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

"What" and "where" in the intraparietal sulcus: an FMRI study of object identity and location in visual short-term memory

The intraparietal sulcus (IPS) has been closely linked to limitations of visual short-term memory capacity (VSTM; Todd and Marois 2004; Xu and Chun 2006). It is not clearly known, however, to what extent IPS activation reflects VSTM for object identity (What) versus spatial location (Where) information. The present study was designed to manipulate selectively the amount of What and Where information retained in VSTM in order to determine, using functional magnetic resonance imaging, the effect of VSTM for each of these 2 dimensions on IPS activation. The results showed an increase in IPS activation only in response to increasing Where memory load, with no effect of What load suggesting that capacity-related activation in the IPS primarily reflects the amount of spatial information retained in VSTM.

Brain mechanisms for representing what another person sees

We used functional magnetic resonance imaging (fMRI) and a naturalistic joint attention scenario to evaluate two, alternative hypotheses concerning the social brain. The first, Content Specific Attribution hypothesis, was that core regions previously identified as being involved in social cognition also participate in representing the contents of another mind. The second, Dual Role hypothesis, was that extrastriate, category-specific visual regions respond to a visible stimulus of a specific category and to the same stimulus occluded, but when it appears to be the focus of another person's visual attention. Participants viewed category-specific stimuli (Place and Body images) to localize the extrastriate body area (EBA) and parahippocampal place area (PPA). Then, they observed a computerized character viewing each stimulus category, occluded from the participant's view. In support of the Content Specific Attribution hypothesis, whole-brain analyses revealed that viewing someone else looking at an occluded picture of a body activated brain regions previously associated with components of social cognition more than viewing someone else looking at an occluded picture of a place. Counter to the Dual Role hypothesis, functional region of interest (ROI) analyses revealed that the EBA and PPA were not clearly involved in representing what the character was seeing.

Using biologically plausible neural models to specify the functional and neural mechanisms of visual search

We review research from our laboratory that attempts to pull apart the functional and neural mechanisms of visual search using converging, inter-disciplinary evidence from experimental studies with normal participants, neuropsychological studies with brain lesioned patients, functional brain imaging and computational modelling. The work suggests that search is determined by excitatory mechanisms that support the selection of target stimuli, and inhibitory mechanisms that suppress irrelevant distractors. These mechanisms operate through separable though overlapping neural circuits which can be functionally decomposed by imposing model-based analyses on brain imaging data. The chapter highlights the need for inter-disciplinary research for understanding complex cognitive processes at several levels.

Reanalysis of the obesity-related attenuation in the left dorsolateral prefrontal cortex response to a satiating meal using gyral regions-of-interest

OBJECTIVE

The left dorsolateral prefrontal cortex (LDLPFC), which includes the inferior (IFG), middle (MFG), and superior (SFG) frontal gyri, has been implicated in satiation. Using a voxel-based approach, we previously identified an LDLPFC region (as reported as peak voxel) in which a reduced neuronal response to a meal was associated with obesity. In this study, we sought to determine which gyri in the LDLPFC best distinguished the neuronal responses to a meal using a different statistical approach.

METHODS

We reanalyzed brain responses to a meal using the hypothesis-driven region-of-interest-based (ROI) approach. Regional cerebral blood flow (rCBF), a marker of neuronal activity in the LDLPFC and its 3 gyri, was acquired in 2 conditions (hunger and after the satiating meal) using (15)O-water positron emission tomography scans. rCBF was extracted and estimated using masks of the 3 gyri that were created in MRIcro and Statistical Parametric Mapping 5 software.

RESULTS

Using the ROI approach, a satiation-related reduction in LDLPFC rCBF was observed in the obese (p = 0.04) and tended to be significantly greater than that in lean subjects (p = 0.07). The rCBF reduction was greater in the obese subjects than in the lean subjects in the left IFG (p = 0.03) and MFG (p = 0.004) after adjustment was made for age, sex, and number of voxels in these gyri, but not in the SFG (p = 0.5).

CONCLUSIONS

Our results are consistent with those obtained by the voxel-based approach in showing the association between obesity and a satiation-related reduction in LDLPFC activity. This LDLPFC response preferentially involves the IFG and MFG. We suggest that these brain regions could be targeted by new therapeutic interventions.

Coding categorical and coordinate spatial relations in visual–spatial short-term memory

Four experiments explored the coding of categorical and coordinate spatial relations in visual–spatial short-term memory (VSSTM). Participants judged whether two stimuli presented successively on a computer screen were the same or different. On positional change trials the two stimuli differed in the position of one element. Positional changes were of two types, coordinate and categorical. On coordinate trials the position of one element changed by a small amount, but retained the categorical relationships (above, below, left of, right of) to all other elements. On categorical trials one element moved by the same amount but additionally changed its categorical relationship to one of the other elements (e.g., changed from below to above). Participants detected categorical changes more accurately than coordinate changes when the elements were independent locations marked by squares, indicating that the categorical relationships amongst the squares were encoded in memory. Furthermore, this categorical advantage was unmodulated by either the suppression of articulation (Experiment 2) or by the requirement to retain either colour–position associations or positions only (Experiment 3). When the elements to be remembered were the vertices of simple outline polygons (Experiment 4) there was no categorical advantage, establishing the effect as spatial in nature. Contrary to predictions derived from Postma and De Haan (1996), the employment of categorical relations appears not to be specifically linked to either verbal coding or to the requirement to associate objects with positions. The results suggest that the categorical relations are an intrinsic property of the representation of spatial configurations.

