The neural correlates of updating information in verbal working memory

Characterising the unity and diversity of executive functions in a within-subject fMRI study

Behavioural studies investigating the relationship between Executive Functions (EFs) demonstrated evidence that different EFs are correlated with each other, but also that they are partially independent from each other. Neuroimaging studies investigating such an interrelationship with respect to the functional neuroanatomical correlates are sparse and have revealed inconsistent findings. To address this question, we created four tasks derived from the same basic paradigm, one each for updating, inhibition, switching, and dual-tasking. We assessed brain activity through functional magnetic resonance imaging (fMRI) in twenty-nine participants while they performed the four EF tasks plus control tasks. For the analysis, we first determined the neural correlates of each EF by subtracting the respective control tasks from the EF tasks. We tested for unity in EF tasks by calculating the conjunction across these four "EF-minus-control" contrasts. This identified common areas including left lateral frontal cortices [middle and superior frontal gyrus (BA 6)], medial frontal cortices (BA 8) as well as parietal cortices [inferior and superior parietal lobules (BA 39/7)]. We also observed areas activated by two or three EF tasks only, such as frontoparietal areas [e.g., SFG (BA8) right inferior parietal lobule (BA 40), left precuneus (BA 7)], and subcortical regions [bilateral thalamus (BA 50)]. Finally, we found areas uniquely activated for updating [bilateral MFG (BA 8) and left supramarginal gyrus (BA 39)], inhibition (left IFG BA 46), and dual-tasking [left postcentral gyrus (BA 40)]. These results demonstrate that the functional neuroanatomical correlates of the four investigated EFs show unity as well as diversity.

Brain Substrates of Time-Based Prospective Memory Decline in Aging: A Voxel-Based Morphometry and Diffusion Tensor Imaging Study

Time-based prospective memory (TBPM) allows us to remember to perform intended actions at a specific time in the future. TBPM is sensitive to the effects of age, but the neural substrates of this decline are still poorly understood. The aim of the present study was thus to better characterize the brain substrates of the age-related decline in TBPM, focusing on macrostructural gray matter and microstructural white matter integrity. We administered a TBPM task to 22 healthy young (26 ± 5.2 years) and 23 older (63 ± 5.9 years) participants, who also underwent volumetric magnetic resonance imaging and diffusion tensor imaging scans. Neuroimaging analyses revealed lower gray matter volumes in several brain areas in older participants, but these did not correlate with TBPM performance. By contrast, an age-related decline in fractional anisotropy in several white-matter tracts connecting frontal and occipital regions did correlate with TBPM performance, whereas there was no significant correlation in healthy young subjects. According to the literature, these tracts are connected to the anterior prefrontal cortex and the thalamus, 2 structures involved in TBPM. These results confirm the view that a disconnection process occurs in aging and contributes to cognitive decline.

Ecological relevance of the Iowa gambling task in patients with Alzheimer's disease and mild cognitive impairment

BACKGROUND

In spite of their extensive use, the ecological relevance of tasks dedicated to assessing real-world decision-making in a laboratory setting remains unclear.

OBJECTIVES

Our study aimed to evaluate the relationship between decision-making and behavioral competency and awareness of limitations.

METHODS

A total of 20 patients with Alzheimer's disease (AD), 20 with amnestic mild cognitive impairment (aMCI) and 20 healthy controls (HC) were assessed for decision-making using the Iowa Gambling Task (IGT). Behavioral competency was evaluated by the Patient Competency Rating Scale (PCRS), which requires each participant and a relative to answer the same 30 questions on participant's competency and to rate each item, while awareness of limitations was evaluated by subtracting the self-rated score from the relative-rated score.

