing strategies in healthy seniors with poor comprehension: an fMRI study

We used fMRI to examine patterns of brain recruitment in 22 healthy seniors, half of whom had selective comprehension difficulty for grammatically complex sentences. We found significantly reduced recruitment of left posterolateral temporal [Brodmann area (BA) 22/21] and left inferior frontal (BA 44/6) cortex in poor comprehenders compared to the healthy seniors with good sentence comprehension, cortical regions previously associated with language comprehension and verbal working memory, respectively. The poor comprehenders demonstrated increased activation of left prefrontal (BA 9/46), right dorsal inferior frontal (BA 44/6), and left posterior cingulate (BA 31/23) cortices for the grammatically simpler sentences that they understood. We hypothesize that these brain regions support an alternate, nongrammatical strategy for processing complex configurations of symbolic information. Moreover, these observations emphasize the crucial role of the left perisylvian network for grammatically guided sentence processing in subjects with good comprehension.

Covert reorienting and inhibition of return: an event-related fMRI study

Using event-related fMRI, we analyzed the functional neuroanatomy of covert reorienting and inhibition of return (IOR). Covert reorienting to a target appearing within 250 msec after an invalid contralateral location cue elicited increased activation in the left fronto-polar cortex (LFPC), right anterior and left posterior middle frontal gyrus, and right cerebellum, areas that have previously been associated with attentional processes, specifically attentional change. In contrast, IOR, which leads to prolonged response times to targets that appear at the cued location at a stimulus-onset-asynchrony (SOA)>250 msec, was accompanied by increased activation in brain areas involved in oculomotor programming, such as the right medial frontal gyrus (supplementary eye field; SEF) and the right inferior precentral sulcus (frontal eye field; FEF), supporting the oculomotor bias theory of IOR. Pre-SEF and pre-FEF areas were involved both in covert reorienting and IOR. The supramarginal gyri were bilaterally involved in IOR, with the right supramarginal gyrus additionally involved in covert reorienting.

Age-related changes in working memory during sentence comprehension: an fMRI study

Sentence comprehension declines with age, but the neural basis for this change is unclear. We monitored regional brain activity in 13 younger subjects and 11 healthy seniors matched for sentence comprehension accuracy while they answered a simple probe about written sentences. The sentences varied in their grammatical features (subject-relative vs object-relative subordinate clause) and their verbal working memory (WM) demands (short vs long antecedent noun-gap linkage). We found that young and senior subjects both recruit a core written sentence processing network, including left posterolateral temporal and bilateral occipital cortex for all sentences, and ventral portions of left inferior frontal cortex for object-relative sentences with a long noun-gap linkage. Differences in activation patterns for seniors compared to younger subjects were due largely to changes in brain regions associated with a verbal WM network. While seniors had less left parietal recruitment than younger subjects, left premotor cortex, and dorsal portions of left inferior frontal cortex showed greater activation in seniors compared to younger subjects. Younger subjects recruited right posterolateral temporal cortex for sentences with a long noun-gap linkage. Seniors additionally recruited right parietal cortex for this sentence-specific form of WM. Our findings are consistent with the hypothesis that the neural basis for sentence comprehension includes dissociable but interactive large-scale neural networks supporting core written sentence processes and related cognitive resources involved in WM. Seniors with good comprehension appear to up-regulate portions of the neural substrate for WM during sentence processing to achieve comprehension accuracy that equals young subjects.

Inversion and contrast polarity reversal affect both encoding and recognition processes of unfamiliar faces: a repetition study using ERPs