Selective involvement of superior frontal cortex during working memory for shapes

A spatial/nonspatial functional dissociation between the dorsal and ventral visual pathways is well established and has formed the basis of domain-specific theories of prefrontal cortex (PFC). Inconsistencies in the literature regarding prefrontal organization, however, have led to questions regarding whether the nature of the dissociations observed in PFC during working memory are equivalent to those observed in the visual pathways for perception. In particular, the dissociation between dorsal and ventral PFC during working memory for locations versus object identities has been clearly present in some studies but not in others, seemingly in part due to the type of objects used. The current study compared functional MRI activation during delayed-recognition tasks for shape or color, two object features considered to be processed by the ventral pathway for perceptual recognition. Activation for the shape-delayed recognition task was greater than that for the color task in the lateral occipital cortex, in agreement with studies of visual perception. Greater memory-delay activity was also observed, however, in the parietal and superior frontal cortices for the shape than for the color task. Activity in superior frontal cortex was associated with better performance on the shape task. Conversely, greater delay activity for color than for shape was observed in the left anterior insula and this activity was associated with better performance on the color task. These results suggest that superior frontal cortex contributes to performance on tasks requiring working memory for object identities, but it represents different information about those objects than does the ventral frontal cortex.

Two qualitatively different impairments in making rotation operations

It is widely recognized that mental rotation is a cognitive process which engages a distributed cortical network including the frontal, premotor and parietal regions. Like other visual-spatial transformations it could require operations on both metric and categorical spatial representations. Previous reports have implicated respectively the right hemisphere being involved in the metric processing and the left hemisphere in the categorical processing. By using a modified version of the Bricolo et al.'s task (2000), we attempted to establish the cortical regions relevant for the categorical and metric aspects of mental rotation transformations. Two groups of patients were found to be impaired in our study, namely the left prefrontal and the right parietal. In particular, whereas the right parietal group made poor use of categorical information, the left prefrontal patients showed a broader mental rotation impairment with a significant number of metric errors. The results are discussed in terms of the model of Kosslyn et al. (1989) about the possible mental transformation impairments following brain lesions.

Electrophysiological evidence for an early processing of human voices

BACKGROUND

Previous electrophysiological studies have identified a "voice specific response" (VSR) peaking around 320 ms after stimulus onset, a latency markedly longer than the 70 ms needed to discriminate living from non-living sound sources and the 150 ms to 200 ms needed for the processing of voice paralinguistic qualities. In the present study, we investigated whether an early electrophysiological difference between voice and non-voice stimuli could be observed.

RESULTS

ERPs were recorded from 32 healthy volunteers who listened to 200 ms long stimuli from three sound categories - voices, bird songs and environmental sounds - whilst performing a pure-tone detection task. ERP analyses revealed voice/non-voice amplitude differences emerging as early as 164 ms post stimulus onset and peaking around 200 ms on fronto-temporal (positivity) and occipital (negativity) electrodes.

CONCLUSION

Our electrophysiological results suggest a rapid brain discrimination of sounds of voice, termed the "fronto-temporal positivity to voices" (FTPV), at latencies comparable to the well-known face-preferential N170.

Neural Correlates of Novel Object and Novel Location Recognition Behavior in the Mouse Anterior Cingulate Cortex

The anterior cingulate cortex (ACC) is a component of the limbic system implicated in a wide variety of functions spanning motor and sensory information processing, memory, attention, novelty detection, and comparisons of expectation versus outcome. It remains unclear how much of this functional diversity stems from differences in methodology or interpretation versus truly reflecting the range of processes in which the ACC is involved. In the present study, ACC neuronal activity was examined in freely behaving mice (C57BL6/J) under conditions allowing investigation of many of the cited functions in conditions free from externally applied rules: tests of novel object and novel location recognition memory. Behavioral activity and neuronal activity were recorded first in the open field, during the initial exposure and subsequent familiarization to two identical objects, and finally during the recognition memory tests. No discernible stable firing correlates of ACC neurons were found in the open field, but the addition of objects led to lasting changes in the firing patterns of many ACC neurons around one or both of the object locations. During the novel location test, some neurons followed the familiar object to its new location, others fired exclusively where the object had been, and yet others fired to both current and former object locations. Many of these same features were observed during tests of object recognition memory. However, the magnitude of the neuronal preference for the novel or the familiar object was markedly greater than that observed during either the tests of location recognition or novel object preferences in animals that did not exhibit the expected behavior. The present study reveals, for the first time, single-neuron correlates of object and location recognition behaviors in the rodent ACC and suggests that neurons of the ACC provide a distributed representation of all of the salient features of a task.

