Similarities and differences in the neural correlates of episodic memory retrieval and working memory

Dissociation of cortical regions modulated by both working memory load and sleep deprivation and by sleep deprivation alone

Working memory is an important mental capacity that is compromised following sleep deprivation (SD). To understand how working memory load interacts with state to influence brain activation in load-sensitive regions, and the extent to which SD-related changes are common across different loads, we used fMRI to study twelve healthy subjects following 24 h of SD using a verbal n-back task with three load levels. Performance decline was observed by way of reduced accuracy and slower response times following SD. The left prefrontal region and thalamus showed load dependent activity modulation that interacted with state. The right parietal and anterior medial frontal regions showed load dependent changes in activity as well as an effect of state. The anterior cingulate and occipital regions showed activation that displayed state effects that were independent of working memory load. These findings represent a step toward identifying how different brain regions exhibit varying vulnerability to the deleterious effects of SD on working memory.

Hippocampal and neocortical activation during repetitive encoding in older persons

Episodic memory function is known to decline in the course of normal aging; however, compensatory techniques can improve performance significantly in older persons. We investigated the effects of the memory enhancing technique of repetition encoding on brain activation using event-related functional magnetic resonance imaging (fMRI). Twelve healthy older adults without cognitive impairment were studied with fMRI during repetitive encoding of face-name pairs. During the first encoding trials of face-name pairs that were subsequently remembered correctly, activation of the hippocampus and multiple neocortical regions, including prefrontal, parietal and fusiform cortices, was observed. The second and third encoding trials resulted in continued activation in neocortical regions, but no task-related response within the hippocampus. Functional imaging of successful memory processes thus permits us to detect regionally specific responses in the aging brain. Our findings suggest that hippocampal function is preserved in normal aging and that repetition-based memory enhancing techniques may engage primarily neocortical attentional networks.

Frontal lobe mechanisms that resolve proactive interference

Memory of a past experience can interfere with processing during a subsequent experience, a phenomenon termed proactive interference (PI). Neuroimaging and neuropsychological evidence implicate the left mid-ventrolateral prefrontal cortex (mid-VLPFC) in PI resolution during short-term item recognition, though the precise mechanisms await specification. The present functional magnetic resonance imaging (fMRI) experiment sought to further constrain theorizing regarding PI resolution. On each trial, subjects maintained a target set of words, and then decided if a subsequent probe was contained in the target set (positive) or not (negative). Importantly, for half of the negative and half of the positive trials, the probe had been contained in the previous target set (recent). Relative to non-recent trials, negative-recent trials produced an increase in response times and error rates, behavioral markers of PI. In fMRI measures, negative recency was associated with increased activation in the left mid-VLPFC, as well as in the bilateral fronto-polar cortex, providing evidence for multiple components in PI resolution. Furthermore, recency effects were evident during both negative and positive trials, with the magnitude of the recency effect in the mid-VLPFC being greater on negative trials. Collectively, these results serve to specify and constrain proposed models of PI resolution.

Neural representation of interval encoding and decision making

Our perception of time depends on multiple psychological processes that allow us to anticipate events. In this study, we used event-related functional magnetic resonance imaging (fMRI) to differentiate neural systems involved in formulating representations of time from processes associated with making decisions about their duration. A time perception task consisting of two randomly presented standard intervals was used to ensure that intervals were encoded on each trial and to enhance memory requirements. During the encoding phase of a trial, activation was observed in the right caudate nucleus, right inferior parietal cortex and left cerebellum. Activation in these regions correlated with timing sensitivity (coefficient of variation). In contrast, encoding-related activity in the right parahippocampus and hippocampus correlated with the bisection point and right precuneus activation was associated with a measure of memory distortion. Decision processes were studied by examining brain activation during the decision phase of a trial that was associated with the difficulty of interval discriminations. Activation in the right parahippocampus was greater for easier than harder discriminations. In contrast, activation was greater for harder than easier discriminations in systems involved in working memory (left middle-frontal and parietal cortex) and auditory rehearsal (left inferior-frontal and superior-temporal cortex). Activity in the auditory rehearsal network correlated with memory distortion. Our results support the independence of systems that mediate interval encoding and decision processes. The results also suggest that distortions in memory for time may be due to strategic processing in cortical systems involved in either encoding or rehearsal.

An information-processing model of three cortical regions: evidence in episodic memory retrieval

ACT-R (Anderson, J.R., et al., 2003. An information-processing model of the BOLD response in symbol manipulation tasks. Psychon. Bull. Rev. 10, 241-261) relates the inferior dorso-lateral prefrontal cortex to a retrieval buffer that holds information retrieved from memory and the posterior parietal cortex to an imaginal buffer that holds problem representations. Because the number of changes in a problem representation is not necessarily correlated with retrieval difficulties, it is possible to dissociate prefrontal-parietal activations. In two fMRI experiments, we examined this dissociation using the fan effect paradigm. Experiment 1 compared a recognition task, in which representation requirement remains the same regardless of retrieval difficulty, with a recall task, in which both representation and retrieval loads increase with retrieval difficulty. In the recognition task, the prefrontal activation revealed a fan effect but not the parietal activation. In the recall task, both regions revealed fan effects. In Experiment 2, we compared visually presented stimuli and aurally presented stimuli using the recognition task. While only the prefrontal region revealed the fan effect, the activation patterns in the prefrontal and the parietal region did not differ by stimulus presentation modality. In general, these results provide support for the prefrontal-parietal dissociation in terms of retrieval and representation and the modality-independent nature of the information processed by these regions. Using ACT-R, we also provide computational models that explain patterns of fMRI responses in these two areas during recognition and recall.