Using ERPs in a face recognition task, we investigated whether inversion and contrast reversal, which seem to disrupt different aspects of face configuration, differentially affected encoding and memory for faces. Upright, inverted, and negative (contrast-reversed) unknown faces were either immediately repeated (0-lag) or repeated after 1 intervening face (1-lag). The encoding condition (new) consisted of the first presentation of items correctly recognized in the two repeated conditions. 0-lag faces were recognized better and faster than 1-lag faces. Inverted and negative pictures elicited longer reaction times, lower hit rates, and higher false alarm rates than upright faces. ERP analyses revealed that negative and inverted faces affected both early (encoding) and late (recognition) stages of face processing. Early components (N170, VPP) were delayed and enhanced by both inversion and contrast reversal which also affected P1 and P2 components. Amplitudes were higher for inverted faces at frontal and parietal sites from 350 to 600 ms. Priming effects were seen at encoding stages, revealed by shorter latencies and smaller amplitudes of N170 for repeated stimuli, which did not differ depending on face type. Repeated faces yielded more positive amplitudes than new faces from 250 to 450 ms frontally and from 400 to 600 ms parietally. However, ERP differences revealed that the magnitude of this repetition effect was smaller for negative and inverted than upright faces at 0-lag but not at 1-lag condition. Thus, face encoding and recognition processes were affected by inversion and contrast-reversal differently.

Cultural variation in verbal versus spatial neuropsychological function across the life span

Established culture-invariant measures are needed for cross-cultural assessment of verbal and visuospatial speed of processing and working memory across the life span. In this study, 32 younger and 32 older adults from China and from the United States were administered numerically based and spatially based measures of speed of processing and working memory. Chinese superiority on the numerically based tasks was found for younger adults. Age and increasing task demands diminished this cultural effect, as predicted by the framework proposed by D. C. Park, R. Nisbett, and T. Hedden (1999). However, the visuospatial measures of both working memory and speed of processing did not differ cross-culturally for either age group. The authors concluded that these visuospatial measures provide culture-invariant estimates of cognitive processes in East Asian and Western cultures, but that numerically based tasks show evidence of cultural and linguistic biases in performance levels.

Dissociating a common working memory network from different neural substrates of phonological and spatial stimulus processing

Positron emission tomography was used to investigate common versus specific cortical regions for the maintenance of spatial versus phonological information in working memory (WM). Group and single-subject analyses of regional cerebral blood flow during a new 2 x 2 factorial n-back task were performed. Eight subjects had to memorize either phonological features or the location of serially presented syllables. Brain activation during phonological judgment and spatial judgment (0-back) was compared with that during two corresponding WM conditions (2-back). We observed a common network associated with the requirement of maintaining and sequencing items in WM. Seven or more subjects activated (posterior) superior frontal sulcus (pSFS, BA 6/8, global maximum) and/or adjacent gyri, posterior parietal cortex, and precuneus (BA 7). Less consistently, bilateral middle frontal gyrus (BA 9/46) was involved. Bilateral anterior (BA 39/40) and posterior (BA 7) intraparietal sulcus, as well as right pSFS, exhibited dominance for spatial WM. Although underlying stimulus processing pathways for both types of information were different, no region specific for phonological WM was found. Robust activation within the left inferior frontal gyrus (BA 44 and 45) was present, during both phonological WM and phonological judgment. We conclude that the controversial left prefrontal lateralization for verbal WM reflects more general phonological processing strategies, not necessarily required by tasks using letters. We propose a stimulus-independent role for the bilateral pSFS and its vicinity for maintenance and manipulation of different context-dependent information within working memory.

Age-related changes in regional activation during working memory in young adults: an fMRI study

Several lines of evidence suggest that working memory diminishes with advancing age, with concomitant functional changes in associated neuronal activation in frontal cortical regions and hippocampi. No studies to date, however, have investigated age-related changes in neuronal activation in these regions during performance of a working memory task in younger subjects without working memory deficits. In this study, we utilized fMRI to examine changes in brain activation with increasing age in specific regions-of-interest. Eleven healthy subjects performed a "two-back" working memory task and a matched "zero-back" attention task during fMRI. There was no association between age and performance on either task. Left hippocampal activation significantly correlated with age (P = 0.01) and right hippocampal activation showed an association with age (P = 0.09). This study demonstrates that increasing age is associated with increased activation of hippocampus even in young patients without evidence of working memory deficits and suggests that functional changes may precede overt evidence of working memory deficits.