Aneuploidy: from a physiological mechanism of variance to Down syndrome

Quantitative differences in gene expression emerge as a significant source of variation in natural populations, representing an important substrate for evolution and accounting for a considerable fraction of phenotypic diversity. However, perturbation of gene expression is also the main factor in determining the molecular pathogenesis of numerous aneuploid disorders. In this review, we focus on Down syndrome (DS) as the prototype of "genomic disorder" induced by copy number change. The understanding of the pathogenicity of the extra genomic material in trisomy 21 has accelerated in the last years due to the recent advances in genome sequencing, comparative genome analysis, functional genome exploration, and the use of model organisms. We present recent data on the role of genome-altering processes in the generation of diversity in DS neural phenotypes focusing on the impact of trisomy on brain structure and mental retardation and on biological pathways and cell types in target brain regions (including prefrontal cortex, hippocampus, cerebellum, and basal ganglia). We also review the potential that genetically engineered mouse models of DS bring into the understanding of the molecular biology of human learning disorders.

TMS over human frontal eye fields disrupts trans-saccadic memory of multiple objects

We recently showed that transcranial magnetic stimulation (TMS) over the right parietal eye fields disrupts memory of object features and locations across saccades. We applied TMS over the frontal eye fields (FEF) as subjects compared the feature details of visual targets presented either within a single eye fixation (Fixation Task) or across a saccade (Saccade Task). TMS pulses were randomly delivered at one of 3 time intervals around the time of the saccade, or at equivalent times in the Fixation Task. A No-TMS control confirmed that subjects could normally retain approximately 3 visual features. TMS in the Fixation Task had no effect compared with No-TMS, but differences among TMS times were found during right FEF stimulation. TMS over either the right or left FEF disrupted memory performance in the Saccade Task when stimulation coincided most closely with the saccade. The capacity to compare pre-and postsaccadic features was reduced to 1-2 objects, as expected if the spatial aspect of memory was disrupted. These findings suggest that the FEF plays a role in the spatial processing involved in trans-saccadic memory of visual features. We propose that this process employs saccade-related feedback signals similar to those observed in spatial updating.

Do Children with Developmental Dyslexia Show a Selective Visual Attention Deficit?

The authors investigated the performance of children with developmental dyslexia on a number of visual tasks requiring selective visual attention. Dyslexic children did not show the overestimation of the left visual field (pseudo-neglect) characteristic of normal adult vision. The performance of dyslexic children in texture segmentation and feature search tasks was identical to that of control children matched for age, gender and intelligence. However, when tested on conjunction tasks for orientation and form, dyslexic children showed shorter reaction times and a dramatically increased number of errors. Differences between the two groups decreased with increasing age. These results suggest that children with developmental dyslexia present selective deficits in visual attention.

Attentional demands predict short-term memory load response in posterior parietal cortex

Limits to the capacity of visual short-term memory (VSTM) indicate a maximum storage of only 3 or 4 items. Recently, it has been suggested that activity in a specific part of the brain, the posterior parietal cortex (PPC), is correlated with behavioral estimates of VSTM capacity and might reflect a capacity-limited store. In three experiments that varied the delay period and the stimuli to be stored, we found dissociations between functional magnetic resonance imaging (fMRI) activity in PPC and behavioral measures of capacity. When the delay length increased, fMRI activity in this area increased with memory load beyond the behaviorally determined limits of capacity. The results suggest that activity in PPC may reflect the attentional demands of short-term memory rehearsal processes rather than capacity limitations, and imply that a larger number of items than that determined by behavioral measures of capacity may be rehearsed during STM tasks. This account is consistent with the role of PPC in attentional processes and with the close correlation between brain areas that are involved in attention and those that mediate STM.

Evidence of a modality-dependent role of the cerebellum in working memory? An fMRI study comparing verbal and abstract n-back tasks

In working memory (WM), functional imaging studies demonstrate cerebellar involvement indicating a cognitive role of the cerebellum. These cognitive contributions were predominantly interpreted as part of the phonological loop within the Baddeley model of WM. However, those underlying investigations were performed in the context of visual verbal WM which could pose a bias when interpreting the results. The aim of this fMRI study was to address the question of whether the cerebellum supports additional aspects of WM in the context of higher cognitive functions. Furthermore, laterality effects were investigated to further disentangle the cerebellar role in the context of the phonological loop and the visuospatial sketchpad. A direct comparison of verbal and abstract visual WM was performed in 17 young volunteers by applying a 2-back paradigm and extracting the % change in BOLD signal from the fMRI data. To minimize potential verbal strategies, Attneave and Arnoult shapes of non-nameable objects were chosen for the abstract condition. The analyses revealed no significant differences in verbal vs. abstract WM. Moreover, no laterality effects were demonstrated in both verbal and abstract WM. These results provide further evidence of a broader cognitive involvement of the cerebellum in WM that is not only confined to the phonological loop but also supports central executive subfunctions. The fact that no lateralization effects are found might be attributed to the characteristics of the n-back paradigm which emphasizes central executive subfunctions over the subsidiary slave systems.

