Mapping neural interactivity onto regional activity: an analysis of semantic processing and response mode interactions

Circuit activity underlying a distinct modulator of prepulse inhibition

Prepulse inhibition (PPI), the diminished eye blink response to a startling pulse induced by a prepulse, is regulated by brainstem, and modulated by cerebral, processes. Attentional modulation by the prepulse (AMP), a potential biomarker of psychotic disorders, differs from other modulatory processes because it only occurs if the interval between the prepulse and pulse exceeds 100 ms (>PP100). Videotaped eye blinks were measured during fMRI scanning in 15 healthy subjects hearing 64 pulse alone, 64 PP60 and 64 PP120 trials in a rapid event-related design. Because attentional influences on PPI vary spontaneously, we posited AMP could be isolated by comparing eye blink and Blood Oxygen Level Dependent covariation during the two PP trial types. Behavioral regressor coefficients reflecting significant covariation covered the insula and auditory cortices during PP120 but not PP60 trials. Clusters within the right anterior insula and auditory cortex were specific to AMP. Functional connections (FCs) between cerebral ROIs implicated in PPI were stronger during PP120 trials. The four FCs that were individually stronger during PP120 trials involved the right insula or auditory cortex and three were not present during PP60 trials. Converging evidence indicates the right insula is the hub of a network underlying AMP.

A linear structural equation model for covert verb generation based on independent component analysis of FMRI data from children and adolescents

Human language is a complex and protean cognitive ability. Young children, following well defined developmental patterns learn language rapidly and effortlessly producing full sentences by the age of 3 years. However, the language circuitry continues to undergo significant neuroplastic changes extending well into teenage years. Evidence suggests that the developing brain adheres to two rudimentary principles of functional organization: functional integration and functional specialization. At a neurobiological level, this distinction can be identified with progressive specialization or focalization reflecting consolidation and synaptic reinforcement of a network (Lenneberg, 1967; Muller et al., 1998; Berl et al., 2006). In this paper, we used group independent component analysis and linear structural equation modeling (McIntosh and Gonzalez-Lima, 1994; Karunanayaka et al., 2007) to tease out the developmental trajectories of the language circuitry based on fMRI data from 336 children ages 5–18 years performing a blocked, covert verb generation task. The results are analyzed and presented in the framework of theoretical models for neurocognitive brain development. This study highlights the advantages of combining both modular and connectionist approaches to cognitive functions; from a methodological perspective, it demonstrates the feasibility of combining data-driven and hypothesis driven techniques to investigate the developmental shifts in the semantic network.

Babbling, chewing, and sucking: oromandibular coordination at 9 months

PURPOSE

The ontogeny of mandibular control is important for understanding the general neurophysiologic development for speech and alimentary behaviors. Prior investigations suggest that mandibular control is organized distinctively across speech and nonspeech tasks in 15-month-olds and adults and that, with development, these extant forms of motor control primarily undergo refinement and rescaling. The present investigation was designed to evaluate whether these coordinative infrastructures for alimentary behaviors and speech are evident during the earliest period of their co-occurrence.

METHOD

Electromyographic (EMG) signals were obtained from the mandibular muscle groups of 15 typically developing 9-month-old children during sucking, chewing, and speech.

RESULTS

Unlike prior investigations of 12- and 15-month-olds and adults, 9-month-olds' analyses of peak correlations among agonist and antagonist comparisons of mandibular EMG data revealed weak coupling during sucking, chewing, and babble; associated lag values for antagonist muscle groups indicated greater synchrony during alimentary behaviors and less synchrony during babble. Unlike the speech data of 15-month-olds, 9-month-olds exhibited consistent results across speech subtasks.

CONCLUSION

These findings were consistent with previous results in which mandibular coordination across behaviors was more variable for younger age groups, whereas the essential organization of each behavior closely reflected that seen in older infants and adults.