Evidence for dissociation of spatial and nonspatial auditory information processing

Several lines of evidence suggest that visual information processing is segregated into the ventral "what" and dorsal "where" pathways. But the question whether information processing in the auditory system is also parceled to spatial and nonspatial domains remains open. In the present study, we performed simultaneous EEG and MEG recordings during auditory location and pitch delayed matching-to-sample tasks to find out whether working memory processing of the auditory stimulus attribute affects the transient components of the evoked potentials. In both tasks, identical blocks of tone stimuli of one of two frequencies were presented in one of two locations; the only difference between the tasks was the instruction to attend either to the frequency or to the location. In the match condition, the N1 latency was shorter and the N1m amplitude larger in the location task compared to the pitch task. Furthermore, the right-hemisphere generator of N1m elicited in the match condition of the location task was situated significantly medially to the N1m generator in the match condition of the pitch task. Latency and amplitude task-related differences in the N1/N1m components as well as the source location differences indicate at least partial segregation of neuronal mechanisms involved in working memory processing of spatial and nonspatial auditory information.

Cerebellar contribution to spatial event processing: involvement in procedural and working memory components

Spatial function is one of the cognitive functions altered in the presence of cerebellar lesions. We investigated the cerebellar contribution to the acquisition of spatial procedural and working memory components by means of a radial maze. To establish whether a cerebellar lesion would cause a deficit in solving the radial maze, a first experiment was carried out by using a full-baited maze procedure in different experimental groups, with or without cerebellar lesion and with or without pretraining. Non-pretrained hemicerebellectomized (HCbed) animals exhibited impaired performances in all (motor, spatial and procedural) task aspects. Pre-trained HCbed animals performed similarly to control animals in the task aspects linked to the processing of spatial and procedural factors. To distinguish procedural from working memory components, a forced-choice paradigm of the radial maze was used in the second experiment. Non-pretrained HCbed rats continued to make a lot of errors and show severe perseverative tendencies, already observed in the first experiment, supporting a specific cerebellar role in acquiring new behaviours and in modifying them in relation to the context. Interestingly, hindered from putting the acquired explorative patterns into action and compelled to use only working memory abilities, the pretrained HCbed group exhibited a dramatic worsening of performance. In conclusion, the present findings demonstrate that cerebellar damage induces a specific behaviour in radial maze tasks, characterized by an inflexible use of the procedures (if indeed any procedure was acquired before the lesion) and by a severe impairment in working memory processes.

Interference with performance of a response selection task that has no working memory component: an rTMS comparison of the dorsolateral prefrontal and medial frontal cortex

It has been suggested that the dorsolateral prefrontal cortex (DLPFC) is involved in free selection (FS), the process by which subjects themselves decide what action to perform. Evidence for this proposal has been provided by imaging studies showing activation of the DLPFC when subjects randomly generate responses. However, these response selection tasks have a hidden working memory element and it has been widely reported that the DLPFC is activated when subjects perform tasks which involve working memory. The primary aim of this experiment was to establish if the DLPFC is genuinely involved in response selection. We used repetitive transcranial magnetic stimulation (rTMS) to investigate whether temporary interference of the DLPFC could disrupt performance of a response selection task that had no working memory component. Subjects performed tasks in which they made bimanual sequences of eight nonrepeating finger movements. In the FS task, subjects chose their movements at random while a computer monitor displayed these moves. This visual feedback obviated the need for subjects to maintain their previous moves "on-line." No selection was required for the two control tasks as responses were cued by the visual display. The attentional demands of the control tasks varied. In the high load (HL) version, subjects had to maintain their attention throughout the sequence, but this requirement was absent in the low load (LL) task. rTMS over the DLPFC slowed response times on the FS task and at the end of the sequence on the HL task, but had no effect on the LL task. rTMS over the medial frontal cortex (MFC) slowed response times on the FS task but had no effect on the HL task. This suggests that a response selection task without a working memory load will depend on the DLPFC and the MFC. The difference appears to be that the DLPFC is important when selecting between competing responses or when concentrating if there is a high attentional demand, but that the MFC is only important during the response selection task.