To the influence of general slowing and medication on identity- and location-based priming effects in patients with Parkinson's disease

Previous studies on inhibitory mechanisms assessed by negative priming (NP) paradigms in patients suffering from Parkinson's disease (PD) have yielded highly ambiguous results. The present study examined two possible reasons for this heterogeneity: general slowing and anti-Parkinsonian medication. Their effects on identity and location NP and positive priming (PP) were investigated. Twenty medicated PD patients and 20 PD patients after drug withdrawal were compared to 20 sex- and age-matched healthy controls. The influence of PD patients' general slowing on priming effects was statistically controlled. Location NP was found not to be affected by PD, whereas identity NP was reduced in medicated PD patients compared to non-medicated PD patients and healthy controls. At first, identity and location PP appeared to be enhanced in both PD groups. After controlling for general slowing, however, differences between PD patients and healthy controls disappeared. These findings endorse the notion that uncontrolled effects of both, PD-related general slowing and anti-Parkinsonian medication may have contributed to previously conflicting results on priming effects in PD patients.

Working memory impairment in people with Williams syndrome: effects of delay, task and stimuli

Williams syndrome (WS) is a neurodevelopmental disorder associated with impaired visuospatial representations subserved by the dorsal stream and relatively strong object recognition abilities subserved by the ventral stream. There is conflicting evidence on whether this uneven pattern in WS extends to working memory (WM). The present studies provide a new perspective, testing WM for a single stimulus using a delayed recognition paradigm in individuals with WS and typically developing children matched for mental age (MA matches). In three experiments, participants judged whether a second stimulus 'matched' an initial sample, either in location or identity. We first examined memory for faces, houses and locations using a 5s delay (Experiment 1) and a 2s delay (Experiment 2). We then tested memory for human faces, houses, cat faces, and shoes with a 2s delay using a new set of stimuli that were better controlled for expression, hairline and orientation (Experiment 3). With the 5s delay (Experiment 1), the WS group was impaired overall compared to MA matches. While participants with WS tended to perform more poorly than MA matches with the 2s delay, they also exhibited an uneven profile compared to MA matches. Face recognition was relatively preserved in WS with friendly faces (Experiment 2) but not when the faces had a neutral expression and were less natural looking (Experiment 3). Experiment 3 indicated that memory for object identity was relatively stronger than memory for location in WS. These findings reveal an overall WM impairment in WS that can be overcome under some conditions. Abnormalities in the parietal lobe/dorsal stream in WS may damage not only the representation of spatial location but may also impact WM for visual stimuli more generally.

The Influence of Similarity on Visual Working Memory Representations

In verbal memory, similarity between items in memory often leads to interference and impaired memory performance. The present study sought to determine whether analogous interference effects would be observed in visual working memory by varying the similarity of the to-be-remembered objects in a color change-detection task. Instead of leading to interference and impaired performance, increased similarity among the items being held in memory led to improved performance. Moreover, when two similar colors were presented along with one dissimilar color, memory performance was better for the similar colors than for the dissimilar color. Similarity produced better performance even when the objects were presented sequentially and even when memory for the first item in the sequence was tested. These findings show that similarity does not lead to interference between representations in visual working memory. Instead, similarity may lead to improved task performance, possibly due to increased stability or precision of the memory representations during maintenance.

Functional magnetic resonance imaging outcomes from a comprehensive magnetic resonance study of children with fetal alcohol spectrum disorders

A comprehensive neuropsychological/psychiatric, MR imaging, (MRI), MR spectroscopy (MRS), and functional MRI (fMRI) assessment was administered to children with fetal alcohol spectrum disorders (FASD) to determine if global and/or focal abnormalities could be identified, and distinguish diagnostic subclassifications across the spectrum. The four study groups included: 1. FAS/Partial FAS; 2. Static Encephalopathy/Alcohol Exposed (SE/AE); 3. Neurobehavioral Disorder/Alcohol Exposed (ND/AE); and 4. healthy peers with no prenatal alcohol exposure. fMRI outcomes are reported here. The neuropsychological/psychiatric, MRI, and MRS outcomes are reported separately. fMRI was used to assess activation in seven brain regions during performance of N-back working memory tasks. Children across the full spectrum of FASD exhibited significant working memory deficits and altered activation patterns in brain regions that are known to be involved in working memory. These results demonstrate the potential research and diagnostic value of this non-invasive MR tool in the field of FASD.