Combining path analysis with time-resolved functional magnetic resonance imaging: the neurocognitive network underlying mental rotation

There is strong evidence to suggest that the complex cognitive process underlying mental rotation does not have a discrete neural correlate, but is represented as a distributed neural system. Although the neuroanatomical nodes of this so-called rotation network are well established, there is as yet little empirical evidence to indicate how these nodes interact during task performance. Using an optimized, event-related paradigm, this study aimed to test a previously proposed hypothetical neurocognitive network for mental rotation in female subjects with path analysis, and to examine changes in effective connections across different levels of task difficulty. Path analysis was carried out in combination with a time-resolved functional magnetic resonance imaging (fMRI) analysis in order to relate the observed changes on the network level to changes in specific temporal characteristics of the hemodynamic response function on the level of individual neuroanatomical nodes. Overall, it was found that the investigated sequential model did not provide an adequate fit to the data and that a model with parallel information processing was superior to the serial model. This finding challenges traditional cognitive models describing the complex cognitive process underlying mental rotation by a set of sequentially organized, functionally distinct processing stages. It was further demonstrated that the observed in interregional effective connectivity changes with the level of task demand. These changes were directly related to the time course of the experimental paradigm. The results of path analysis in fMRI should therefore only be interpreted in the light of a specific experimental design and should not be considered as general indicators of effective connections.

Unified structural equation modeling approach for the analysis of multisubject, multivariate functional MRI data

The ultimate goal of brain connectivity studies is to propose, test, modify, and compare certain directional brain pathways. Path analysis or structural equation modeling (SEM) is an ideal statistical method for such studies. In this work, we propose a two-stage unified SEM plus GLM (General Linear Model) approach for the analysis of multisubject, multivariate functional magnetic resonance imaging (fMRI) time series data with subject-level covariates. In Stage 1, we analyze the fMRI multivariate time series for each subject individually via a unified SEM model by combining longitudinal pathways represented by a multivariate autoregressive (MAR) model, and contemporaneous pathways represented by a conventional SEM. In Stage 2, the resulting subject-level path coefficients are merged with subject-level covariates such as gender, age, IQ, etc., to examine the impact of these covariates on effective connectivity via a GLM. Our approach is exemplified via the analysis of an fMRI visual attention experiment. Furthermore, the significant path network from the unified SEM analysis is compared to that from a conventional SEM analysis without incorporating the longitudinal information as well as that from a Dynamic Causal Modeling (DCM) approach.

Age-related connectivity changes in fMRI data from children listening to stories

The way humans comprehend narrative speech plays an important part in human development and experience. A group of 313 children with ages 5-18 were subjected to a large-scale functional magnetic resonance imaging (fMRI) study in order to investigate the neural correlates of auditory narrative comprehension. The results were analyzed to investigate the age-related brain activity changes involved in the narrative language comprehension circuitry. We found age-related differences in brain activity which may either reflect changes in local neuroplasticity (of the regions involved) in the developing brain or a more global transformation of brain activity related to neuroplasticity. To investigate this issue, Structural Equation Modeling (SEM) was applied to the results obtained from a group independent component analysis (Schmithorst, V.J., Holland, S.K., et al., 2005. Cognitive modules utilized for narrative comprehension in children: a functional magnetic resonance imaging study. NeuroImage) and the age-related differences were examined in terms of changes in path coefficients between brain regions. The group Independent Component Analysis (ICA) had identified five bilateral task-related components comprising the primary auditory cortex, the mid-superior temporal gyrus, the most posterior aspect of the superior temporal gyrus, the hippocampus, the angular gyrus and the medial aspect of the parietal lobule (precuneus/posterior cingulate). Furthermore, a left-lateralized network (sixth component) was also identified comprising the inferior frontal gyrus (including Broca's area), the inferior parietal lobule, and the medial temporal gyrus. The components (brain regions) for the SEM were identified based on the ICA maps and the results are discussed in light of recent neuroimaging studies corroborating the functional segregation of Broca's and Wernicke's areas and the important role played by the right hemisphere in narrative comprehension. The classical Wernicke-Geschwind (WG) model for speech processing is expanded to a two-route model involving a direct route between Broca's and Wernicke's area and an indirect route involving the parietal lobe.

Effect of unpleasant loud noise on hippocampal activities during picture encoding: An fMRI study

The functional link between the amygdala and hippocampus in humans has not been well documented. We examined the effect of unpleasant loud noise on hippocampal and amygdaloid activities during picture encoding by means of fMRI, and on the correct response in humans. The noise reduced activity in the hippocampus during picture encoding, decreased the correct response and increased the activity of the amygdala. A path diagram using structural equation modeling suggested that hippocampus activity might be depressed by high amygdala activity. Therefore, noise should diminish memory by reducing hippocampal activity, which might be depressed by high amygdala activity.