Differential Anterior Prefrontal Activation during the Recognition Stage of a Spatial Working Memory Task

Neuroimaging studies commonly show widespread activations in the prefrontal cortex during various forms of working memory and long-term memory tasks. However, the anterior prefrontal cortex (aPFC, Brodmann area 10) has been mainly associated with retrieval in episodic memory, and its role in working memory is less clear. We conducted an event-related functional magnetic resonance imaging study to examine brain activations in relation to recognition in a spatial delayed-recognition task. Similar to the results from previous findings, several frontal areas were strongly activated during the recognition phase of the task, including the aPFC, the lateral PFC and the anterior cingulate cortex. Although the aPFC was more active during the recognition phase, it was also active during the delay phase of the spatial working memory task. In addition, the aPFC showed greater activity in response to negative probes (non-targets) than to positive probes (targets). While our analyses focused on examining signal changes in the aPFC, other prefrontal regions showed similar effects and none of the areas were more active in response to the positive probes than to the negative probes. Our findings support the conclusion that the aPFC is involved in working memory and particularly in processes that distinguish target and non-target stimuli during recognition.

Is medial temporal lobe activation specific for encoding long-term memories?

Several neuroimaging studies have consistently demonstrated the critical involvement of prefrontal cortices and medial temporal lobes during long-term encoding. While the contribution of prefrontal lobes to working memory is well established, the role of the MTL structures remains controversial. To address this issue, we registered the neuromagnetic brain patterns of eight adult volunteers while they performed two working memory tasks (verbal and spatial) using magnetoencephalography (MEG). MEG recordings can provide real-time measures of brain activity, thus allowing detailed tracking of the time-course of brain activation during the encoding phase. We detected sustained and material-specific activity on the MTLs during the encoding phase of a working memory task, based on verbal and spatial information. Two peaks of activation were noted in the left MTL during word encoding in two non-consecutive time periods (500-600 ms and 700-800 ms after stimulus onset). Right MTL laterality was found for encoding locations when we collapsed activity sources in a wider time period (400-800 ms). In addition, we provided the spatiotemporal profiles of what seems to be two different brain circuits specific for each type of material. Finally, following an emerging conceptualization of working memory, we hypothesized that encoding processes mediated by the MTL to long-term memory would also apply to working memory.

Abnormal brain activity in prefrontal brain regions in abstinent marijuana users

We used PET (15)O and a modified version of the Stroop task to determine if 25-day abstinent heavy marijuana (MJ) users have persistent deficits in executive cognitive functioning (ECF) and brain activity. Performance on a modified version of the Stroop task and brain activity was compared between 25-day abstinent, heavy marijuana users (n = 11), and a matched comparison group (n = 11). The 25-day abstinent marijuana users showed no deficits in performance on the modified version of the Stroop task when compared to the comparison group. Despite the lack of performance differences, the marijuana users showed hypoactivity in the left perigenual anterior cingulate cortex (ACC) and the left lateral prefrontal cortex (LPFC) and hyperactivity in the hippocampus bilaterally, when compared to the comparison group. These results suggest that marijuana users display persistent metabolic alterations in brain regions responsible for ECF. It may be that marijuana users recruit an alternative neural network as a compensatory mechanism during performance on a modified version of the Stroop task. These differences in brain activity may be a common denominator in the evolution of maladaptive behaviors such as substance abuse and other neuropsychiatric disorders.

Representation of attitudinal knowledge: role of prefrontal cortex, amygdala and parahippocampal gyrus

It has been proposed that behavior is influenced by representations of different types of knowledge: action representations, event knowledge, attitudes and stereotypes. Attitudes (representations of a concept or object and its emotional evaluation) allow us to respond quickly to a given stimulus. In this study, we explored the representation and inhibition of attitudes. We show that right dorsolateral prefrontal cortex mediates negative attitudes whereas left ventrolateral prefrontal cortex mediates positive attitudes. Parahippocampal regions and amygdala mediate evaluative processing. Furthermore, anxiety modulates right dorsolateral prefrontal activation during negative attitude processing. Inhibition of negative attitudes activates left orbitofrontal cortex: a region that when damaged is associated with socially inappropriate behavior in patients. Inhibition of positive attitudes activates a brain system involving right inferior frontal gyrus and bilateral anterior cingulate. Thus, we show that there are dissociable networks for the representation and inhibition of attitudes.

Common fronto-parietal activity in attention, memory, and consciousness: shared demands on integration?

Fronto-parietal activity has been frequently observed in fMRI and PET studies of attention, working memory, and episodic memory retrieval. Several recent fMRI studies have also reported fronto-parietal activity during conscious visual perception. A major goal of this review was to assess the degree of anatomical overlap among activation patterns associated with these four functions. A second goal was to shed light on the possible cognitive relationship of processes that relate to common brain activity across functions. For all reviewed functions we observed a consistent and overlapping pattern of brain activity. The overlap was most pronounced for the bilateral parietal cortex (BA 7 and BA 40; close to the intraparietal sulcus), and dorsolateral prefrontal cortex (right BA 9 and left BA 6). The common fronto-parietal activity will be discussed in terms of processes related to integration of distributed representations in the brain.

An integrated theory of the mind

Adaptive control of thought-rational (ACT-R; J. R. Anderson & C. Lebiere, 1998) has evolved into a theory that consists of multiple modules but also explains how these modules are integrated to produce coherent cognition. The perceptual-motor modules, the goal module, and the declarative memory module are presented as examples of specialized systems in ACT-R. These modules are associated with distinct cortical regions. These modules place chunks in buffers where they can be detected by a production system that responds to patterns of information in the buffers. At any point in time, a single production rule is selected to respond to the current pattern. Subsymbolic processes serve to guide the selection of rules to fire as well as the internal operations of some modules. Much of learning involves tuning of these subsymbolic processes. A number of simple and complex empirical examples are described to illustrate how these modules function singly and in concert.

