Active representation of shape and spatial location in man

The neural correlates of value representation: From single items to bundles

One of the core questions in Neuro-economics is to determine where value is represented. To date, most studies have focused on simple options and identified the ventromedial prefrontal cortex (VMPFC) as the common value region. We report the findings of an fMRI study in which we asked participants to make pairwise comparisons involving options of varying complexity: single items (Control condition), bundles made of the same two single items (Scaling condition) and bundles made of two different single items (Bundling condition). We construct a measure of choice consistency to capture how coherent the choices of a participant are with one another. We also record brain activity while participants make these choices. We find that a common core of regions involving the left VMPFC, the left dorsolateral prefrontal cortex (DLPFC), regions associated with complex visual processing and the left cerebellum track value across all conditions. Also, regions in the DLPFC, the ventrolateral prefrontal cortex (VLPFC) and the cerebellum are differentially recruited across conditions. Last, variations in activity in VMPFC and DLPFC value-tracking regions are associated with variations in choice consistency. This suggests that value based decision-making recruits a core set of regions as well as specific regions based on task demands. Further, correlations between consistency and the magnitude of signal change with lateral portions of the PFC suggest the possibility that activity in these regions may play a causal role in decision quality.

The Quest for Hemispheric Asymmetries Supporting and Predicting Executive Functioning

This narrative review addresses the neural bases of two executive functions: criterion setting, that is, the capacity to flexibly set up and select task rules and associations between stimuli, responses, and nonresponses, and monitoring, that is, the process of continuously evaluating whether task rules are being applied optimally. There is a documented tendency for criterion setting and monitoring to differentially recruit left and right lateral prefrontal regions and connected networks, respectively, above and beyond the specific task context. This model, known as the ROtman-Baycrest Battery to Investigate Attention (ROBBIA) model, initially sprung from extensive neuropsychological work led by Don Stuss. In subsequent years, multimodal lines of empirical investigation on both healthy individuals and patients with brain damage, coming from functional neuroimaging, EEG, neurostimulation, individual difference approaches, and, again, neuropsychology, so to "complete the circle," corroborated the functional mapping across the two hemispheres as predicted by the model. More recent electrophysiological evidence has further shown that hemispheric differences in intrinsic prefrontal dynamics are able to predict cognitive performance in tasks tapping these domain-general functions. These empirical contributions will be presented together with contrasting evidence, limits, and possible future directions to better fine-tune this model and extend its scope to new fields.

The Different Brain Mechanisms of Object and Spatial Working Memory: Voxel-Based Morphometry and Resting-State Functional Connectivity

In working memory (WM), the ability to concurrently integrate different types of information and to maintain or manipulate them promotes the flow of ongoing tasks. WM is a key component of normal human cognition. In this study, we applied a combined voxel-based morphometry (VBM) and resting-state functional connectivity (rsFC) analysis to investigate the relationship between the ability of object and spatial working memory (WM), and regional gray matter density (GMD), as well as intrinsic functional connectivity. The VBM analysis showed a positive correlation between the individual difference of object WM and GMD in the right middle occipital gyrus (MOG) and the left superior temporal gyrus (STG), which are responsible for coding object information and processing the shape of an object. The individual difference of the spatial WM was positively related to GMD in the right middle frontal gyrus (MFG) located in the dorsolateral prefrontal cortex (dlPFC), which confirmed that it is an important region for memory stores and maintains WM spatial representations. Further functional connectivity analysis revealed that the individual difference of object WM was significantly correlated with the rsFC of right intraparietal sulcus (IPS) - left postcentral gyrus (PostCG)/right precentral gyrus (PreCG)/left Supplementary Motor Area (SMA). While the capacity of spatial WM was significantly associated with the FC strength of the left dlPFC - left precuneus, right dlPFC - right MFG, and the left superior frontal sulcus (SFS) - left SMA/ right inferior parietal lobe (IPL). Our findings suggest that object WM is associated with the structure and functional organization of the brain regions involved in the ventral pathway (occipital - temporal regions) and the capacity of spatial WM is related to the dorsal pathway (frontal - parietal regions).

Functional Connectivity of Cognitive Control and Visual Regions During Verb Generation Is Related to Improved Reading in Children

Reading is a complex cognitive ability, which relies on visual and language processing as well as on executive functions (EFs). Recent studies have demonstrated that increased reading ability in children aged 7-17 years is related to greater activation of cognitive control regions during verb generation, a task which merges linguistic and cognitive control ability. The aim of the current study is to determine the relationships between neural circuits specifically related to EF and reading ability. We focused on functional connectivity between the dorsolateral prefrontal cortex (DLPFC), a region involved in EF and is part of the frontoparietal network during a verb generation task, and reading ability in seventeen 8-12-year-old typical readers. Results show positive functional connectivity between the left and right DLPFCs and regions related to cognitive control and visual processing while generating verbs. Increased reading ability was positively correlated with greater functional connectivity between the left and right DLPFCs and right-lateralized visual processing regions. The current study highlights the importance of neural circuits related to EF during both verb generation and reading and points to the role of the right occipital cortex in generating verbs as well as automatic word recognition in typical readers.

The Effects of Smoking Abstinence on Incentivized Spatial Working Memory

BACKGROUND AND OBJECTIVE

Reward processing and working memory (WM) underlie value-based decision-making; consequently, joint examination of these systems may further our understanding of why smokers choose to smoke again following a quit attempt (relapse). While previous studies have demonstrated altered reward and WM function associated with nicotine exposure, little is known about the effects of abstinence on the joint function of these systems. The current study aims to address this gap.