Functional network of the basal ganglia and cerebellar motor loops in vivo: Different activation patterns between self-initiated and externally triggered movements

The basal ganglia and cerebellar loops are known to participate differently in self-initiated (SI) and externally triggered (ET) movements. However, no previous neuroimaging studies have illustrated functional organization of these loops in vivo. Here, we aimed to functionally visualize these loops during motor execution using functional magnetic resonance imaging (fMRI) with structural equation modeling (SEM). Twelve normal subjects (24-29 years old) were scanned while performing five different frequencies of sequential left finger movements using either SI or ET movements. Random effect analysis combined with a parametric approach revealed a significant positive linear dependence of cerebral activation upon movement rate in the right Put, GPi, VL, SMC and SMA during SI tasks. During ET tasks, significant positive linear relationships were found in the right SMC, VPL, left CB and DN, whereas tendency for linear relationships was seen in the right PMv. SEM further showed significant interactions within the right basal ganglia-thalamo-motor loop during SI tasks. In contrast, there were significant interactions within the entire right cerebral hemisphere-left cerebellar loop involving CB, DN, VPL, PMv and SMC during ET tasks. Therefore, our modeling approach enabled identification of different contributions of the motor loops of basal ganglia and cerebellum to SI and ET tasks during motor execution.

Investigating the neural basis for functional and effective connectivity. Application to fMRI

Viewing cognitive functions as mediated by networks has begun to play a central role in interpreting neuroscientific data, and studies evaluating interregional functional and effective connectivity have become staples of the neuroimaging literature. The neurobiological substrates of functional and effective connectivity are, however, uncertain. We have constructed neurobiologically realistic models for visual and auditory object processing with multiple interconnected brain regions that perform delayed match-to-sample (DMS) tasks. We used these models to investigate how neurobiological parameters affect the interregional functional connectivity between functional magnetic resonance imaging (fMRI) time-series. Variability is included in the models as subject-to-subject differences in the strengths of anatomical connections, scan-to-scan changes in the level of attention, and trial-to-trial interactions with non-specific neurons processing noise stimuli. We find that time-series correlations between integrated synaptic activities between the anterior temporal and the prefrontal cortex were larger during the DMS task than during a control task. These results were less clear when the integrated synaptic activity was haemodynamically convolved to generate simulated fMRI activity. As the strength of the model anatomical connectivity between temporal and frontal cortex was weakened, so too was the strength of the corresponding functional connectivity. These results provide a partial validation for using fMRI functional connectivity to assess brain interregional relations.

Abnormal effective connectivity of dopamine D2 receptor binding in schizophrenia

Receptor binding has been examined region by region in both in vitro and in vivo studies, but less attention has been paid to the connectivity of regional receptor binding despite the fact that neurophysiological studies have indicated an extensive inter-regional connectivity. In this study, we investigated the connectivity of regional dopamine D2 receptor binding in positron emission tomography data from 10 drug-naive patients with schizophrenia and 19 healthy controls. We applied a structural equation method to regional receptor binding. The results indicated that the network models of the patients and normal subjects were significantly different. As to the individual path coefficients, (a) connectivity between cortical regions was different between groups; (b) connectivity from the prefrontal cortex, parietal cortex, and thalamus to the anterior cingulate differed from that in controls; and (c) connectivity from the prefrontal cortex to the anterior cingulate and thalamus via the hippocampus was observed in normal subjects but not in patients. These results suggest that a systems-level change reflected in the connectivity of D2 receptor binding is present in schizophrenia.

Hemispheric asymmetry in supplementary motor area connectivity during unilateral finger movements

Studies of unilateral finger movement in right-handed subjects have shown asymmetrical patterns of activation in primary motor cortex. Some studies have measured a similar asymmetry in the supplementary motor area (SMA), but others have not. To shed more light on the symmetry of function in the SMA, we used path analysis of functional MRI data to investigate effective connectivity during a unilateral finger movement task. We observed a slight asymmetry in task activation: left SMA was equally active during movement of either hand, while right SMA was more active for left-hand movement, suggesting a dominant role of left SMA. In addition, we tested for a corresponding asymmetry in the influence of SMA on sensorimotor cortex (SMC) using a path model based on the well-established principle that SMA is involved in motor control and SMC in execution. We observed that the influence of left SMA on left SMC increased during right-hand movement, and the influence of left SMA on right SMC increased during left-hand movement. However, there was no significant hand-dependent change in the influences of the right SMA. This asymmetry in connectivity implies that left SMA does play a dominant role in unilateral movements of either hand in right handers. The experiment also provides a basis for further studies of motor system connectivity in healthy or patient populations.