Event-related FMRI evidence of frontotemporal involvement in aberrant response inhibition and task switching in attention-deficit/hyperactivity disorder

OBJECTIVE

Response inhibition deficits are characteristic of individuals with attention-deficit/hyperactivity disorder (ADHD). Previous functional magnetic resonance imaging (fMRI) studies investigating the neural correlates of this dysfunction have used block designs, making it difficult to disentangle activation differences specifically related to response inhibition from activation differences related to subprocesses involved in task performance. The current study was designed to further enhance our understanding of this critical function in individuals with ADHD using event-related fMRI.

METHOD

Ten adolescent boys diagnosed with ADHD, combined type, and 12 typically developing controls completed a Go/NoGo task modified to control for novelty processing.

RESULTS

The ADHD group made significantly more errors of omission and more errors of commission than the control group. Further, compared with controls, individuals with ADHD showed marked abnormalities in brain activation during response inhibition, including hypoactivation of the anterior/mid-cingulate cortex extending to the supplementary motor area and hyperactivation of the left temporal gyrus.

CONCLUSIONS

The authors suggest that underactivation in frontal regions reflects core deficits in response/task-switching abilities for the ADHD group.

Visual search and memory search engage extensive overlapping cerebral cortices: an fMRI study

Previous studies have investigated neural correlates of visual search and memory search independently, but none of those studies examined whether cortical regions involved in these searches are overlapping or segregated by directly comparing the two types of search. In this study, we compared the cortical regions involved in visual search and memory search in the same functional magnetic resonance imaging (fMRI) experiment run on the same subjects, using identical stimuli and time courses of stimulus presentation. The right dorsolateral prefrontal cortex (DLPFC), the left frontal eye field (FEF), the right precuneus and cuneus, and the left cerebellum were activated by both visual search and memory search. We suggest that the right DLPFC is associated with the process of monitoring and manipulating multiple elements, while the left FEF is involved in cognitive planning. We also propose that the right precuneus and cuneus as well as the left cerebellum are responsible for both spatial and nonspatial shifts of attention, including attentional shifts in long-term memory, although each of these regions has a slightly different role.

The 3-D prefrontal cortex: Hemispheric asymmetries in prefrontal activity and their relation to memory retrieval processes

Neuroimaging results have raised interest in characterizing hemispheric asymmetries in prefrontal activity during different types of memory retrieval tasks. In this issue, Dobbins et al. and Mitchell et al. report results suggesting that the two hemispheres of the prefrontal cortex may indeed make different contributions to memory retrieval. Here, I discuss these findings within the context of studies characterizing more general processing differences between the cerebral hemispheres and studies characterizing prefrontal organization along the dorsal-ventral and anterior-posterior dimensions.

The relationship between volumetric brain changes and cognitive function: a family study on schizophrenia

BACKGROUND

We examined the cerebral correlates of intelligence, memory, and executive processing in 56 patients with schizophrenia or schizoaffective disorder and 90 of their nonpsychotic relatives to establish whether the pattern of structure--function relationships in these two groups was different from that in 55 control subjects.

METHODS

Magnetic resonance imaging data were acquired, and volumetric measurements were made for whole brain, prefrontal region, lateral ventricles, third ventricle, temporal lobes, hippocampi, and cerebellum.

RESULTS

In the total sample, full intelligence quotient (IQ) and verbal IQ correlated with the volume of the whole brain and right hippocampus; the latter was also associated with performance IQ. Left hippocampal size was associated with verbal IQ and, in control subjects and nonpsychotic relatives only, with estimated full IQ. Delayed verbal memory was linked to cerebellar and inversely to left hippocampal volume. Discrepancies in the relationship pattern emerged in patients with schizophrenia between left hippocampus and measures of IQ and verbal memory.

CONCLUSIONS

The latter data indicate a loss of a normal structure--function relationship in schizophrenia and might reflect a functional compensation occurring secondary to early neurodevelopmental impairment.

Dissociable effects of arousal and valence on prefrontal activity indexing emotional evaluation and subsequent memory: an event-related fMRI study

Prefrontal cortex (PFC) activity associated with emotional evaluation and subsequent memory was investigated with event-related functional MRI (fMRI). Participants were scanned while rating the pleasantness of emotionally positive, negative, and neutral pictures, and memory for the pictures was tested after scanning. Emotional evaluation was measured by comparing activity during the picture rating task relative to baseline, and successful encoding was measured by comparing activity for subsequently remembered versus forgotten pictures (Dm effect). The effect of arousal on these measures was indicated by greater activity for both positive and negative pictures than for neutral ones, and the effect of valence was indicated by differences in activity between positive and negative pictures. The study yielded three main results. First, consistent with the valence hypothesis, specific regions in left dorsolateral PFC were more activated for positive than for negative picture evaluation, whereas regions in right ventrolateral PFC showed the converse pattern. Second, dorsomedial PFC activity was sensitive to emotional arousal, whereas ventromedial PFC activity was sensitive to positive valence, consistent with evidence linking these regions, respectively, to emotional processing and self-awareness or appetitive behavior. Finally, successful encoding (Dm) activity in left ventrolateral and dorsolateral PFC was greater for arousing than for neutral pictures. This finding suggests that the enhancing effect of emotion on memory formation is partly due to an augmentation of PFC-mediated strategic, semantic, and working memory operations. These results underscore the critical role of PFC in emotional evaluation and memory, and disentangle the effects of arousal and valence across PFC regions associated with different cognitive functions.