METHOD

Eighteen daily smokers were tested on a monetarily incentivized memory guided saccade (MGS) task on two separate, counterbalanced occasions, an abstinent and a non-abstinent session. The MGS task is a widely used metric of spatial working memory and enables precise quantification of the effects of rewards and nicotine exposure on behavior.

RESULTS

During the non-abstinent session, participants showed increased accuracy of the initial saccade towards the remembered target location on reward vs. neutral trials. Participants also showed increased accuracy of the final saccade towards the target, across incentive types, only during the non-abstinent condition.

DISCUSSION AND CONCLUSIONS

Our observation that rewards improve the accuracy of the initial memory guided saccade during the non-abstinent but not abstinent condition extends a growing literature indicating reduced motivation towards monetary rewards during abstinence. Further, differences in the accuracy of the final corrective saccade during the non-abstinent but not the abstinent condition suggests smoking abstinence-related effects on WM precision beyond those related to incentive motivation (e.g., sustained attention).

SIGNIFICANCE

This work extends our fundamental understanding of smoking's effects on core affective and cognitive processes.

Neurocognitive characteristics of youth with noncomorbid and comorbid forms of conduct disorder and attention deficit hyperactivity disorder

OBJECTIVE

Studies investigating neurocognitive deficits in youth with conduct disorder (CD) and attention deficit hyperactivity disorder (ADHD) are often confounded by the high rates of comorbidity between the two.

METHOD

Neurocognitive functioning was examined in three diagnostic groups (ADHD only, CD only, comorbid ADHD and CD) matched by age, sex, IQ, and medication status (n=28-32 per group).

RESULTS

No significant differences emerged between the diagnostic groups on measures of risk-taking or response inhibition. Children with CD performed better on a measure of spatial planning than those with comorbid ADHD and CD, and dimensional analyses in the full sample (n=265) revealed a small association between ADHD symptoms and poorer spatial planning.

CONCLUSION

These results suggest that deficits in spatial planning may be more pronounced in individuals with ADHD, but that the neurocognitive functioning of youth with noncomorbid and comorbid CD and ADHD are largely similar.

Nonverbal auditory working memory: Can music indicate the capacity?

Different working memory (WM) mechanisms that underlie words, tones, and timbres have been proposed in previous studies. In this regard, the present study developed a WM test with nonverbal sounds and compared it to the conventional verbal WM test. A total of twenty-five, non-music major, right-handed college students were presented with four different types of sounds (words, syllables, pitches, timbres) that varied from two to eight digits in length. Both accuracy and oxygenated hemoglobin (oxyHb) were measured. The results showed significant effects of number of targets on accuracy and sound type on oxyHb. A further analysis showed prefrontal asymmetry with pitch being processed by the right hemisphere (RH) and timbre by the left hemisphere (LH). These findings suggest a potential for employing musical sounds (i.e., pitch and timbre) as a complementary stimuli for conventional nonverbal WM tests, which can additionally examine its asymmetrical roles in the prefrontal regions.

Uncertain relational reasoning in the parietal cortex

The psychology of reasoning is currently transitioning from the study of deductive inferences under certainty to inferences that have degrees of uncertainty in both their premises and conclusions; however, only a few studies have explored the cortical basis of uncertain reasoning. Using transcranial magnetic stimulation (TMS), we show that areas in the right superior parietal lobe (rSPL) are necessary for solving spatial relational reasoning problems under conditions of uncertainty. Twenty-four participants had to decide whether a single presented order of objects agreed with a given set of indeterminate premises that could be interpreted in more than one way. During the presentation of the order, 10-Hz TMS was applied over the rSPL or a sham control site. Right SPL TMS during the inference phase disrupted performance in uncertain relational reasoning. Moreover, we found differences in the error rates between preferred mental models, alternative models, and inconsistent models. Our results suggest that different mechanisms are involved when people reason spatially and evaluate different kinds of uncertain conclusions.

Prefrontal neuronal responses during audiovisual mnemonic processing

During communication we combine auditory and visual information. Neurophysiological research in nonhuman primates has shown that single neurons in ventrolateral prefrontal cortex (VLPFC) exhibit multisensory responses to faces and vocalizations presented simultaneously. However, whether VLPFC is also involved in maintaining those communication stimuli in working memory or combining stored information across different modalities is unknown, although its human homolog, the inferior frontal gyrus, is known to be important in integrating verbal information from auditory and visual working memory. To address this question, we recorded from VLPFC while rhesus macaques (Macaca mulatta) performed an audiovisual working memory task. Unlike traditional match-to-sample/nonmatch-to-sample paradigms, which use unimodal memoranda, our nonmatch-to-sample task used dynamic movies consisting of both facial gestures and the accompanying vocalizations. For the nonmatch conditions, a change in the auditory component (vocalization), the visual component (face), or both components was detected. Our results show that VLPFC neurons are activated by stimulus and task factors: while some neurons simply responded to a particular face or a vocalization regardless of the task period, others exhibited activity patterns typically related to working memory such as sustained delay activity and match enhancement/suppression. In addition, we found neurons that detected the component change during the nonmatch period. Interestingly, some of these neurons were sensitive to the change of both components and therefore combined information from auditory and visual working memory. These results suggest that VLPFC is not only involved in the perceptual processing of faces and vocalizations but also in their mnemonic processing.