Changes in effective connectivity models in the presence of task-correlated motion: an fMRI study

We investigated the effects of motion-correction strategy and time course selection method when structural equation modeling is applied to fMRI data in the presence of task-correlated motion. Three motion-correction methods were employed for a group of 12 subjects performing an orthographic lexical retrieval task: (1) a rigid body realignment as implemented in SPM99, (2) a rigid body realignment combined with the inclusion of motion parameters in the statistical model, and (3) the FLIRT motion correction followed by an ICA analysis aiming to identify and remove the motion-related components and the ghosting artifacts. For each motion correction, the time courses of the activated regions were selected in three ways: (1) using the voxels with the highest Z scores, (2) using the average across all the statistically significant voxels in the region of interest, and (3) using a within-region, across-subjects, singular value decomposition. The resulting models of effective connectivity were markedly different, although the activation pattern was not substantially altered by the motion-correction method. Higher values for the path coefficients were obtained for the models fitted to the covariance matrices based on the average time courses than for the covariance matrices based on a single voxel time course. Our results suggest caution with the interpretation of task-induced changes in effective connectivity since, for higher-order cognitive brain functions, multiple models can be fitted to a given data set and these models cannot be rejected on an anatomical or cognitive basis. Hum. Brain Mapping 21:49-63, 2004.

Connectivity analysis with structural equation modelling: an example of the effects of voxel selection

Structural equation modelling (SEM) of neuroimaging data is commonly applied to a network of distributed brain regions. We applied SEM to an fMRI dataset to identify condition-specific effects in a simple experiment composed of visual stimulation and baseline conditions. The visual network was composed of three well-defined anatomical regions (V1, V2, and V5) and three path connections (V1 --> V2, V1 --> V5, and V2 --> V5). This network was used to test four hypotheses: (1) whether the condition-specific effects for all three connections vary according to the data selected for modelling; (2) whether the "summary" measures that are often used are indeed appropriate; (3) whether measures taken from the voxel timecourse can reliably predict the condition-specific effects for each one of the three path connections, and (4) whether all voxels within an anatomical region yield equivalent SEM outcomes. There was some variability in the significance of the condition-specific effects across randomly selected voxels within regions. However, the SEM outcome from the "summary" measures was comparable to the most frequent pattern of condition-specific effects. Magnitude, delay, spread, and goodness-of-fit measures taken from a gamma fit to the voxel time courses predicted reliably the significance of the SEM condition-specific effects for each connection. This result enabled us to identify spatially coherent regions at the boundaries of V2 that displayed different condition-specific effects from those seen in the majority of the voxels. Although the generality of these results awaits further investigation, this example highlights a number of important issues for SEM. We have provided further evidence that the SEM outcome does vary somewhat according to the voxels selected and that, although the use of summary measures can give a generalised view of the connectivity pattern, they could fail to capture functional differences within specialised areas.

Shared and dissociated cortical regions for object and letter processing

The present study determined the extent to which object and letter recognition recruit similar or dissociated neural resources. Participants passively viewed and silently named line drawings of objects, single letters, and visual noise patterns and centrally fixated an asterisk. We used whole-brain functional MRI and a very conservative approach to hypothesis testing that distinguished among brain regions that were selectively activated by different experimental conditions and those that were conjointly activated. The left fusiform gyrus (BA 19 & 37) and left inferior frontal cortex BA(44/6) showed a greater degree of conjoined activation for objects and letters than selective activation for either category, whereas left inferior parietal cortex (BA 40) and the left insula showed a strong letter-selective response. Equal recruitment of left fusiform and inferior frontal regions by objects and letters reflects similar demands on cognitive processing by these two categories and argues against category-specific modules in these regions. However, cortical systems for object and letter processing are not completely shared given the exclusive activation of left inferior parietal cortex by letters.

"Pray or Prey?" dissociation of semantic memory retrieval from episodic memory processes using positron emission tomography and a novel homophone task