Dissociating Confidence and Accuracy: Functional Magnetic Resonance Imaging Shows Origins of the Subjective Memory Experience

Successful memory typically implies both objective accuracy and subjective confidence, but there are instances when confidence and accuracy diverge. This dissociation suggests that there may be distinct neural patterns of activation related to confidence and accuracy. We used event-related functional magnetic resonance imaging to study the encoding of novel face–name associations, assessed with a postscan memory test that included objective measures of accuracy and subjective measures of confidence. We showed specific neural activity in the left inferior prefrontal cortex associated with trials when subjects expressed high confidence that they had chosen the correct name for the face and made a correct identification. Moreover, we found that this region was also associated with imparting high confidence when subjects chose the incorrect name. However, medial temporal lobe regions showed activity only for high-confidence correct trials. Many functional magnetic resonance imaging studies have shown that the medial temporal lobe and left prefrontal regions are particularly important for the successful formation of memories by using a combination of subjective and objective measures. Our findings suggest that these regions may be differentially involved in the objective and subjective components of memory and that the origins of confidence–accuracy dissociations may be related to incomplete activation of the neural pattern seen in successful encoding. These findings may also aid understanding of eyewitness misidentifications and memory distortions.

Functional imaging of working memory after 24 hr of total sleep deprivation

The neurobehavioral effects of 24 hr of total sleep deprivation (SD) on working memory in young healthy adults was studied using functional magnetic resonance imaging. Two tasks, one testing maintenance and the other manipulation and maintenance, were used. After SD, response times for both tasks were significantly slower. Performance was better preserved in the more complex task. Both tasks activated a bilateral, left hemisphere-dominant frontal-parietal network of brain regions reflecting the engagement of verbal working memory. In both states, manipulation elicited more extensive and bilateral (L>R) frontal, parietal, and thalamic activation. After SD, there was reduced blood oxygenation level-dependent signal response in the medial parietal region with both tasks. Reduced deactivation of the anterior medial frontal and posterior cingulate regions was observed with both tasks. Finally, there was disproportionately greater activation of the left dorsolateral prefrontal cortex and bilateral thalamus when manipulation was required. This pattern of changes in activation and deactivation bears similarity to that observed when healthy elderly adults perform similar tasks. Our data suggest that reduced activation and reduced deactivation could underlie cognitive impairment after SD and that increased prefrontal and thalamic activation may represent compensatory adaptations. The additional left frontal activation elicited after SD is postulated to be task dependent and contingent on task complexity. Our findings provide neural correlates to explain why task performance in relatively more complex tasks is better preserved relative to simpler ones after SD.

Inferior temporal, prefrontal, and hippocampal contributions to visual working memory maintenance and associative memory retrieval

Higher order cognition depends on the ability to recall information from memory and hold it in mind to guide future behavior. To specify the neural mechanisms underlying these processes, we used event-related functional magnetic resonance imaging to compare brain activity during the performance of a visual associative memory task and a visual working memory task. Activity within category-selective subregions of inferior temporal cortex reflected the type of information that was actively maintained during both the associative memory and working memory tasks. In addition, activity in the anterior prefrontal cortex and hippocampus was specifically enhanced during associative memory retrieval. These data are consistent with the view that the active maintenance of visual information is supported by activation of object representations in inferior temporal cortex, but that goal-directed associative memory retrieval additionally depends on top-down signals from the anterior prefrontal cortex and medial temporal lobes.

Functional-anatomical validation and individual variation of diffusion tractography-based segmentation of the human thalamus

Parcellation of the human thalamus based on cortical connectivity information inferred from non-invasive diffusion-weighted images identifies sub-regions that we have proposed correspond to nuclei. Here we test the functional and anatomical validity of this proposal by comparing data from diffusion tractography, cytoarchitecture and functional imaging. We acquired diffusion imaging data in eleven healthy subjects and performed probabilistic tractography from voxels within the thalamus. Cortical connectivity information was used to divide the thalamus into sub-regions with highest probability of connectivity to distinct cortical areas. The relative volumes of these connectivity-defined sub-regions correlate well with volumetric predictions based on a histological atlas. Previously reported centres of functional activation within the thalamus during motor or executive tasks co-localize within atlas regions showing high probabilities of connection to motor or prefrontal cortices, respectively. This work provides a powerful validation of quantitative grey matter segmentation using diffusion tractography in humans. Co-registering thalamic sub-regions from 11 healthy individuals characterizes inter-individual variation in segmentation and results in a population-based atlas of the human thalamus that can be used to assign likely anatomical labels to thalamic locations in standard brain space. This provides a tool for specific localization of functional activations or lesions to putative thalamic nuclei.

Prefrontal Cortex Activity Associated with Source Monitoring in a Working Memory Task

Using functional magnetic resonance imaging (fMRI), we investigated prefrontal cortex (PFC) activity during remembering specific source information (format, location judgments) versus remembering that could be based on undifferentiated information, such as familiarity (old/new recognition [ON], recency judgments). A working memory (WM) paradigm with an immediate test yielded greater activation in the lateral PFC for format and location source memory (SM) tasks than ON recognition; this SM-related activity was left lateralized. The same regions of PFC were recruited in Experiment 2 when information was tested immediately and after a filled delay. Substituting recency for location judgments (Experiment 3) resulted in an overall shift in task context that produced greater right PFC activity associated with ON and recency tasks compared to the format task, in addition to left SM-related activity. These data extend to WM previous findings from long-term memory (LTM) indicating that the left and right PFC may be differentially involved in memory attributions depending on the specificity of information evaluated. The findings also provide evidence for the continuity of evaluative processes recruited in WM and LTM.