Developmental changes in category-specific brain responses to numbers and letters in a working memory task

Neuroimaging studies have identified a common network of brain regions involving the prefrontal and parietal cortices across a variety of working memory (WM) tasks. However, previous studies have also reported category-specific dissociations of activation within this network. In this study, we investigated the development of category-specific activation in a WM task with digits, letters, and faces. Eight-year-old children and adults performed a 2-back WM task while their brain activity was measured using functional magnetic resonance imaging (fMRI). Overall, children were significantly slower and less accurate than adults on all three WM conditions (digits, letters, and faces); however, within each age group, behavioral performance across the three conditions was very similar. FMRI results revealed category-specific activation in adults but not children in the intraparietal sulcus for the digit condition. Likewise, during the letter condition, category-specific activation was observed in adults but not children in the left occipital-temporal cortex. In contrast, children and adults showed highly similar brain-activity patterns in the lateral fusiform gyri when solving the 2-back WM task with face stimuli. Our results suggest that 8-year-old children do not yet engage the typical brain regions that have been associated with abstract or semantic processing of numerical symbols and letters when these processes are task-irrelevant and the primary task is demanding. Nevertheless, brain activity in letter-responsive areas predicted children's spelling performance underscoring the relationship between abstract processing of letters and linguistic abilities. Lastly, behavioral performance on the WM task was predictive of math and language abilities highlighting the connection between WM and other cognitive abilities in development.

Encoding of gustatory working memory by orbitofrontal neurons

The content model regarding the functional organization of working memory in prefrontal cortex (PFC) states that different PFC areas encode different types of information in working memory depending on their afferent connections with other brain areas. Previous studies that tested this model focused on visual, auditory and somatosensory information. However, posterior areas processing this information project to widespread and overlapping regions of lateral PFC, making it difficult to establish the veracity of the model. In contrast, gustatory information enters PFC via orbitofrontal cortex (OFC), and so the content model would argue that OFC should be responsible for maintaining gustatory information in working memory. To test this, we recorded the activity of single neurons throughout PFC and gustatory cortex (GUS) from two subjects while they performed a gustatory delayed-match-to-sample task with intervening gustatory distraction. Neurons that encoded the identity of the gustatory stimulus across the delay, consistent with a role in gustatory working memory, were most prevalent in OFC and GUS compared with dorsolateral PFC and ventrolateral PFC. Gustatory information in OFC was more resilient to intervening distraction, paralleling previous findings regarding visual working memory processes in PFC and posterior sensory cortex. Our findings provide support for the content model of working memory organization. Maintaining gustatory information may be one aspect of a wider function for OFC in reward working memory that could contribute to its role in decision-making.

Age effects on load-dependent brain activations in working memory for novel material

Three competing models of cognitive aging (neural compensation, capacity limitations, neural inefficiency) were examined in relation to working memory for novel non-verbal material. To accomplish this goal young (n=25) and old (n=25) participants performed a delayed item recognition (DIR) task while being scanned with bold fMRI. The stimuli in the DIR task consisted of computer-generated closed-curve shapes with each shape presented only once in the testing conditions of each participant. This ensured that both the novelty and appearance of the shapes maximized visual demands and limited the extent of phonologic processing. Behaviorally, as expected, the old participants were slower and less accurate compared to the young participants. Spatial patterns of brain activation that corresponded to load-dependent (stimulus set size ranged from 1 to 3) fMRI signal during the three phases of the DIR task (memory set presentation, retention delay, probe presentation) were evaluated in both age groups. Support for neural compensation and capacity limitation was evident in retention delay and the probe phase, respectively. Data were inconsistent with the neural inefficiency model. The process specific support for the theories we examined is consistent with a large corpus of research showing that the substrates underlying the encoding, retention and probe phases are different. That is, cognitive aging theories can be specific to the neural networks/regions underlying the different phases of working memory. Delineating how these theories work in concert can increase knowledge of age-related effects on working memory.

Using brain-based measures to compose teams: how individual capabilities and team collaboration strategies jointly shape performance

Advances in understanding neural processes open the possibility of using brain-based measures to compose collaborative work teams. Neuroimaging studies have shown that individual differences in patterns of brain activity can predict differences in performance of specific tasks. We extended this finding by examining performance not simply by a single brain, but by pairs of brains. We used measures derived from brain-based studies to compose 100 two-person teams in which members' roles were either congruent or incongruent with their individual abilities. The assessed abilities are rooted in the visual system, which comprises independent "spatial" and "object" subsystems. The team task required one member to navigate through a virtual maze (a spatial task) and the other to remember "tag" repetitions of complex "greebles" (an object-properties task). Teams in which members' role assignments were congruent with their abilities performed better than incongruent teams and teams in which both members scored high on only one of the abilities. In addition, verbal collaboration enabled members of incongruent teams to overcome their compositional disadvantage but did not enhance the performance of congruent teams-and actually impaired performance in teams in which both members were adept in only one of the two necessary abilities. The findings show that knowledge about brain systems can not only be used to compose teams, but also provides insights into how teams can best perform.