One problem in studying the neural basis of semantic memory using functional neuroimaging is that it is often difficult to disentangle activation associated with semantic memory retrieval from that associated with episodic memory encoding and retrieval. To address this issue, a novel homophone task was used in which subjects were PET scanned whilst learning a series of real words (e.g., prey). In a subsequent scan, the subjects were presented with homophone pairs (e.g., prey vs pray) and were required to choose the one that had been shown previously. In two corresponding baseline tasks, the subjects were scanned whilst learning and recognizing pronounceable nonwords. Thus, while all of these tasks recruited either episodic memory encoding or retrieval processes, only the homophone tasks involved semantic memory retrieval. A conjunction analysis designed to isolate activation associated with semantic memory retrieval, revealed changes in several left lateral frontal regions (BA 9/10, 9/45), the left middle temporal cortex (BA 21), and in the left inferior temporoparietal cortex (BA 39). In contrast, a conjunction analysis designed to isolate activation associated with episodic memory encoding, revealed significant changes in the left hippocampus, as well as in the frontopolar cortex (BA 10) bilaterally, the left inferior parietal cortex (BA 40), and the left superior temporal gyrus (BA 22, 28). The present results clarify and extend recent attempts to understand the neural basis of semantic memory retrieval, by actively controlling for the confounding effects of episodic memory encoding and retrieval processes.

GABA-ergic modulation of prefrontal spatio-temporal activation pattern during emotional processing: a combined fMRI/MEG study with placebo and lorazepam

Various prefrontal cortical regions have been shown to be activated during emotional stimulation, whereas neurochemical mechanisms underlying emotional processing in the prefrontal cortex remain unclear. We therefore investigated the influence of the GABA-A potentiator lorazepam on prefrontal cortical emotional-motor spatio-temporal activation pattern in a combined functional magnetic resonance imaging/magnetoencephalography study. Lorazepam led to the reversal in orbito-frontal activation pattern, a shift of the early magnetic field dipole from the orbito-frontal to medial prefrontal cortex, and alterations in premotor/motor cortical function during negative and positive emotional stimulation. It is concluded that negative emotional processing in the orbito-frontal cortex may be modulated either directly or indirectly by GABA-A receptors. Such a modulation of orbito-frontal cortical emotional function by lorazepam has to be distinguished from its effects on cortical motor function as being independent from the kind of processing either emotional or nonemotional.

A PET study of stimulus- and task-induced semantic processing

To investigate the neural correlates of semantic processing, previous functional imaging studies have used semantic decision and generation tasks. However, in addition to activating semantic associations these tasks also involve executive functions that are not specific to semantics. The study reported in this paper aims to dissociate brain activity due to stimulus-driven semantic associations and task-induced semantic and executive processing by using repetition and semantic decision on auditorily presented words in a cognitive conjunction design. The left posterior inferior temporal, inferior frontal (BA 44/45), and medial orbital gyri were activated by both tasks, suggesting a general role in stimulus-driven semantic and phonological processing. In addition, semantic decision increased activation in (i) left ventral inferior frontal cortex (BA 47), right cerebellum, and paracingulate, which have all previously been implicated in executive functions, and (ii) a ventral region in the left anterior temporal pole which is commonly affected in patients with semantic impairments. We attribute activation in this area to the effortful linkage of semantic features. Thus, our study replicated the functional dissociation between dorsal and ventral regions of the left inferior frontal cortex. Moreover, it also dissociated the semantic functions of the left posterior inferior temporal gyrus and anterior temporal pole: The posterior region subserves stimulus-driven activation of semantic associations and the left anterior region is involved in task-induced association of semantic information.

Can meaningful effective connectivities be obtained between auditory cortical regions?

Structural equation modeling (SEM) of neuroimaging data can be evaluated both for the goodness of fit of the model and for the strength of path coefficients (as an index of effective connectivity). SEM of auditory fMRI data is made difficult by the necessary sparse temporal sampling of the time series (to avoid contamination of auditory activation by the response to scanner noise) and by the paucity of well-defined anatomical information to constrain the functional model. We used SEM (i.e., a model incorporating latent variables) to investigate how well fMRI data in four adjacent cortical fields can be described as an auditory network. Seven of the 14 models (2 hemispheres x (6 subjects and 1 group)) produced a plausible description of the measured data. Since the auditory model to be tested is not fully validated by anatomical data, our approach requires that goodness of fit be confirmed to ensure generalizability of connectivity patterns. For good-fitting models, connectivity patterns varied significantly across subjects and were not replicable across stimulus conditions. SEM of central auditory function therefore appears to be highly sensitive to the voxel-selection procedure and/or the sampling of the time series.