Functional dissociations within the inferior parietal cortex in verbal working memory

Neuroimaging studies of working memory have revealed two sites in the left supramarginal gyrus that may support the short-term storage of phonological information. Activation in the left dorsal aspect of the inferior parietal cortex (DIPC) has been observed in contrasts of working memory load, whereas activation in the ventral aspect of the inferior parietal cortex (VIPC) has been found primarily in contrast of information type (verbal vs. nonverbal). Our goal was to determine whether these two areas are functionally distinct or if instead they are part of a homogeneous region with large variations in the focus of peak activity. Toward this end, we used fMRI to assess the neural response in two working memory tasks (N-back and item recognition) in which we also manipulated memory load and the type of information to be recalled (verbal vs. nonverbal). We found both DIPC and VIPC activation in the same group of subjects and further demonstrated that they have differential sensitivity to our experimental factors. Only the DIPC showed robust load effects, whereas only the VIPC showed reliable effects of information type. These results help to account for the differences observed in between-subject comparisons, and they indicate that the two regions are functionally dissociable. In contrast to the DIPC, activity of the VIPC was also recruited in the fixation and low-load conditions, a surprising result that has not been fully explored in prior studies. Despite their distinctive patterns of performance, neither of these regions displayed a pattern of activity that entirely corresponds to common assumptions of a dedicated phonological short-term store (STS). Instead, we hypothesize that the DIPC may support domain-general executive processes, while the VIPC may support phonological encoding-recoding processes central to a variety of language tasks.

Normal genetic variation, cognition, and aging

This article reviews the modulation of cognitive function by normal genetic variation. Although the heritability of "g" is well established, the genes that modulate specific cognitive functions are largely unidentified. Application of the allelic association approach to individual differences in cognition has begun to reveal the effects of single nucleotide polymorphisms on specific and general cognitive functions. This article proposes a framework for relating genotype to cognitive phenotype by considering the effect of genetic variation on the protein product of specific genes within the context of the neural basis of particular cognitive domains. Specificity of effects is considered, from genes controlling part of one receptor type to genes controlling agents of neuronal repair, and evidence is reviewed of cognitive modulation by polymorphisms in dopaminergic and cholinergic receptor genes, dopaminergic enzyme genes, and neurotrophic genes. Although allelic variation in certain genes can be reliably linked to cognition--specifically to components of attention, working memory, and executive function in healthy adults--the specificity, generality, and replicability of the effects are not fully known.

Exploratory EEG data analysis for psychophysiological experiments

A method for the exploratory analysis of electroencephalographic (EEG) data for neurophysiological experiments is presented. It is based on a time-frequency decomposition of the EEG time series, which is measured by several electrodes in the scalp surface, and includes the computation of a statistic that measures the deviations of the log-power with respect to the pre-stimulus average; the computation of a significance index for these deviations; a new type of display (the time-frequency-topography plot) for the visualization of these indices, and the segmentation of the time-frequency plane into regions with uniform activation patterns. As a particular example, an experiment to study EEG changes during figure and word categorization is analyzed in detail.

The Relationship of Three Cortical Regions to an Information-Processing Model

This research tests a model of the computational role of three cortical regions in tasks like algebra equation solving. The model assumes that there is a left parietal region-of-interest (ROI) where the problem expression is represented and transformed, a left prefrontal ROI where information for solving the task is retrieved, and a motor ROI where hand movements to produce the answer are programmed. A functional magnetic resonance imaging (fMRI) study of an abstract symbolmanipulation task was performed to articulate the roles of these three regions. Participants learned to associate words with instructions for transforming strings of letters. The study manipulated the need to retrieve these instructions, the need to transform the strings, and whether there was a delay between calculation of the answer and the output of the answer. As predicted, the left parietal ROI mainly reflected the need for a transformation and the left prefrontal ROI the need for retrieval. Homologous right ROIs showed similar but weaker responses. Neither the prefrontal nor the parietal ROIs responded to delay, but the motor ROI did respond to delay, implying motor rehearsal over the delay. Except for the motor ROI, these patterns of activity did not vary with response hand. In an ACT-R model, it was shown that the activity of an imaginal buffer predicted the blood oxygen level-dependent (BOLD) response of the parietal ROI, the activity of a retrieval buffer predicted the response of the prefrontal ROI, and the activity of a manual buffer predicted the response of the motor ROI.

The change of the brain activation patterns as children learn algebra equation solving

In a brain imaging study of children learning algebra, it is shown that the same regions are active in children solving equations as are active in experienced adults solving equations. As with adults, practice in symbol manipulation produces a reduced activation in prefrontal cortex area. However, unlike adults, practice seems also to produce a decrease in a parietal area that is holding an image of the equation. This finding suggests that adolescents' brain responses are more plastic and change more with practice. These results are integrated in a cognitive model that predicts both the behavioral and brain imaging results.

Functional MR imaging: paradigms for clinical preoperative mapping

Clinical applications of functional MR imaging include mapping of brain functions in relationship to intracranial tumors, seizure foci, or vascular malformations to determine the risk for performing surgical excision, the need for intraoperative mapping during excision, and selecting the optimal surgical approach to a lesion. A variety of paradigms are used to produce a blood-oxygen-level-dependent response in various brain regions, which can be identified with functional MR imaging. The paradigms used include active motor, language, or cognitive tasks, and passive tactile, auditory, or visual stimuli. Activation usually indicates the location of eloquent cortex. Lack of function in a region cannot be assumed when functional MR imaging shows absence of activation within the region.