RESULTS

Using the median-split approach, the proportion of disadvantageous decision-makers was higher in both the MCI and AD groups than in HC (P=0.02 and P=0.03, respectively), with no differences between clinical groups. The percentage of participants with poorer behavioral competency was also higher in the MCI and AD than in the HC (self-rated: P=0.025 and P=0.01, respectively; relative-rated: P=0.008 and P=0.008, respectively), again with no differences between MCI and AD. All groups were comparable in awareness. For all participants, disadvantageous decision-making was associated with both reduced behavioral competency and poor awareness of limitations (OR: 3.47, P=0.03 and OR: 5.4, P=0.004, respectively).

CONCLUSION

Our findings support the ecological relevance of the IGT. Behavioral competency integrity and awareness of limitations are both associated with advantageous decision-making profiles.

Estimating the executive demands of a one-back choice reaction time task by means of the selective interference paradigm

The present study proposes a new executive task, the one-back choice reaction time (RT) task, and implements the selective interference paradigm to estimate the executive demands of the processing components involved in this task. Based on the similarities between a one-back choice RT task and the n-back updating task, it was hypothesized that one-back delaying of a choice reaction involves executive control. In three experiments, framed within Baddeley's (1986) working-memory model, a one-back choice RT task, a choice RT task, articulatory suppression, and matrix tapping were performed concurrently with primary tasks involving verbal, visuospatial, and executive processing. The results demonstrate that one-back delaying of a choice reaction interferes with tasks requiring executive control, while the potential interference at the level of the verbal or visuospatial working memory slave systems remains minimal.

Updating of working memory in ecstasy polydrug users: Findings from fNIRS

AIMS/OBJECTIVES

Cognitive deficits are now well documented in ecstasy (MDMA) users with type and relative demand of task emerging as important factors. The updating component of executive processes appears to be particularly affected. The study reported here used functional near infrared spectroscopy imaging to investigate changes in cortical haemodynamics during memory updating.

METHOD

Twenty ecstasy users and 20 non-users completed verbal and spatial memory updating tasks and brain blood oxygenation and deoxygenation change was measured using functional near infrared spectroscopy.

RESULTS

There was no interaction between group and difficulty on the updating tasks, though there was a significant main effect of difficulty on both tasks. The effects of group approached significance on the verbal updating task. There were significant differences in blood oxygenation and deoxygenation change at optodes centred over the right and left dorsolateral prefrontal cortex, with ecstasy users showing greater blood oxygenation than the other groups.

DISCUSSION

The lack of a behavioural difference on both tasks but presence of blood oxygenation and deoxygenation changes in letter updating provides support for the notion that ecstasy-polydrug users are investing more effort to achieve the same behavioural output. Total lifetime dose was high, and recency of use was significantly related to most changes, suggesting that heavy and recent use may be particularly detrimental.

The neural basis of updating: Distinguishing substitution processes from other concurrent processes

Most previous studies of updating processes have not been able to contrast processes of substituting items in memory with other concurrent processes. In the present investigation, we used a new task called "number updating" and an fMRI protocol to contrast the activation of trials that require item substitution (adding a new item to the working memory representation and suppressing an old item) with trials that involve no substitution (discarding the new item). Trials that require item substitution activated the dorsolateral prefrontal cortex, the posterior medial frontal cortex and the parietal lobes, areas typically seen activated for working memory tasks in general. Trials that do not require substitution activated the anterior medial frontal cortex. Studies examining executive functions have associated this area with cognitive conflict, and may represent suppression of the substitution processes.

Brain training in progress: a review of trainability in healthy seniors

The cognitive deterioration associated with aging is accompanied by structural alterations and loss of functionality of the frontostriatal dopamine system. The question arises how such deleterious cognitive effects could be countered. Brain training, currently highly popular among young and old alike, promises that users will improve on certain neurocognitive skills, and this has indeed been confirmed in a number of studies. Based on these results, it seems reasonable to expect beneficial effects of brain training in the elderly as well. A selective review of the existing literature suggests, however, that the results are neither robust nor consistent, and that transfer and sustained effects thus far appear limited. Based on this review, we argue for a series of elements that hold potential for progress in successful types of brain training: (1) including flexibility and novelty as features of the training, (2) focusing on a number of promising, yet largely unexplored domains, such as decision-making and memory strategy training, and (3) tailoring the training adaptively to the level and progress of the individual. We also emphasize the need for covariance-based MRI methods in linking structural and functional changes in the aging brain to individual differences in neurocognitive efficiency and trainability in order to further uncover the underlying mechanisms.