Neural sources of visual working memory maintenance in human parietal and ventral extrastriate visual cortex

Maintaining information in visual working memory is reliably indexed by the contralateral delay activity (CDA) - a sustained modulation of the event-related potential (ERP) with a topographical maximum over posterior scalp regions contralateral to the memorized input. Based on scalp topography, it is hypothesized that the CDA reflects neural activity in the parietal cortex, but the precise cortical origin of underlying electric activity was never determined. Here we combine ERP recordings with magnetoencephalography based source localization to characterize the cortical current sources generating the CDA. Observers performed a cued delayed match to sample task where either the color or the relative position of colored dots had to be maintained in memory. A detailed source-localization analysis of the magnetic activity in the retention interval revealed that the magnetic analog of the CDA (mCDA) is generated by current sources in the parietal cortex. Importantly, we find that the mCDA also receives contribution from current sources in the ventral extrastriate cortex that display a time-course similar to the parietal sources. On the basis of the magnetic responses, forward modeling of ERP data reveals that the ventral sources have non-optimal projections and that these sources are therefore concealed in the ERP by overlapping fields with parietal projections. The present observations indicate that visual working memory maintenance, as indexed by the CDA, involves the parietal cortical regions as well as the ventral extrastriate regions, which code the sensory representation of the memorized content.

BOLD response to working memory not related to cortical thickness during early adolescence

BACKGROUND

Significant cortical thinning and neural resource allocation changes emerge during adolescence; however, little is known of how morphometric changes influence neural response to cognitive demands. This study used a novel multimodal imaging registration technique to examine the relationship between brain structure and function during adolescence.

METHODS

156 healthy 12-14 year-olds (44% female) participants underwent structural and functional magnetic resonance imaging. Cortical surface reconstruction was performed via FreeSurfer, and neural activation was measured from a blood oxygen level dependent (BOLD) contrast during visual working memory (VWM) via AFNI. AFNI Surface Mapper aligned segmented volumetric and functional datasets to a common template space. Hierarchical linear regressions determined the effect of cortical thickness on VWM BOLD contrast in brain regions that activated during the VWM task, controlling for age, pubertal development, gender, IQ, and intracranial volume.

RESULTS

Power analyses suggest this study was able to detect small effect sizes. However, in no region was cortical thickness related to BOLD activation (ps>.01; R(2)Δ<.02). Gender did not moderate effects.

CONCLUSIONS

Cortical thickness, although variable across individuals, was not related to BOLD response, suggesting that structural and functional maturation do not have the same developmental trajectory during early adolescence. These findings are important, as imaging studies that report group differences in regards to cortical thickness should not necessarily assume co-occurring behavioral or functional changes. The methodology used in this study could be of interest to other developmental neuroimaging researchers using multimodal imaging to understand adolescent brain development.

Brain response to working memory over three years of adolescence: influence of initiating heavy drinking

OBJECTIVE

Many adolescents engage in heavy alcohol use. The aim of this study was to disentangle whether brain abnormalities seen in adolescent heavy drinkers are a consequence of heavy drinking, a preexisting risk factor for initiation of alcohol use, or both.

METHOD

Study 1 used cross-sectional functional magnetic resonance imaging (fMRI) visual working-memory (VWM) data from 15- to 19-year-olds (20 heavy drinkers, 20 controls) to identify brain regions affected by heavy adolescent alcohol use. Study 2 used longitudinal fMRI VWM data from 12- to 16-year-olds imaged before the onset of drinking and imaged again on the same scanner approximately 3 years later. Those who had transitioned into heavy drinking (n = 20) were matched to continuous nondrinkers (n = 20) on baseline alcohol risk and developmental factors (N = 40; 80 scans).

RESULTS

Study 1 found that heavy drinkers exhibited more frontal and parietal but less occipital activation than controls, defining the regions of interest for Study 2. In Study 2, adolescents who later transitioned into heavy drinking showed less fMRI response contrast at baseline than continuous nondrinkers, which increased after the onset of heavy drinking, in frontal (1,431 μL, p = .003; η² = .19) and parietal (810 μL, p = .005; η²= .23) regions, as in Study 1. Lower baseline activation in the frontal and parietal regions predicted subsequent substance use, more so than commonly observed predictors of youth drinking (p < .05).

CONCLUSIONS

Adolescents who initiated heavy drinking showed different brain activation before the onset of drinking, then less efficient information processing after high-dose alcohol use started. This suggests neural response patterns that could be risk factors for future substance use and also supports prior neuropsychological reports indicating that initiating heavy episodic drinking in adolescence may be followed by subtle alterations in brain functioning.

Interactive effects of three core goal pursuit processes on brain control systems: goal maintenance, performance monitoring, and response inhibition

Goal attainment relies in part on one’s ability to maintain a cognitive representation of the desired goal (goal maintenance), monitor the current state vis-à-vis the targeted end state and remain vigilant for lapses in progress (performance monitoring), and inhibit counter-goal behaviors (response inhibition). Because neurocognitive studies have typically examined these three processes in isolation from one another, little is known regarding if and how they interact during goal pursuit. However, these processes frequently co-occur during online, real-world goal pursuit. The present study employed a novel task to investigate how goal maintenance, performance monitoring, and response inhibition interact with one another. We identified functional activations distinct to each of the processes that correspond to results of prior investigations. In addition, we report interactive effects between response inhibition and goal maintenance in the dorsal anterior cingulate cortex and between performance monitoring and goal maintenance in the superior frontal gyrus and supramarginal gyrus. Implications for studying the neural systems of in situ goals include the need for both experimental designs that distinguish between process, but also more complex, realistic tasks to begin to map interactions among these neurocognitive processes and how they are altered by the presence or absence of one another.