A functional MRI study of visual oddball: evidence for frontoparietal dysfunction in subjects at risk for alcoholism

Attending to rare stimuli interspersed among repetitive frequent stimuli produces a positive scalp potential at 300 to 600 ms after the target stimulus onset; this potential is known as the P300 wave. Although there is clear evidence of low visual P300 in subjects at high risk (HR) for developing alcoholism, the functional neuroanatomical correlates have not been studied. Functional and high-resolution anatomical magnetic resonance images were collected during the performance of a visual oddball task, from six control (low risk-LR) subjects with high P300s and eight HR subjects with low P300s. All the HR subjects were offspring of male alcoholics. The data were analyzed using a randomization-based statistical method that accounts for multiple comparisons, requires no assumptions about the noise structure of the data, and does not require spatial or temporal smoothing. Target counts showed that all subjects performed the task comparably. Analysis of the functional magnetic resonance imaging (fMRI) data revealed two areas with significantly lower activation in the HR group when compared to the LR group: the bilateral inferior parietal lobule (BA 40), and the bilateral inferior frontal gyrus (BA 44). Inferior parietal lobule showed significantly lower activation in the HR group in contrast to the LR group, and inferior frontal gyrus was not activated in the HR group but was only activated in the LR group. This finding indicates that a dysfunctional frontoparietal circuit may underlie the low P300 responses seen in HR subjects. This perhaps implies a deficiency in the rehearsal component of the working memory system.

fMRI differences in encoding and retrieval of pictures due to encoding strategy in the elderly

Functional MRI (fMRI) was used to examine the neural correlates of depth of processing during encoding and retrieval of photographs in older normal volunteers (n = 12). Separate scans were run during deep (natural vs. man-made decision) and shallow (color vs. black-and-white decision) encoding and during old/new recognition of pictures initially presented in one of the two encoding conditions. A baseline condition consisting of a scrambled, color photograph was used as a contrast in each scan. Recognition accuracy was greater for the pictures on which semantic decisions were made at encoding, consistent with the expected levels of processing effect. A mixed-effects model was used to compare fMRI differences between conditions (deep-baseline vs. shallow-baseline) in both encoding and retrieval. For encoding, this contrast revealed greater activation associated with deep encoding in several areas, including the left parahippocampal gyrus (PHG), left middle temporal gyrus, and left anterior thalamus. Increased left hippocampal, right dorsolateral, and inferior frontal activations were found for recognition of items that had been presented in the deep relative to the shallow encoding condition. We speculate that the modulation of activity in these regions by the depth of processing manipulation shows that these regions support effective encoding and successful retrieval. A direct comparison between encoding and retrieval revealed greater activation during retrieval in the medial temporal (right hippocampus and bilateral PHG), anterior cingulate, and bilateral prefrontal (inferior and dorsolateral). Most notably, greater right posterior PHG was found during encoding compared to recognition. Focusing on the medial temporal lobe (MTL) region, our results suggest a greater involvement of both anterior MTL and prefrontal regions in retrieval compared to encoding.

The contribution of executive processes to deceptive responding

We measured behavioral responses (RT) and recorded event-related brain potentials (ERPs) when participants made truthful and deceptive responses about perceived and remembered stimuli. Participants performed an old/new recognition test under three instructional conditions: Consistent Truthful, Consistent Deceptive and Random Deceptive. Compared to Consistent Truthful responses, Consistent Deceptive responses to both perceived and remembered stimuli produced the same pattern of less accurate, slower and more variable responses and larger medial frontal negativities (MFN). The MFN is thought to reflect activity in anterior cingulate cortex, a brain area involved in monitoring actions and resolving conflicting response tendencies. The Random Deceptive condition required participants to strategically monitor their long-term response patterns to accommodate a deceptive strategy. Even compared to the Consistent Deceptive condition, RTs in the Random Deceptive condition were significantly slower and more variable and MFN activity increased significantly. MFN scalp distribution results revealed the presence of three different patterns of brain activity; one each for truthful responses, deceptive responses and strategic monitoring. Thus, the data indicate that anterior cingulate cortex plays a key role in making deceptive responses.

Putting names to faces: successful encoding of associative memories activates the anterior hippocampal formation

The ability to form associations between previously unrelated items of information, such as names and faces, is an essential aspect of episodic memory function. The neural substrate that determines success vs. failure in learning these associations remains to be elucidated. Using event-related functional MRI during the encoding of novel face-name associations, we found that successfully remembered face-name pairs showed significantly greater activation in the anterior hippocampal formation bilaterally and left inferior prefrontal cortex, compared to pairs that were forgotten. Functional connectivity analyses revealed significant correlated activity between the right and left hippocampus and neocortical regions during successful, but not attempted, encoding. These findings suggest that anterior regions of the hippocampal formation, in particular, are crucial for successful associative encoding and that the degree of coordination between hippocampal and neocortical activity may predict the likelihood of subsequent memory.

Dementia rating and nicotinic receptor expression in the prefrontal cortex in schizophrenia

BACKGROUND

The etiology of dementia that occurs in patients with schizophrenia is not well understood. Nicotinic acetylcholine receptors have been implicated in cognitive function, and deficits in these receptors have been reported in schizophrenia.

METHODS

The present study investigates possible associations of nicotinic receptor subunit expression in the dorsal lateral prefrontal cortex, an area known to be affected in schizophrenia, and dementia rating.

RESULTS

alpha7 immunoreactivity was reduced by 20% to 28% and [(3)H]epibatidine binding was increased twofold in groups of patients with schizophrenia compared to normal control subjects matched for age, postmortem delay, and low levels of brain nicotine and cotinine. In contrast, no significant differences in alpha4, alpha3, or beta2 immunoreactivity or alpha7 messenger RNA expression were observed in schizophrenia patients compared with control subject values. Clinical dementia ratings in patients with schizophrenia were correlated with neither [(3)H]epibatidine binding nor nicotinic receptor subunit expression.

CONCLUSIONS

These data indicate no relationship between the trend for reduced neocortical alpha7 subunit protein expression in schizophrenia and dementia. Further investigations are required to establish whether the reduction in alpha7 protein in the dorsal lateral prefrontal cortex is associated with clinical features other than dementia in schizophrenia.