The effects of heavy social drinking on executive function: a systematic review and meta-analytic study of existing literature and new empirical findings

BACKGROUND

Previous investigations of executive function in alcohol dependent and in social drinkers have not always produced consistent results and have not utilised key indicators of recent theoretical models of Executive Function (EF). The present paper reports the results of two studies that seek to address these limitations.

METHOD

Study 1 took the form of a systematic review and meta-analysis of studies examining EF in social drinkers. In Study 2, 41 participants completed an alcohol use inventory and were assigned to either light or heavy alcohol use via median split of average weekly dose. Participants completed measures of the fractionated executive processes of updating, shifting, inhibition and access to semantic memory.

RESULTS

Study 1 only identified seven studies of EF in social drinkers, and the mean effect size was non-significant. In study 2, the heavy alcohol use group performed worse on all measures of executive functioning except memory updating.

CONCLUSIONS

To our knowledge, this is the first systematic investigation of executive functioning in social drinkers. Given that the participants were non-treatment seeking social drinking students, the EF deficit in the heavy user group is particularly worrying and could increase the likelihood of developing an alcohol use disorder.

The role of executive functions in human prospective interval timing

Human timing is thought to be based on the output of an internal clock. Whilst the functioning of this clock is well documented, it is unclear which other cognitive resources may moderate timing. Brown (2006) and Rattat (2010) suggest that the central executive of working memory may be recruited during timing. However it seems likely that the fractionated executive component processes identified by Miyake et al. (2000) and Fisk and Sharp (2004) may differentially contribute to timing performance; further exploration of this was the aim of the present study. An interference paradigm was employed in which participants completed an interval production task, and tasks which have been shown to tap the four key executive component processes (shifting, inhibition, updating and access) under single and dual-task conditions. Comparison of single and dual-task performance indicated that timing always became more variable when concurrently performing a second task. Bidirectional interference only occurred between the interval production task and the memory updating task, implying that both tasks are competing for the same executive resource of updating. There was no evidence in the current study to suggest that switching, inhibition or access was involved in timing, however they may be recruited under more difficult task conditions.

The hemispheric lateralization for processing geometric word/shape combinations: the Stroop-shape effect

The authors conducted 4 experiments to test whether hemispheric lateralization occurs for the processing of geometric word-shape combinations. In 3 experiments, participants responded to geometric shapes combined with geometric words (square, circle, triangle). In the 4th experiment, stimuli were combinations of geometric shapes and non-geometric words. The authors predicted that it would take longer to respond in incongruent conditions (e.g., the word "square" combined with the shape of a circle) than in congruent conditions. The authors found the strongest incongruency effects for the dominant hemisphere--that is, the left hemisphere for responding to words and the right hemisphere for responding to shapes. A Shape Interfering Properties hypothesis (SIP) is a possible explanation for these results.

Intracranial electroencephalography reveals two distinct similarity effects during item recognition

Behavioral studies of visual recognition memory indicate that old/new decisions reflect both the similarity of the probe to the studied items (probe-item similarity) and the similarities among the studied items themselves (list homogeneity). Recording intracranial electroencephalography from 1,155 electrodes across 15 patients, we examined the oscillatory correlates of probe-item similarity and homogeneity effects in short-term recognition memory for synthetic faces. Frontal areas show increases in low-frequency oscillations with both probe-item and item-item similarity, whereas temporal lobe areas show distinct oscillatory correlates for probe-item similarity and homogeneity in the gamma band. We discuss these frontal low-frequency effects and the dissociation in the temporal lobe in terms of recent computational models of visual recognition memory.