An fMRI study of phonological and spatial working memory using identical stimuli

The aim of the present fMRI study was to localize brain areas that were uniquely activated for phonological versus spatial working memory. Previous studies have reported inconsistent results, most likely because of methodological heterogeneity varying both stimuli and instructions in the same study. Here, identical consonant-vowel-consonant non-words were visually presented to the subjects in a 2-back paradigm under two different instructions; the subjects either had to memorize the non-words per se or their location. The results give evidence for a hemispheric organization of working memory, with dominance for processing of phonological information in the left hemisphere and frontal cortex, and spatial information in the right hemisphere and parietal cortex. The results also reflect a certain overlap between the neuronal network for working memory and processing of verbal and spatial material. These findings are discussed with regard to processing specificity and the extent that activated areas also may reflect perceptual processes.

Rhinal and dorsolateral prefrontal cortex lesions produce selective impairments in object and spatial learning and memory in canines

To examine the effects of rhinal and dorsolateral prefrontal cortex lesions on object and spatial recognition memory in canines, we used a protocol in which both an object (delayed nonmatching to sample, or DNMS) and a spatial (delayed nonmatching to position or DNMP) recognition task were administered daily. The tasks used similar procedures such that only the type of stimulus information to be remembered differed. Rhinal cortex (RC) lesions produced a selective deficit on the DNMS task, both in retention of the task rules at short delays and in object recognition memory. By contrast, performance on the DNMP task remained intact at both short and long delay intervals in RC animals. Subjects who received dorsolateral prefrontal cortex (dlPFC) lesions were impaired on a spatial task at a short, 5-second delay, suggesting disrupted retention of the general task rules; however, this impairment was transient, and long-term spatial memory performance was unaffected in dlPFC subjects. The present results provide support for the involvement of the RC in object, but not visuospatial, processing and recognition memory, whereas the dlPFC appears to mediate retention of a nonmatching rule. These findings support theories of functional specialization within the medial temporal lobe and frontal cortex and suggest that rhinal and dorsolateral prefrontal cortices in canines are functionally similar to analogous regions in other mammals.

Dynamic premotor-to-parietal interactions during spatial imagery

The neurobiological processes underlying mental imagery are a matter of debate and controversy among neuroscientists, cognitive psychologists, philosophers, and biologists. Recent neuroimaging studies demonstrated that the execution of mental imagery activates large frontoparietal and occipitotemporal networks in the human brain. These previous imaging studies, however, neglected the crucial interplay within and across the widely distributed cortical networks of activated brain regions. Here, we combined time-resolved event-related functional magnetic resonance imaging with analyses of interactions between brain regions (functional and effective brain connectivity) to unravel the premotor-parietal dynamics underlying spatial imagery. Participants had to sequentially construct and spatially transform a mental visual object based on either verbal or visual instructions. By concurrently accounting for the full spatiotemporal pattern of brain activity and network connectivity, we functionally segregated an early from a late premotor-parietal imagery network. Moreover, we revealed that the modality-specific information upcoming from sensory brain regions is first sent to the premotor cortex and then to the medial-dorsal parietal cortex, i.e., top-down from the motor to the perceptual pole during spatial imagery. Importantly, we demonstrate that the premotor cortex serves as the central relay station, projecting to parietal cortex at two functionally distinct stages during spatial imagery. Our approach enabled us to disentangle the multicomponential cognitive construct of mental imagery into its different cognitive subelements. We discuss and explicitly assign these mental subprocesses to each of the revealed effective brain connectivity networks and present an integrative neurobiological model of spatial imagery.

A tale of two recognition systems: implications of the fusiform face area and the visual word form area for lateralized object recognition models

Two areas of current intense interest in the neuroimaging literature are that of the visual word form area (VWFA) and of the fusiform face area (FFA) and their roles in word and face perception, respectively. These two areas are of particular relevance to laterality research because visual word identification and face identification have long been shown to be especially lateralized to the left hemisphere and the right hemisphere, respectively. This review therefore seeks to evaluate their significance for the broader understanding of lateralization of object recognition. A multi-level model of lateralized object recognition is proposed based on a combination of behavioral and neuroimaging findings. Rather than seek to characterize hemispheric asymmetries according to a single principle (e.g., serial-parallel), it is suggested that current observations can be understood in terms of three asymmetric levels of processing, using the framework of the Janus model of hemispheric function. It is suggested that the left hemisphere represents features using an abstract-category code whereas the RH utilizes a specific-exemplar code. The relationships between these features are also coded asymmetrically, with the LH relying on associative co-occurrence values and the RH relying on spatial metrics. Finally, the LH controlled selection system focuses on isolating features and the RH focuses on conjoining features. It is suggested that each hemisphere utilizes efficient (apparently parallel) processing when stimuli are congruent with its preferred processing style and inefficient (apparently serial) processing when they are not, resulting in the typical left-lateralization for orthographic analysis and right-lateralization for face analysis.