A combined neuropsychological and neuroimaging study of topographical and non-verbal memory in semantic dementia

A combined neuropsychological and neuroimaging investigation was carried out on a patient (O.I.) with semantic dementia who had asymmetrical temporal lobe atrophy, greater on the left. His performance on tests of verbal memory was gravely impaired. Similarly, his visual memory as indexed by recognition of unfamiliar faces was impaired. By contrast, his recognition memory for topographical memoranda (e.g. buildings, landscapes) and ability to find his way around was preserved. In order to identify the neural substrates supporting the preserved recognition of static topographical memoranda, O.I. was scanned using positron emission tomography (PET) during the encoding and recognition of building and landscape stimuli. In common with control subjects, during encoding O.I. activated parahippocampal cortex bilaterally, along with bilateral temporo-parietal, retrosplenial and left frontal cortices. During recognition, both patient and controls activated right parahippocampal, right superior parietal and right frontal cortices. Notably, control subjects, but not O.I., also activated at encoding the precuneus and at recognition the retrosplenial cortex. This allows the conclusion that these two areas while involved may not be necessary for topographical memory. Interestingly, the patient also activated regions that were not evident in control subjects both during encoding and recognition. These additional areas of activation may be necessary in a compensatory role. Overall, these data represent the first reported assessment of the functional integrity of degenerating brain tissue and its contribution to preserved topographical memory. The combination of the neuropsychological and neuroimaging approaches may provide insights into the functional-anatomy of memory while having clinical utility for the assessment of residual brain tissue.

A new electrophysiological index for working memory load in humans

Working memory, the ability to store and simultaneously manipulate information, is affected in several neuropsychiatric disorders which lead to severe cognitive and functional deficits. An electrophysiological marker for this process could help identify early cerebral function abnormalities. In subjects performing working memory-specific n-back tasks, event-related potential analysis revealed a positive-negative waveform (PNwm) component modulated in amplitude by working memory load. It occurs in the expected time range for this process, 140-280 ms after stimulus onset, superimposed on the classical P200 and N200 components. Independent Component Analysis extracted two functional components with latencies and topographical scalp distributions similar to the PNwm. Our results imply that the PNwm represents a new electrophysiological index for working memory load in humans.

Four facets of a single brain: behaviour, cerebral blood flow/metabolism, neuronal activity and neurotransmitter dynamics

Is functional neuroimaging a royal way to understand brain function or is it a new phrenology without an exact understanding what we measure? After two decades of imaging revolution, more and more authors ask this question. Brain functions are multidimensional, which can be approached from the point of (1) behavioural measures, (2) brain activation as reflected by blood flow and metabolic changes, (3) electrical activity of cells and cell-populations, and (4) neurotransmitter dynamics (release, receptor binding and reuptake). Using imaging techniques, we must take into consideration that even during the simplest task all of these processes operate in a closely interacting manner. Therefore, before drawing final conclusions about brain functions on the basis of a single aspect of these mechanisms, we must clarify the exact relationship among them. In this paper, we address this issue in order to draw attention to a number of uncertainties and controversies in the relationship of the four facets of brain functions.

Competition and representation during memory retrieval: roles of the prefrontal cortex and the posterior parietal cortex

In this functional-MRI study we examined the hypothesis that the prefrontal cortex responds differently to the extent of competition during retrieval, whereas the parietal cortex is responsible for problem representation that should not be directly related to the competition. Participants mastered arbitrary person-location pairs, and their recognition memory was tested in a functional-MRI session. The pairs were constructed such that a person was associated with one, two, or three different locations and vice versa. The recognition time increased with the number of associations, reflecting increased competition. A confirmatory analysis of imaging data with prespecified prefrontal and parietal regions showed that, although both regions were highly involved during memory retrieval, only the prefrontal region responded to the levels of competition. This result was consistent with predictions of an information-processing model as well as with an exploratory identification of regions of interest.

Common prefrontal activations during working memory, episodic memory, and semantic memory

Regions of the prefrontal cortex (PFC) are typically activated in many different cognitive functions. In most studies, the focus has been on the role of specific PFC regions in specific cognitive domains, but more recently similarities in PFC activations across cognitive domains have been stressed. Such similarities may suggest that a region mediates a common function across a variety of cognitive tasks. In this study, we compared the activation patterns associated with tests of working memory, semantic memory and episodic memory. The results converged on a general involvement of four regions across memory tests. These were located in left frontopolar cortex, left mid-ventrolateral PFC, left mid-dorsolateral PFC and dorsal anterior cingulate cortex. These findings provide evidence that some PFC regions are engaged during many different memory tests. The findings are discussed in relation to theories about the functional contribution of the PFC regions and the architecture of memory.

Attention-related activity during episodic memory retrieval: a cross-function fMRI study

In functional neuroimaging studies of episodic retrieval (ER), activations in prefrontal, parietal, anterior cingulate, and thalamic regions are typically attributed to episodic retrieval processes. However, these activations are also frequent during visual attention (VA) tasks, suggesting that their role in ER may reflect attentional rather than mnemonic processes. To investigate this possibility, we directly compared brain activity during ER and VA tasks using event-related fMRI. The ER task was a word recognition test with a retrieval mode component, and the VA task was a target detection task with a sustained attention component. The study yielded three main findings. First, a common fronto-parietal-cingulate-thalamic network was found for ER and VA, suggesting that the involvement of these regions during ER reflects general attentional processes. This idea is compatible with some of the interpretations proposed in the ER literature (e.g. postretrieval monitoring), which may be rephrased in terms of attentional processes. Second, several subregions were differentially involved in ER versus VA. For example, the frontopolar cortex and the precuneus were more activated for ER than for VA, possibly reflecting retrieval mode and processing of internally generated stimuli, respectively. Finally, the study yielded an unexpected finding: some medial temporal lobe regions were similarly activated for ER and VA. This finding suggests that the medial temporal lobes may be involved in indexing representations within the focus of consciousness, regardless of whether they are mnemonic or perceptual. Overall, the present results suggest that many of the activations attributed to specific cognitive processes, such as episodic memory, may actually reflect more general cognitive operations.