Evidence for selective executive function deficits in ecstasy/polydrug users

Previous research has suggested that the separate aspects of executive functioning are differentially affected by ecstasy use. Although the inhibition process appears to be unaffected by ecstasy use, it is unclear whether this is true of heavy users under conditions of high demand. Tasks loading on the updating process have been shown to be adversely affected by ecstasy use. However, it remains unclear whether the deficits observed reflect the executive aspects of the tasks or whether they are domain general in nature affecting both verbal and visuo-spatial updating. Fourteen heavy ecstasy users (mean total lifetime use 1000 tablets), 39 light ecstasy users (mean total lifetime use 150 tablets) and 28 non-users were tested on tasks loading on the inhibition executive process (random letter generation) and the updating component process (letter updating, visuo-spatial updating and computation span). Heavy users were not impaired in random letter generation even under conditions designed to be more demanding. Ecstasy-related deficits were observed on all updating measures and were statistically significant for two of the three measures. Following controls for various aspects of cannabis use, statistically significant ecstasy-related deficits were obtained on all three updating measures. It was concluded that the inhibition process is unaffected by ecstasy use even among heavy users. By way of contrast, the updating process appears to be impaired in ecstasy users with the deficit apparently domain general in nature.

Ecstasy-related deficits in the updating component of executive processes

AIMS

Research shows that users of ecstasy (MDMA) exhibit deficits in executive processes. The updating component appears to be particularly susceptible. Less is known about the precise nature of such deficits. The present study sought to determine if ecstasy-related deficits in memory updating are related to serial position of items presented, or length of the list of items.

METHOD

Seventy-three ecstasy/polydrug users and seventy-three non-ecstasy users completed tasks of verbal and spatial memory running memory, recalling the most recent items, in lists of varying and unknown length. Participants were categorised according to letter and spatial span (four, five or six), producing six sub-samples for analysis.

RESULTS

Ecstasy-polydrug users were impaired in four out of the six sub-sample analyses. Three of these were due to impaired recall of earlier serial positions.

CONCLUSIONS

The results of the present study provide further support for updating deficits in ecstasy-polydrug users. The results are suggestive of a breakdown in the maintenance of information in working memory in terms of chunking; it appears that ecstasy/polydrug users are as able as non-ecstasy users to form memory 'chunks' from the items, but that such chunks are not retained as effectively.

Investigating the role of conflict resolution in memory updating by means of the one-back choice RT task

The current study is inspired by recent findings, which suggest that conflict is involved in the updating of memory representations. It directly addresses the relation between memory updating and conflict resolution by means of the one-back choice reaction time (RT) task, an updating task, which requires participants to postpone their response to stimulus n until the subsequent stimulus n + 1 has been presented. In three experiments, a more detailed analysis of the one-back choice RT task is presented in order to further identify the role of conflict resolution in memory updating. The findings demonstrate that the one-back choice RT task, which allows motor preparation just like a simple RT task, is in fact performed slower than a simple RT task because it additionally involves conflict resolution. It is further shown that also the response-stimulus interval of the one-back task involves processes that affect the amount of conflict in the task. In the "General discussion", the theoretical relevance of these findings for the concept of updating is discussed.

Effects of acute exercise on executive processing, short-term and long-term memory

In the present study, we evaluated the effects of a brief bout of exercise on executive function, short-term memory, and long-term memory tests. Eighteen young adults (mean age 22.2 years, s = 1.6) performed a set-switching test, a Brown-Peterson test, and a free-recall memory test before and after 40 min of moderate aerobic exercise on a cycle ergometer, and two control conditions. Exercise did not facilitate set switching or short-term memory, which suggests that exercise-induced arousal does not influence executive function processes involved in the reconfiguration of information in working memory. Exercise did alter specific aspects of delayed long-term memory. Free recall of items in the primacy and recency portions of the word list declined following the rest and non-exercise conditions, but was maintained after exercise, which suggests that exercise-induced arousal may facilitate the consolidation of information into long-term memory.