The central role of the parietal lobes in consciousness

There are now various approaches to understand where and how in the brain consciousness arises from neural activity, none of which is universally accepted. Difficulties among these approaches are reviewed, and a missing ingredient is proposed here to help adjudicate between them, that of "perspectivalness." In addition to a suitable temporal duration and information content of the relevant bound brain activity, this extra component is posited as being a further important ingredient for the creation of consciousness from neural activity. It guides the development of what is termed the "Central Representation," which is supposed to be present in all mammals and extended in humans to support self-consciousness as well as phenomenal consciousness. Experimental evidence and a theoretical framework for the existence of the central representation are presented, which relates the extra component to specific buffer working memory sites in the inferior parietal lobes, acting as attentional coordinators on the spatial maps making up the central representation. The article closes with a discussion of various open questions.

Interconnected large-scale systems for three fundamental cognitive tasks revealed by functional MRI

The specific brain areas required to execute each of three fundamental cognitive tasks - object naming, same-different discrimination, and integer computation - are determined by whole-brain functional magnetic resonance imaging (fMRI) using a novel technique optimized for the isolation of neurocognitive systems. This technique (1) conjoins the activity associated with identical or nearly identical tasks performed in multiple sensory modalities (conjunction) and (2) isolates the activity conserved across multiple subjects (conservation). Cortical regions isolated by this technique are, thus, presumed associated with cognitive functions that are both distinguished from primary sensory processes and from individual differences. The object-naming system consisted of four brain areas: left inferior frontal gyrus, Brodmann's areas (BAs) 45 and 44; left superior temporal gyrus, BA 22; and left medial frontal gyrus, BA 6. The same-different discrimination system consisted of three brain areas: right inferior parietal lobule, BA 40; right precentral gyrus, BA 6; and left medial frontal gyrus, BA 6. The integer computation system consisted of five brain areas: right middle frontal gyrus, BA 6; right precentral gyrus, BA 6; left inferior parietal lobule, BA 40; left inferior frontal gyrus, BA 44; and left medial frontal gyrus, BA 6. All three neurocognitive systems shared one common cortical region, the left medial frontal gyrus, the object-naming and integer computation systems shared the left inferior frontal gyrus, and the integer computation and same-different discrimination systems shared the right precentral gyrus. These results are consistent with connectionist models of cognitive processes where specific sets of remote brain areas are assumed to be transiently bound together as functional units to enable these functions, and further suggest a superorganization of neurocognitive systems where single brain areas serve as elements of multiple functional systems.

Towards a network theory of cognition

For cognitive neuroscience to go forward a more explicit effort is needed to use neurophysiology to constrain how the brain produces human mental functions. This review begins with the suggestion that two fundamental features may be critical for this effort. The first is the connectivity of the brain, which occupies an intermediate position between complete redundant interconnections and independence. The term semniconnected is presented as a designation, which is an obvious derivation of the term semiconductors as used in engineering. The second is transient response plasticity where a given neuron or collection of neurons may show rapid changes in response characteristics depending on experience. Response plasticity is a ubiquitous property of the brain rather than a unique characteristic of "neurocognitive" regions. These two properties may be brought together when brain areas interact such that their aggregate function embodies cognition. Three examples are used to illustrate these general principles and to develop the idea that a particular region in isolation may not act as a reliable index for a particular cognitive function. Instead, the neural context in which an area is active may define the cognitive function. Neural context emphasizes that the particular spatiotemporal pattern of neural interactions may hold the key to bridge between brain and mind.

Decomposing memory: functional assignments and brain traffic in paired word associate learning

The recent covariance structural equation model for word-pair associate encoding and retrieval (Krause, Horwitz, Taylor, Schmidt, Mottaghy, Halsband et al., 1998; Krause, Horwitz, Taylor, Schmidt, Mottaghy, Herzog et al., 1999) is analysed to deduce possible functional assignments of the various brain modules used by subjects in solving the task. Specific processing aspects are considered, in particular, that of long-term working memory sites and how they are coupled to buffer working memory sites to enable deposition and manipulation of remembered associates. The new concept of 'brain traffic' is introduced as an aid to the assessment of how important are various brain modules. A set of functional assignments is produced for the relevant modules.

Neural modeling, functional brain imaging, and cognition

The richness and complexity of data sets acquired from PET or fMRI studies of human cognition have not been exploited until recently by computational neural-modeling methods. In this article, two neural-modeling approaches for use with functional brain imaging data are described. One, which uses structural equation modeling, estimates the functional strengths of the anatomical connections between various brain regions during specific cognitive tasks. The second employs large-scale neural modeling to relate functional neuroimaging signals in multiple, interconnected brain regions to the underlying neurobiological time-varying activities in each region. Delayed match-to-sample (visual working memory for form) tasks are used to illustrate these models.