Dissociation between appearance and location within visuo-spatial working memory

Previous research has demonstrated separation between systems supporting memory for appearance and memory for location. However, the interpretation of these results is complicated by a confound occurring because of the simultaneous presentation of objects in multiple-item arrays when assessing memory for appearance and the sequential presentation of items when assessing memory for location. This paper reports an experiment in which sequential or simultaneous modes of presentation were factorially manipulated with memory for visual appearance or memory for location. Spatial interference (tapping) or visual interference (dynamic visual noise) were presented during retention. Appearance versus location interacted with the type of interference task, but mode of presentation did not. These results are consistent with the view that different subsystems within visuo-spatial working memory support memory for appearance and memory for location.

Common coding of auditory and visual spatial information in working memory

We compared spatial short-term memory for visual and auditory stimuli in an event-related slow potentials study. Subjects encoded object locations of either four or six sequentially presented auditory or visual stimuli and maintained them during a retention period of 6 s. Slow potentials recorded during encoding were modulated by the modality of the stimuli. Stimulus related activity was stronger for auditory items at frontal and for visual items at posterior sites. At frontal electrodes, negative potentials incrementally increased with the sequential presentation of visual items, whereas a strong transient component occurred during encoding of each auditory item without the cumulative increment. During maintenance, frontal slow potentials were affected by modality and memory load according to task difficulty. In contrast, at posterior recording sites, slow potential activity was only modulated by memory load independent of modality. We interpret the frontal effects as correlates of different encoding strategies and the posterior effects as a correlate of common coding of visual and auditory object locations.

A neural basis for percept stabilization in binocular rivalry

When the same visual input has conflicting interpretations, conscious perception can alternate spontaneously between each competing percept. Surprisingly, such bistable perception can be stabilized by intermittent stimulus removal, suggesting the existence of perceptual "memory" across interruptions in stimulation. The neural basis of such a process remains unknown. Here, we studied binocular rivalry, one type of bistable perception, in two linked experiments in human participants. First, we showed, in a behavioral experiment using binocular rivalry between face and grating stimuli, that the stabilizing effect of stimulus removal was specific to perceptual alternations evoked by rivalry, and did not occur following physical alternations in the absence of rivalry. We then used functional magnetic resonance imaging to measure brain activity in a variable delay period of stimulus removal. Activity in the fusiform face area during the delay period following removal of rivalrous stimuli was greater following face than grating perception, whereas such a difference was absent during removal of non-rivalrous stimuli. Moreover, activity in areas of fronto-parietal regions during the delay period correlated with the degree to which individual participants tended to experience percept stabilization. Our findings suggest that percept-related activity in specialized extrastriate visual areas help to stabilize perception during perceptual conflict, and that high-level mechanisms may determine the influence of such signals on conscious perception.

Common and unique components of inhibition and working memory: an fMRI, within-subjects investigation

Behavioural findings indicate that the core executive functions of inhibition and working memory are closely linked, and neuroimaging studies indicate overlap between their neural correlates. There has not, however, been a comprehensive study, including several inhibition tasks and several working memory tasks, performed by the same subjects. In the present study, 11 healthy adult subjects completed separate blocks of 3 inhibition tasks (a stop task, a go/no-go task and a flanker task), and 2 working memory tasks (one spatial and one verbal). Activation common to all 5 tasks was identified in the right inferior frontal gyrus, and, at a lower threshold, also the right middle frontal gyrus and right parietal regions (BA 40 and BA 7). Left inferior frontal regions of interest (ROIs) showed a significant conjunction between all tasks except the flanker task. The present study could not pinpoint the specific function of each common region, but the parietal region identified here has previously been consistently related to working memory storage and the right inferior frontal gyrus has been associated with inhibition in both lesion and imaging studies. These results support the notion that inhibitory and working memory tasks involve common neural components, which may provide a neural basis for the interrelationship between the two systems.

Double dissociation of 'what' and 'where' processing in auditory cortex

Studies of cortical connections or neuronal function in different cerebral areas support the hypothesis that parallel cortical processing streams, similar to those identified in visual cortex, may exist in the auditory system. However, this model has not yet been behaviorally tested. We used reversible cooling deactivation to investigate whether the individual regions in cat nonprimary auditory cortex that are responsible for processing the pattern of an acoustic stimulus or localizing a sound in space could be doubly dissociated in the same animal. We found that bilateral deactivation of the posterior auditory field resulted in deficits in a sound-localization task, whereas bilateral deactivation of the anterior auditory field resulted in deficits in a pattern-discrimination task, but not vice versa. These findings support a model of cortical organization that proposes that identifying an acoustic stimulus ('what') and its spatial location ('where') are processed in separate streams in auditory cortex.