Self reported sleep quality and cognitive performance in ecstasy users

OBJECTIVES

Research suggests that ecstasy users exhibit psychobiological changes relative to nonusers such as altered sleep patterns and cognitive deficits. In turn, it has been suggested that sleep quality may be a mediator of such cognitive deficits in ecstasy users. The present study sought to investigate this proposed relationship.

METHODS

Aspects of cognitive functioning in 104 ecstasy users and 103 nonusers obtained from our previous studies were reanalysed to explore the extent to which ecstasy-related group differences were attributable to differences in sleep quality. Cognitive function was assessed via the computation span test, consonant updating, paired associate learning, syllogistic reasoning and word fluency. Sleep quality was measured via the Epworth Sleepiness Scale (ESS), and the Karolinska Sleepiness Scale (KSS).

RESULTS

Ecstasy users performed worse than nonusers on all cognitive measures. While no differences were observed on the ESS, ecstasy users reported greater tiredness at the beginning of testing than nonusers. When the sleep variables were included as covariates, the effects of ecstasy on all cognitive measures remained significant.

CONCLUSIONS

The results of the present study suggest little evidence for the mediating effects of sleep on cognitive function in ecstasy users.

Does caffeine modulate verbal working memory processes? An fMRI study

To assess the effect of caffeine on the functional MRI signal during a 2-back verbal working memory task, we examined blood oxygenation level-dependent regional brain activity in 15 healthy right-handed males. The subjects, all moderate caffeine consumers, underwent two scanning sessions on a 1.5-T MR-Scanner separated by a 24- to 48-h interval. Each participant received either placebo or 100 mg caffeine 20 min prior to the performance of the working memory task in blinded crossover fashion. The study was implemented as a blocked-design. Analysis was performed using SPM2. In both conditions, the characteristic working memory network of frontoparietal cortical activation including the precuneus and the anterior cingulate could be shown. In comparison to placebo, caffeine caused an increased response in the bilateral medial frontopolar cortex (BA 10), extending to the right anterior cingulate cortex (BA 32). These results suggest that caffeine modulates neuronal activity as evidenced by fMRI signal changes in a network of brain areas associated with executive and attentional functions during working memory processes.

Neural system for updating object working memory from different sources: sensory stimuli or long-term memory

Working memory (WM) is the active maintenance of currently relevant information so that it is available for use. A crucial component of WM is the ability to update the contents when new information becomes more relevant than previously maintained information. New information can come from different sources, including from sensory stimuli (SS) or from long-term memory (LTM). Updating WM may involve a single neural system regardless of source, distinct systems for each source, or a common network with additional regions involved specifically in sensory or LTM processes. The current series of experiments indicates that a single fronto-parietal network (including supplementary motor area, parietal, left inferior frontal junction, middle frontal gyrus) is active in updating WM regardless of the source of information. Bilateral cuneus was more active during updating WM from LTM than updating from SS, but the activity in this region was attributable to recalling information from LTM regardless of whether that information was to be entered into WM for future use or not. No regions were found to be more active during updating from SS than updating from LTM. Functional connectivity analysis revealed that different regions within this common update network were differentially more correlated with visual processing regions when participants updated from SS, and more correlated with LTM processing regions when participants updated from the contents of LTM. These results suggest that a single neural mechanism is responsible for controlling the contents of WM regardless of whether that information originates from a sensory stimulus or from LTM. This network of regions involved in updating WM interacts with the rest of the brain differently depending on the source of newly relevant information.