Spatiotemporal dynamics of brain activity during the transition from visually guided to memory-guided force control

It is well established that the prefrontal cortex is involved during memory-guided tasks whereas visually guided tasks are controlled in part by a frontal-parietal network. However, the nature of the transition from visually guided to memory-guided force control is not as well established. As such, this study examines the spatiotemporal pattern of brain activity that occurs during the transition from visually guided to memory-guided force control. We measured 128-channel scalp electroencephalography (EEG) in healthy individuals while they performed a grip force task. After visual feedback was removed, the first significant change in event-related activity occurred in the left central region by 300 ms, followed by changes in prefrontal cortex by 400 ms. Low-resolution electromagnetic tomography (LORETA) was used to localize the strongest activity to the left ventral premotor cortex and ventral prefrontal cortex. A second experiment altered visual feedback gain but did not require memory. In contrast to memory-guided force control, altering visual feedback gain did not lead to early changes in the left central and midline prefrontal regions. Decreasing the spatial amplitude of visual feedback did lead to changes in the midline central region by 300 ms, followed by changes in occipital activity by 400 ms. The findings show that subjects rely on sensorimotor memory processes involving left ventral premotor cortex and ventral prefrontal cortex after the immediate transition from visually guided to memory-guided force control.

Space representation in the prefrontal cortex

The representation of space and its function in the prefrontal cortex have been examined using a variety of behavioral tasks. Among them, since the delayed-response task requires the temporary maintenance of spatial information, this task has been used to examine the mechanisms of spatial representation. In addition, the concept of working memory to explain prefrontal functions has helped us to understand the nature and functions of space representation in the prefrontal cortex. The detailed analysis of delay-period activity observed in spatial working memory tasks has provided important information for understanding space representation in the prefrontal cortex. Directional delay-period activity has been shown to be a neural correlate of the mechanism for temporarily maintaining information and represent spatial information for the visual cue and the saccade. In addition, many task-related prefrontal neurons exhibit spatially selective activities. These neurons are also important components of spatial information processing. In fact, information flow from sensory-related neurons to motor-related neurons has been demonstrated, along with a change in spatial representation as the trial progresses. The dynamic functional interactions among neurons exhibiting different task-related activities and representing different aspects of information could play an essential role in information processing. In addition, information provided from other cortical or subcortical areas might also be necessary for the representation of space in the prefrontal cortex. To better understand the representation of space and its function in the prefrontal cortex, we need to understand the nature of functional interactions between the prefrontal cortex and other cortical and subcortical areas.

Functional MRI evidence for the importance of visual short-term memory in logographic reading

Logographic symbols are visually complex, and thus children's abilities for visual short-term memory (VSTM) predict their reading competence in logographic systems. In the present study, we investigated the importance of VSTM in logographic reading in adults, both behaviorally and by means of fMRI. Outside the scanner, VSTM predicted logographic Kanji reading in native Japanese adults (n=45), a finding consistent with previous observations in Japanese children. In the scanner, participants (n=15) were asked to perform a visual one-back task. For this fMRI experiment, we took advantage of the unique linguistic characteristic of the Japanese writing system, whereby syllabic Kana and logographic Kanji can share the same sound and meaning, but differ only in the complexity of their visual features. Kanji elicited greater activation than Kana in the cerebellum and two regions associated with VSTM, the lateral occipital complex and the superior intraparietal sulcus, bilaterally. The same regions elicited the highest activation during the control condition (an unfamiliar, unpronounceable script to the participants), presumably due to the increased VSTM demands for processing the control script. In addition, individual differences in VSTM performance (outside the scanner) significantly predicted blood oxygen level-dependent signal changes in the identified VSTM regions, during the Kanji and control conditions, but not during the Kana condition. VSTM appears to play an important role in reading logographic words, even in skilled adults, as evidenced at the behavioral and neural level, most likely due to the increased VSTM/visual attention demands necessary for processing complex visual features inherent in logographic symbols.

The commonality of neural networks for verbal and visual short-term memory

Although many neuroimaging studies have considered verbal and visual short-term memory (STM) as relying on neurally segregated short-term buffer systems, the present study explored the existence of shared neural correlates supporting verbal and visual STM. We hypothesized that networks involved in attentional and executive processes, as well as networks involved in serial order processing, underlie STM for both verbal and visual list information, with neural specificity restricted to sensory areas involved in processing the specific items to be retained. Participants were presented sequences of nonwords or unfamiliar faces, and were instructed to maintain and recognize order or item information. For encoding and retrieval phases, null conjunction analysis revealed an identical fronto-parieto-cerebellar network comprising the left intraparietal sulcus, bilateral dorsolateral prefrontal cortex, and the bilateral cerebellum, irrespective of information type and modality. A network centered around the right intraparietal sulcus supported STM for order information, in both verbal and visual modalities. Modality-specific effects were observed in left superior temporal and mid-fusiform areas associated with phonological and orthographic processing during the verbal STM tasks, and in right hippocampal and fusiform face processing areas during the visual STM tasks, wherein these modality effects were most pronounced when storing item information. The present results suggest that STM emerges from the deployment of modality-independent attentional and serial ordering processes toward sensory networks underlying the processing and storage of modality-specific item information.