Prefrontal activity associated with working memory and episodic long-term memory

Many recent neuroimaging studies have highlighted the role of prefrontal regions in the sustained maintenance and manipulation of information over short delays, or working memory (WM). In addition, neuroimaging findings have highlighted the role of prefrontal regions in the formation and retrieval of memories for events, or episodic long-term memory (LTM), but it remains unclear whether these regions are distinct from those that support WM. We used event-related functional magnetic resonance imaging (fMRI) to identify patterns of prefrontal activity associated with encoding and recognition during WM and LTM tasks performed by the same subjects. Results showed that the same bilateral ventrolateral prefrontal regions (at or near Brodmann's Areas [BA] 6, 44, 45, and 47) and dorsolateral prefrontal regions (BA 9/46) were engaged during encoding and recognition within the context of WM and LTM tasks. In addition, a region situated in the left anterior middle frontal gyrus (BA 10/46) was engaged during the recognition phases of the WM and LTM tasks. These results support the view that the same prefrontal regions implement reflective processes that support both WM and LTM.

Lateralization of prefrontal activity during episodic memory retrieval: evidence for the production-monitoring hypothesis

We propose a new hypothesis concerning the lateralization of prefrontal cortex (PFC) activity during verbal episodic memory retrieval. The hypothesis states that the left PFC is differentially more involved in semantically guided information production than is the right PFC, and that the right PFC is differentially more involved in monitoring and verification than is the left PFC. This "production-monitoring hypothesis" differs from the existing "systematic-heuristic hypothesis," which proposes that the left PFC is primarily involved in systematic retrieval operations, and the right PFC in heuristic retrieval operations. To compare the two hypotheses, we measured PFC activity using positron emission tomography (PET) during the performance of four episodic retrieval tasks: stem cued recall, associative cued recall, context recognition (source memory), and item recognition. Recall tasks emphasized production processes, whereas recognition tasks emphasized monitoring processes. Stem cued recall and context-recognition tasks underscored systematic operations, whereas associative cued recall and item-recognition tasks underscored heuristic operations. Consistent with the production-monitoring hypothesis, the left PFC was more activated for recall than for recognition tasks and the right PFC was more activated for recognition than for recall tasks. Inconsistent with the systematic-heuristic hypothesis, the left PFC was more activated for heuristic than for systematic tasks and the right PFC showed the converse result. Additionally, the study yielded activation differences outside the PFC. In agreement with a previous recall/recognition PET study, anterior cingulate, cerebellar, and striatal regions were more activated for recall than for recognition tasks, and the converse occurred for posterior parietal regions. A right medial temporal lobe region was more activated for stem cued recall and context recognition than for associative cued recall and item recognition, possibly reflecting perceptual integration. In sum, the results provide evidence for the production-monitoring hypothesis and clarify the role of different brain regions typically activated in PET and functional magnetic resonance imaging (fMRI) studies of episodic retrieval.

Functional connectivity in the resting brain: a network analysis of the default mode hypothesis

Functional imaging studies have shown that certain brain regions, including posterior cingulate cortex (PCC) and ventral anterior cingulate cortex (vACC), consistently show greater activity during resting states than during cognitive tasks. This finding led to the hypothesis that these regions constitute a network supporting a default mode of brain function. In this study, we investigate three questions pertaining to this hypothesis: Does such a resting-state network exist in the human brain? Is it modulated during simple sensory processing? How is it modulated during cognitive processing? To address these questions, we defined PCC and vACC regions that showed decreased activity during a cognitive (working memory) task, then examined their functional connectivity during rest. PCC was strongly coupled with vACC and several other brain regions implicated in the default mode network. Next, we examined the functional connectivity of PCC and vACC during a visual processing task and show that the resultant connectivity maps are virtually identical to those obtained during rest. Last, we defined three lateral prefrontal regions showing increased activity during the cognitive task and examined their resting-state connectivity. We report significant inverse correlations among all three lateral prefrontal regions and PCC, suggesting a mechanism for attenuation of default mode network activity during cognitive processing. This study constitutes, to our knowledge, the first resting-state connectivity analysis of the default mode and provides the most compelling evidence to date for the existence of a cohesive default mode network. Our findings also provide insight into how this network is modulated by task demands and what functions it might subserve.

Brain imaging of human memory systems: between-systems similarities and within-system differences

There is much evidence for the existence of multiple memory systems. However, it has been argued that tasks assumed to reflect different memory systems share basic processing components and are mediated by overlapping neural systems. Here we used multivariate analysis of PET-data to analyze similarities and differences in brain activity for multiple tests of working memory, semantic memory, and episodic memory. The results from two experiments revealed between-systems differences, but also between-systems similarities and within-system differences. Specifically, support was obtained for a task-general working-memory network that may underlie active maintenance. Premotor and parietal regions were salient components of this network. A common network was also identified for two episodic tasks, cued recall and recognition, but not for a test of autobiographical memory. This network involved regions in right inferior and polar frontal cortex, and lateral and medial parietal cortex. Several of these regions were also engaged during the working-memory tasks, indicating shared processing for episodic and working memory. Fact retrieval and synonym generation were associated with increased activity in left inferior frontal and middle temporal regions and right cerebellum. This network was also associated with the autobiographical task, but not with living/non-living classification, and may reflect elaborate retrieval of semantic information. Implications of the present results for the classification of memory tasks with respect to systems and/or processes are discussed.