Working memory for social cues recruits orbitofrontal cortex and amygdala: a functional magnetic resonance imaging study of delayed matching to sample for emotional expressions

During everyday interactions, we continuously monitor and maintain information about different individuals and their changing emotions in memory. Yet to date, working memory (WM) studies have primarily focused on mechanisms for maintaining face identity, but not emotional expression, and studies investigating the neural basis of emotion have focused on transient activity, not delay related activity. The goal of this functional magnetic resonance imaging study was to investigate WM for two critical social cues: identity and emotion. Subjects performed a delayed match-to-sample task that required them to match either the emotional expression or the identity of a face after a 10 s delay. Neuroanatomically, our predictions focused on the orbitofrontal cortex (OFC) and the amygdala, as these regions have previously been implicated in emotional processing and long-term memory, and studies have demonstrated sustained OFC and medial temporal lobe activity during visual WM. Consistent with previous studies, transient activity during the sample period representing emotion and identity was found in the superior temporal sulcus and inferior occipital cortex, respectively. Sustained delay-period activity was evident in OFC, amygdala, and hippocampus, for both emotion and identity trials. These results suggest that, although initial processing of emotion and identity is accomplished in anatomically segregated temporal and occipital regions, sustained delay related memory for these two critical features is held by the OFC, amygdala and hippocampus. These regions share rich connections, and have been shown previously to be necessary for binding features together in long-term memory. Our results suggest a role for these regions in active maintenance as well.

I. Longitudinal changes in aging brain function

Changes in brain activity over time were evaluated in a group of older adults in the Baltimore Longitudinal Study of Aging who maintained good physical and cognitive health. Participants underwent PET scans during rest and delayed verbal and figural recognition memory performance at year 1 baseline and at year 9. While memory performance remained stable over the 8 years, longitudinal changes in regional cerebral blood flow were observed within each scan condition. Further analyses revealed distinctive patterns of change related specifically to verbal or figural recognition, as well as longitudinal changes common to all scan conditions. These findings demonstrate that the older brain undergoes functional reorganization with increasing age in healthy, cognitively stable individuals. In view of the stable memory performance, the task-dependent results suggest that age-related changes in brain activity help maintain cognitive function with advancing age.

Origins of spatial working memory deficits in schizophrenia: an event-related FMRI and near-infrared spectroscopy study

Abnormal prefrontal functioning plays a central role in the working memory (WM) deficits of schizophrenic patients, but the nature of the relationship between WM and prefrontal activation remains undetermined. Using two functional neuroimaging methods, we investigated the neural correlates of remembering and forgetting in schizophrenic and healthy participants. We focused on the brain activation during WM maintenance phase with event-related functional magnetic resonance imaging (fMRI). We also examined oxygenated hemoglobin changes in relation to memory performance with the near-infrared spectroscopy (NIRS) using the same spatial WM task. Distinct types of correct and error trials were segregated for analysis. fMRI data indicated that prefrontal activation was increased during WM maintenance on correct trials in both schizophrenic and healthy subjects. However, a significant difference was observed in the functional asymmetry of frontal activation pattern. Healthy subjects showed increased activation in the right frontal, temporal and cingulate regions. Schizophrenic patients showed greater activation compared with control subjects in left frontal, temporal and parietal regions as well as in right frontal regions. We also observed increased ‘false memory’ errors in schizophrenic patients, associated with increased prefrontal activation and resembling the activation pattern observed on the correct trials. NIRS data replicated the fMRI results. Thus, increased frontal activity was correlated with the accuracy of WM in both healthy control and schizophrenic participants. The major difference between the two groups concerned functional asymmetry; healthy subjects recruited right frontal regions during spatial WM maintenance whereas schizophrenic subjects recruited a wider network in both hemispheres to achieve the same level of memory performance. Increased “false memory” errors and accompanying bilateral prefrontal activation in schizophrenia suggest that the etiology of memory errors must be considered when comparing group performances. Finally, the concordance of fMRI and NIRS data supports NIRS as an alternative functional neuroimaging method for psychiatric research.

Functional developmental similarities and differences in the neural correlates of verbal and nonverbal working memory tasks

Relatively little is known about the functional development of verbal and nonverbal working memory during adolescence. Behavioral studies have demonstrated that WM capacity increases with age, yet relatively few studies have assessed the relationship between brain-activity and age-related changes in WM capacity, especially as it differs across multiple domains. The present study used an n-back task and functional magnetic resonance imaging to assess age-related differences in the neural correlates of word and face working memory tasks. Seventy-eight individuals between the ages of 14 and 27 underwent scans while performing word and face "n-back" working memory tasks. We found very little evidence for age-related differences in accuracy and reaction time. We did find similarities and differences between adolescents and adults in the neural correlates of word and face working memory tasks, even in the absence of performance differences. More specifically, we found similar age-related differences in left superior parietal cortex for both word and face stimuli. We also found that age-related differences in a number of other regions (including left inferior frontal lobe, left supramarginal gyrus, left rolandic sulcus, right cerebellum and left fusiform gyrus) differed according to stimulus type. Our results provide further evidence for continued functional development through adolescence and into adulthood.