Oscillatory activity in parietal and dorsolateral prefrontal cortex during retention in visual short-term memory: additive effects of spatial attention and memory load

We used whole-head magnetoencephalography to study the representation of objects in visual short-term memory (VSTM) in the human brain. Subjects remembered the location and color of either two or four colored disks that were encoded from the left or right visual field (equal number of distractors in the other visual hemifield). The data were analyzed using time-frequency methods, which enabled us to discover a strong oscillatory activity in the 8-15 Hz band during the retention interval. The study of the alpha power variation revealed two types of responses, in different brain regions. The first was a decrease in alpha power in parietal cortex, contralateral to the stimuli, with no load effect. The second was an increase of alpha power in parietal and lateral prefrontal cortex, as memory load increased, but without interaction with the hemifield of the encoded stimuli. The absence of interaction between side of encoded stimuli and memory load suggests that these effects reflect distinct underlying mechanisms. A novel method to localize the neural generators of load-related oscillatory activity was devised, using cortically-constrained distributed source-localization methods. Some activations were found in the inferior intraparietal sulcus (IPS) and intraoccipital sulcus (IOS). Importantly, strong oscillatory activity was also found in dorsolateral prefrontal cortex (DLPFC). Alpha oscillatory activity in DLPFC was synchronized with the activity in parietal regions, suggesting that VSTM functions in the human brain may be implemented via a network that includes bilateral DLPFC and bilateral IOS/IPS as key nodes.

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.

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.

Visual-spatial processing in children and adolescents with Down's syndrome: a computerized assessment of memory skills

BACKGROUND

Several studies have identified better visual-spatial than verbal memory skills in children with Down's syndrome (DS); however, research in both typical development and DS points to a relative dissociation between visual and spatial memory processing, questioning the notion of a unitary visual-spatial memory construct. The insufficient and often contradictory results regarding the visual-spatial memory domain probably reflect the heterogeneity of memory tests employed by these studies and the different memory systems that they evaluate.

METHOD

We administered five visual-spatial memory tasks from the Cambridge Neuropsychological Test Automated Battery (CANTAB) to 25 children with DS and to 25 controls matched for mental age (MA) and basic psychomotor speed and accuracy. The memory tasks measure spatial span, visual and spatial recognition, paired associates learning and self-ordered search abilities.

RESULTS

The results confirm the relative sparing of the spatial short-term memory (STM) capacity in children with DS; however, as memory load increases, in recognition tasks, or when visual and spatial demands are combined, their performance is impaired compared with MA controls. The same impairment is generated by additional executive demands in the self-ordered search task, although search strategy is similar to the one presented by MA controls.

CONCLUSIONS

We did not find support for a visual vs. spatial dissociation in recognition memory. Performance impairment in the visual-spatial domain parallels the increase in working memory (WM) load or in the executive demands of the task. Possible neurobiological implications of the observed performance on the CANTAB tasks are also considered.

Domain-dependent activation during spatial and nonspatial auditory working memory

Visual system has been proposed to be divided into two, the ventral and dorsal, processing streams. The ventral pathway is thought to be involved in object identification whereas the dorsal pathway processes information regarding the spatial locations of objects and the spatial relationships among objects. Several studies on working memory (WM) processing have further suggested that there is a dissociable domain-dependent functional organization within the prefrontal cortex for processing of spatial and nonspatial visual information. Also the auditory system is proposed to be organized into two domain-specific processing streams, similar to that seen in the visual system. Recent studies on auditory WM have further suggested that maintenance of nonspatial and spatial auditory information activates a distributed neural network including temporal, parietal, and frontal regions but the magnitude of activation within these activated areas shows a different functional topography depending on the type of information being maintained. The dorsal prefrontal cortex, specifically an area of the superior frontal sulcus (SFS), has been shown to exhibit greater activity for spatial than for nonspatial auditory tasks. Conversely, ventral frontal regions have been shown to be more recruited by nonspatial than by spatial auditory tasks. It has also been shown that the magnitude of this dissociation is dependent on the cognitive operations required during WM processing. Moreover, there is evidence that within the nonspatial domain in the ventral prefrontal cortex, there is an across-modality dissociation during maintenance of visual and auditory information. Taken together, human neuroimaging results on both visual and auditory sensory systems support the idea that the prefrontal cortex is organized according to the type of information being maintained in WM.

Behavioural evidence for separating components within visuo-spatial working memory

Several different sources of evidence support the idea that visuo-spatial working memory can be segregated into separate cognitive subsystems. However, the nature of these systems remains unclear. Recently we reported data from neurological patients suggesting that information about visual appearance is retained in a different subsystem from information about spatial location. In this paper we report latency data from neurologically intact participants showing an experimental double dissociation between memory for appearance and memory for location. This was achieved by use of a selective dual task interference technique. This pattern provides evidence supporting the segregation of visuo-spatial memory between two systems, one of which supports memory for stimulus appearance and the other which supports memory for spatial location.

Binding of what and where during working memory maintenance

Prefrontal cortex (PFC) supports the maintenance of currently relevant information in working memory (WM). How the PFC is organized for the maintenance of disparate information, how this information is conjoined into a unified whole, and how the representation may change with task demands is still debated. The pattern of neural activity during maintenance of either abstract visual patterns, locations, or their "conjunction" was measured in two experiments using functional magnetic resonance imaging (fMRI). During delays, common regions in PFC were active, but a dorsal-ventral/spatial-nonspatial functional topography distinguished among the three delay types. During conjunction delays, no additional neural architecture was recruited. Instead, conjunction delays were characterized by a significant reduction compared to the response of that cortical region while maintaining its "preferred" information. A model is presented, extending the principles of "biased competition" to the PFC and the dynamic maintenance of information in WM, that accounts for current and seemingly contradictory previous results from both imaging and physiological studies. In this schema, the PFC is not only the source of biasing signals targeting earlier processing regions, but is also the target of these signals. This model stands as an alternative to traditional "domain specific" and "domain general" models of frontal organization of WM, and as an extension of earlier models of PFC mechanisms related to the cognitive control of goal directed behavior.