Mapping the updating process: common and specific brain activations across different versions of the running span task

Neuroimaging studies exploring the neural substrates of executive functioning have only rarely investigated whether the non-executive characteristics of the experimental executive tasks could contribute to the observed brain activations. The aim of this study was to determine cerebral activity in three different tasks involving the updating executive function. The experimental updating tasks required subjects to process strings of items (respectively letters, words, and sounds) of unknown lengths, and then to recall or identify a specific number of presented items. Conjunction and functional connectivity analyses demonstrated that the cerebral areas activated by all three experimental tasks are the left frontopolar cortex, bilateral dorsolateral prefrontal and premotor cortex, bilateral intraparietal sulcus, right inferior parietal lobule and cerebellum. Some regions of this network appear to be more specific to each updating task. These results clearly indicate that the neural substrates underlying a specific executive process (in this case, updating) are modulated by the exact requirements of the task (such as the material to process or the kind of response) and the specific cognitive processes associated with updating.

Neural correlates of age-related verbal episodic memory decline: a PET study with combined subtraction/correlation analysis

Using PET, we have determined the neural substrates of age-related verbal episodic memory decline. Twelve young and twelve older healthy volunteers (mean age; 22 and 59 years, respectively) were scanned while performing encoding and retrieval tasks. Retrieval performance was lower in old than in young subjects. The PET data were analyzed using a combined subtraction/correlation approach. Classic subtraction disclosed prefrontal rCBF increases common to both groups, distributed bilaterally during encoding and exclusively right-sided during retrieval, without between-group differences. The correlation analysis between PET activity during encoding and subsequent retrieval performance revealed significant correlations for the left hippocampal region in both groups, but for the right inferior frontal gyrus in the older subjects only. Thus, lower performance in older subjects during an episodic retrieval task may reflect a combination of (i) subtle encoding dysfunction, evidenced by more widespread activity-performance correlations and (ii) less efficient retrieval, as evidenced by unaltered activation pattern (as revealed by the classic subtraction method) despite reduced performance. These exploratory findings suggest the aged brain may be unable to compensate for reduced efficiency of right prefrontal cortex by additional left frontal activation.

Exploration of the neural substrates of executive functioning by functional neuroimaging

This review presents neuroimaging studies that have explored the cerebral substrates of executive functioning. These studies have demonstrated that different executive functions not only recruit various frontal areas but also depend upon posterior (mainly parietal) regions. These results are in accordance with the hypothesis that executive functioning relies on a distributed cerebral network that is not restricted to anterior cerebral areas. However, there exists an important heterogeneity in the cerebral areas associated with these different processes, and also between different tasks assessing the same process. Since these discrepant results could be due to the paradigms used (subtraction designs), recent results obtained with conjunction and interaction analyses are presented, which confirm the role of parietal areas in executive functioning and also demonstrate the existence of some specificity in the neural substrates of the executive processes of updating, shifting and inhibition. Finally, functional magnetic resonance imaging studies show that the activity in cerebral areas involved in executive tasks can be transient or sustained. Consequently, to better characterize the functional role of areas associated with executive functioning, it is important to take into account not only the localization of cerebral activity but also the temporal pattern of this activity.

Neural System for Controlling the Contents of Object Working Memory in Humans

Working memory (WM), the active maintenance of currently relevant information, is a flexible system allowing for fast and frequent goal-directed changes of rehearsed information. Successful WM maintenance prevents interference from distracting stimuli while allowing new task-relevant information to update the contents of WM. We used functional magnetic resonance imaging to show that when WM contents were updated, regardless of stimulus type (faces or houses), a frontoparietal network showed transient increases in activation. Some of these regions are highly similar to those identified in studies of shifting attention, supporting the idea that updating WM involves a change in the attentional priority afforded to the current perceptual input. A region within the mid-ventrolateral prefrontal cortex, near the junction of the inferior frontal sulcus and precentral sulcus (inferior frontal junction), that has previously been implicated in cognitive control, demonstrated transient increases in activity during updating as well as sustained maintenance activity. A more anterior prefrontal region, middle frontal gyrus, previously implicated in protecting the contents of WM from interfering stimuli during maintenance, demonstrated transient increases in activity during updating. The current study suggests that updating WM results from a combination of increased attention to the visual stimulus and a change in the system's interference protection state.