Behavioral inhibition and prefrontal cortex in decision-making

Every day we make innumerable decisions; some require no effort at all whereas others require considerable deliberation and weighing of various options. Despite the importance of decision-making in our lives and increased research interest, the specific neural mechanisms underlying decision-making remain unclear. We propose that the brain has at least two cortical pathways that independently generate a decision about appropriate behavior in given circumstances. These two pathways are extensions of the dorsal and ventral streams of the visual processing pathways. The parieto-premotor (extended dorsal) pathway makes decisions about motor actions in a largely autonomous and automatic fashion whereas the temporo-ventrolateral prefrontal (extended ventral) pathway is involved primarily in deliberate decisions and inhibitory control over behavior through the inhibitory function of the ventrolateral prefrontal cortex.

Learning related interactions among neuronal systems involved in memory processes

Functional neuroimaging techniques using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have provided new insights in our understanding of brain function from the molecular to the systems level. While subtraction strategy based data analyses have revealed the involvement of distributed brain regions in memory processes, covariance analysis based data analysis strategies allow functional interactions between brain regions of a neuronal network to be assessed. The focus of this chapter is to (1) establish the functional topography of episodic and working memory processes in young and old normal volunteers, (2) to assess functional interactions between modules of networks of brain regions by means of covariance based analyses and systems level modelling and (3) to relate neuroimaging data to the underpinning neural networks. Male normal young and old volunteers without neurological or psychiatric illness participated in neuroimaging studies (PET, fMRI) on working and episodic memory. Distributed brain areas are involved in memory processes (episodic and working memory) in young volunteers and show much of an overlap with respect to the network components. Systems level modelling analyses support the hypothesis of bihemispheric, asymmetric networks subserving memory processes and revealed both similarities in general and differences in the interactions between brain regions during episodic encoding and retrieval as well as working memory. Changes in memory function with ageing are evident from studies in old volunteers activating more brain regions compared to young volunteers and revealing more and stronger influences of prefrontal regions. We finally discuss the way in which the systems level models based on PET and fMRI results have implications for the understanding of the underlying neural network functioning of the brain.

Working memory in cigarette smokers: comparison to non-smokers and effects of abstinence

The present study was designed to examine the effect of cigarette smoking and withdrawal on working memory. Participants included 15 smokers and 22 matched non-smokers. For both groups the N-Back Task (of working memory) was administered in two test blocks on each of two days. On one day, smokers were tested after >or=13 h abstinence; on the other day, testing began <or=1 h after smoking. Smokers inhaled one cigarette between the blocks on each test day. Results indicated that performance of smokers after >or=13 h but not <or=1 h abstinence was significantly less accurate than that of non-smokers. A within-subject comparison revealed that in the abstinence session, smokers had significantly longer response latencies (in the 2-back condition) and made more overall errors compared to the satiety session. Smoking between test blocks in the abstinence session did not significantly affect performance although it significantly reduced craving. These findings provide further evidence for a deficit in working memory associated with acute abstinence from smoking, which may contribute to the difficulty of smoking cessation.

Effects of prenatal marijuana on visuospatial working memory: An fMRI study in young adults

The long lasting neurophysiological effects of prenatal marijuana exposure on visuospatial working memory were investigated in 18-22 year olds using functional magnetic resonance imaging (fMRI). The participants are members of the Ottawa Prenatal Prospective Study (OPPS), a longitudinal study that provides a unique body of information collected from each participant over 20 years, including prenatal drug history, detailed cognitive/behavioral performance from infancy to young adulthood, and current and past drug usage. This information allowed for the control of potentially confounding drug exposure variables in the statistical analyses. Thirty-one offspring from the OPPS (16 prenatally exposed and 15 nonexposed) performed a visuospatial 2-back task while neural activity was imaged with fMRI. Cognitive performance data were also collected. No significant performance differences were observed when comparing controls versus exposed participants. Multiple regression analyses (including controls with no exposure) revealed that as the amount of prenatal marijuana exposure increased, there was significantly more neural activity in the left inferior and middle frontal gyri, left parahippocampal gyrus, left middle occipital gyrus and left cerebellum. There was also significantly less activity in right inferior and middle frontal gyri. These results suggest that prenatal marijuana exposure alters neural functioning during visuospatial working memory processing in young adulthood.

fMRI Evidence for a Three-Stage Model of Deductive Reasoning

Deductive reasoning is fundamental to science, human culture, and the solution of problems in daily life. It starts with premises and yields a logically necessary conclusion that is not explicit in the premises. Here we investigated the neurocognitive processes underlying logical thinking with event-related functional magnetic resonance imaging. We specifically focused on three temporally separable phases: (1) the premise processing phase, (2) the premise integration phase, and (3) the validation phase in which reasoners decide whether a conclusion logically follows from the premises. We found distinct patterns of cortical activity during these phases, with initial temporo-occipital activation shifting to the prefrontal cortex and then to the parietal cortex during the reasoning process. Activity in these latter regions was specific to reasoning, as it was significantly decreased during matched working memory problems with identical premises and equal working memory load.

Double dissociation in neural correlates of visual working memory: A PET study

Using positron emission tomography (PET), we investigated the organisation of spatial versus object-based visual working memory in 11 normal human subjects. The paradigm involved a conditional colour-response association task embedded within two visual working memory tasks. The subject had to remember a position (spatial) or shape (object-based) and then use this to recover the colour of the matching element for the conditional association. Activation of the nucleus accumbens and the anterior cingulate cortex was observed during the conditional associative task, indicating a possible role of these limbic structures in associative memory. When the 2 memory tasks were contrasted, we observed activation of 2 distinct cortical networks: (1) The spatial task activated a dorsal stream network distributed in the right hemisphere in the parieto-occipital cortex and the dorsal prefrontal cortex, and (2) The non spatial task activated a ventral stream network distributed in the left hemisphere in the temporo- occipital cortex, the ventral prefrontal cortex and the striatum. These results support the existence of a domain-specific dissociation with dorsal and ventral cortical systems involved respectively in spatial and non spatial working memory functions.

Material-dependent activation in prefrontal cortex: working memory for letters and texture patterns--initial observations

PURPOSE

To prospectively evaluate whether a distinction between verbal and nonverbal short-term memory systems, as predicted with the multicomponent working memory model, is reflected in the material-specific patterns of activation in the prefrontal cortex.

MATERIALS AND METHODS

Informed written consent was obtained from all participants, and the institutional review board approved the study protocol. Echo-planar MR imaging was performed in 12 healthy subjects (five female and seven male subjects), with a mean age of 23.52 years +/- 2.52 (standard deviation) and a range of 20-29 years. A two-back task was used in the verbal and nonverbal versions. In the first version, letters were used as stimuli, and in the second version, the stimuli were abstract texture patterns. Timing parameters for both versions were the same. Statistical analysis of the functional data involved a fixed-effects general linear model. Regions of activation were identified from specific t-statistic contrasts between baseline and active tasks (corrected for whole-brain multiple comparisons).

RESULTS

The following suprathreshold voxels for the verbal condition were observed predominantly in the left hemisphere (middle frontal gyrus, precentral gyrus, middle temporal gyrus, and occipital cortex). Bilateral activations were in inferior frontal gyri, insulae inferior, superior parietal lobules, and cingulate gyri. In the nonverbal condition, suprathreshold voxels were located mostly bilaterally in the following regions: inferior, middle, and medial frontal gyri and inferior parietal lobules. Active regions were also found in the precentral gyrus and precuneate gyrus in the left parietal lobe and the occipital cortex in the right hemisphere.

CONCLUSION

Results of this study are consistent with the multicomponent model of working memory.

When meaningless symbols become letters: Neural activity change in learning new phonograms

Left fusiform gyrus and left angular gyrus are considered to be respectively involved with visual form processing and associating visual and auditory (phonological) information in reading. However, there are a number of studies that fail to show the contribution of these regions in carrying out these aspects of reading. Considerable differences in the type of stimuli and tasks used in the various studies may account for the discrepancy in results. This functional magnetic resonance imaging (fMRI) study attempts to control aspects of experimental stimuli and tasks to specifically investigate brain regions involved with visual form processing and character-to-phonological (i.e., simple grapheme-to-phonological) conversion processing for single letters. Subjects performed a two-back identification task using known Japanese, and previously unknown Korean, and Thai phonograms before and after training on one of the unknown language orthographies. Japanese subjects learned either five Korean or five Thai phonograms. Brain regions related to visual form processing were assessed by comparing activity related to native (Japanese) phonograms with that of non-native (Korean and Thai) phonograms. There was no significant differential brain activity for visual form processing. Brain regions related to character-to-phonological conversion processing were assessed by comparing pre- and post-tests of trained non-native phonograms with that of native phonograms and non-trained non-native phonograms. Significant differential activation post-relative to pre-training exclusively for the trained non-native phonograms was found in left angular gyrus. In addition, psychophysiologic interaction (PPI) analysis revealed greater integration of left angular gyrus with primary visual cortex as well as with superior temporal gyrus for the trained phonograms post-relative to pre-training. The results suggest that left angular gyrus is involved with character-to-phonological conversion in letter perception.

Functional specialization within the medial frontal gyrus for perceptual go/no-go decisions based on "what," "when," and "where" related information: an fMRI study

Cortical systems engaged during executive and volitional functions receive and integrate input from multiple systems. However, these integration processes are not well understood. In particular, it is not known whether these input pathways converge or remain segregated at the executive levels of cortical information processing. If unilateral information streams are conserved within structures that serve high-level executive functions, then the functional organization within these structures would predictably be similarly organized. If, however, unilateral input information streams are integrated within executive-related structures, then activity patterns will not necessarily reflect lower organizations. In this study, subjects were imaged during the performance of a "perceptual go/no-go" task for which instructions were based on spatial ("where"), temporal ("when"), or object ("what") stimulus features known to engage unilateral processing streams, and the expected hemispheric biases were observed for early processing areas. For example, activity within the inferior and middle occipital gyri, and the middle temporal gyrus, during the what and when tasks, was biased toward the left hemisphere, and toward the right hemisphere during the "where" task. We discover a similar lateralization within the medial frontal gyrus, a region associated with high-level executive functions and decision-related processes. This lateralization was observed regardless of whether the response was executed or imagined, and was demonstrated in multiple sensory modalities. Although active during the go/no-go task, the cingulate gyrus did not show a similar lateralization. These findings further differentiate the organizations and functions of the medial frontal and cingulate executive regions, and suggest that the executive mechanisms operative within the medial frontal gyrus preserve fundamental aspects of input processing streams.

Remembering "what" brings along "where" in visual working memory

Does a behavioral and anatomical division exist between spatial and object working memory? In this article, we explore this question by testing human participants in simple visual working memory tasks. We compared a condition in which there was no location change with conditions in which absolute location change and absolute plus relative location change were manipulated. The results showed that object memory was influenced by memory for relative but not for absolute location information. Furthermore, we demonstrated that relative space can be specified by a salient surrounding box or by distractor objects with no touching surfaces. Verbal memory was not influenced by any type of spatial information. Taken together, these results indicate that memory for "where" influences memory for "what." We propose that there is an asymmetry in memory according to which object memory always contains location information.

The effects of acute tyrosine and phenylalanine depletion on spatial working memory and planning in healthy volunteers are predicted by changes in striatal dopamine levels

RATIONALE

Dopamine (DA) is considered important in the modulation of tasks of spatial working memory. However, the findings from studies in humans to date are mixed. While this may be due to the characteristics of the tasks used, it is also possible that these findings are explained by variable central effects of the manipulations used.

OBJECTIVE

To test the effects of acute tyrosine and phenylalanine depletion (TPD, which reduces synthesis and release of brain DA) on cognitive function and relate changes in performance accuracy to the central effects of TPD measured with [11C]raclopride positron emission tomography (PET).

METHODS

Fourteen participants were given tests of spatial working memory, planning, verbal memory span and trial-and-error learning after acute TPD, seven of whom also received PET scans to measure changes in striatal DA levels.

RESULTS

Although TPD produced a clear reduction in tyrosine and phenylalanine availability to the brain, no impairments on any of the cognitive tests were observed. However, changes in spatial working memory and planning accuracy after TPD showed a highly significant relationship with the changes in striatal DA levels.

CONCLUSIONS

Our findings suggest that the effects of TPD on spatial working memory and planning may be unreliable due to the variability of the changes in brain DA levels achieved with this manipulation.

Learning by strategies and learning by drill—evidence from an fMRI study

The present fMRI study investigates, first, whether learning new arithmetic operations is reflected by changing cerebral activation patterns, and second, whether different learning methods lead to differential modifications of brain activation. In a controlled design, subjects were trained over a week on two new complex arithmetic operations, one operation trained by the application of back-up strategies, i.e., a sequence of arithmetic operations, the other by drill, i.e., by learning the association between the operands and the result. In the following fMRI session, new untrained items, items trained by strategy and items trained by drill, were assessed using an event-related design. Untrained items as compared to trained showed large bilateral parietal activations, with the focus of activation along the right intraparietal sulcus. Further foci of activation were found in both inferior frontal gyri. The reverse contrast, trained vs. untrained, showed a more focused activation pattern with activation in both angular gyri. As suggested by the specific activation patterns, newly acquired expertise was implemented in previously existing networks of arithmetic processing and memory. Comparisons between drill and strategy conditions suggest that successful retrieval was associated with different brain activation patterns reflecting the underlying learning methods. While the drill condition more strongly activated medial parietal regions extending to the left angular gyrus, the strategy condition was associated to the activation of the precuneus which may be accounted for by visual imagery in memory retrieval.

Increased MEG activation in OCD reflects a compensatory mechanism specific to the phase of a visual working memory task

We examined spatio-temporal patterns of evoked magnetoencephalographic signals (MEG) in patients with obsessive-compulsive disorder (OCD) during the Encoding, Retention, and Retrieval phases of a Delayed Matching-to-Sample working memory task (DMST). The question was whether the mechanisms of abnormally increased cortical excitability, frequently reported in OCD, relate to a global cortical disinhibition and unselective over-processing of stimuli or, alternatively, to a compensatory mechanism of effortful enhanced inhibitory control. The DMST-related network of activation in OCD was found similar to that of Controls, and to that reported in other neuroimaging studies. The pattern of increased MEG activation in OCD patients was phase specific. During the Encoding phase, the activation was enhanced in the region of anterior insula and reduced in the posterior-inferior parietal cortex. During Retention, the activation was reduced in the occipital, parietal, superior temporal sulcus, and dorsolateral prefrontal cortex (BA 6/8/9). During Retrieval, a significant increase of activation was found in the right anterior insula extending towards the orbital region and right superior temporal sulcus, along with reduced activation in the left parietal cortex. The performance accuracy was high in OCD and comparable to Controls, although the RTs were prolonged. The results are discussed as being consistent with the hypothesis of a compensatory mechanism of effortful inhibitory control. This mechanism may be a major contributor to the increased cortical activation during Encoding and, in particular, Retrieval of the DMST task in patients suffering OCD. The findings do not support the concept of a faulty working memory mechanism per se in OCD.

Cognition in mania and depression: psychological models and clinical implications

Affective disorders, including bipolar disorder and major depressive disorder, are highly prevalent throughout the world and are extremely disabling. Diagnostic and Statistical Manual criteria and psychological models strongly implicate cognitive dysfunctions as being integral to our understanding of these disorders. We review the findings from studies that have used neurocognitive tests and functional imaging techniques to explore abnormal cognition in affective disorders. In particular, we highlight the evidence for cognitive dysfunctions that persist into full clinical remission, and the recent trend toward the use of "hot" processing tasks, involving emotionally charged stimuli, as a means of differentiating between the cognitive underpinnings of mania and depression. The clinical relevance of these developments is discussed.