The differential effects of ecstasy/polydrug use on executive components: shifting, inhibition, updating and access to semantic memory

RATIONALE/OBJECTIVES

Recent theoretical models suggest that the central executive may not be a unified structure. The present study explored the nature of central executive deficits in ecstasy users.

METHODS

In study 1, 27 ecstasy users and 34 non-users were assessed using tasks to tap memory updating (computation span; letter updating) and access to long-term memory (a semantic fluency test and the Chicago Word Fluency Test). In study 2, 51 ecstasy users and 42 non-users completed tasks that assess mental set switching (number/letter and plus/minus) and inhibition (random letter generation).

RESULTS

MANOVA revealed that ecstasy users performed worse on both tasks used to assess memory updating and on tasks to assess access to long-term memory (C- and S-letter fluency). However, notwithstanding the significant ecstasy group-related effects, indices of cocaine and cannabis use were also significantly correlated with most of the executive measures. Unexpectedly, in study 2, ecstasy users performed significantly better on the inhibition task, producing more letters than non-users. No group differences were observed on the switching tasks. Correlations between indices of ecstasy use and number of letters produced were significant.

CONCLUSIONS

The present study provides further support for ecstasy/polydrug-related deficits in memory updating and in access to long-term memory. The surplus evident on the inhibition task should be treated with some caution, as this was limited to a single measure and has not been supported by our previous work.

Exploring the unity and diversity of the neural substrates of executive functioning

Previous studies exploring the neural substrates of executive functioning used task-specific analyses, which might not be the most appropriate approach due to the difficulty of precisely isolating executive functions. Consequently, the aim of this study was to use positron emission tomography (PET) to reexamine by conjunction and interaction paradigms the cerebral areas associated with three executive processes (updating, shifting, and inhibition). Three conjunction analyses allowed us to isolate the cerebral areas common to tasks selected to tap into the same executive process. A global conjunction analysis demonstrated that foci of activation common to all tasks were observed in the right intraparietal sulcus, the left superior parietal gyrus, and at a lower statistical threshold, the left lateral prefrontal cortex. These regions thus seem to play a general role in executive functioning. The right intraparietal sulcus seems to play a role in selective attention to relevant stimuli and in suppression of irrelevant information. The left superior parietal region is involved in amodal switching/integration processes. One hypothesis regarding the functional role of the lateral prefrontal cortex is that monitoring and temporal organization of cognitive processes are necessary to carry out ongoing tasks. Finally, interaction analyses showed that specific prefrontal cerebral areas were associated with each executive process. The results of this neuroimaging study are in agreement with cognitive studies demonstrating that executive functioning is characterized by both unity and diversity of processes.

Updating information in verbal working memory and executive functioning

50 older adults (M age=77.9 yr., SD=7.3; 35 women and 15 men) were tested using the updating working-memory task. They were also given the neuropsychological Wisconsin Card Sorting Test, assumed to evaluate executive functioning and the frontal cortex. A factor analysis with age, education, and verbal ability partialled out was computed on the updating task outcomes and resulted in a two-factor solution, indicating that this task requires two independent processes, interpreted as reflecting a storage component and an updating component. Partial correlations with age, education, and verbal ability partialled out indicated that Wisconsin Card Sorting Test measures were significantly associated with the factor supposed to reflect the updating process. Such results appeared consistent with the model of working memory with a central executive system involved in the updating process and related to the executive-frontal functioning, and a phonological loop system involved in the storage of verbal information and not linked to executive-frontal functions.

Study of Verbal Working Memory in Patients With Parkinson's Disease

The authors examined the nature of the working memory deficit in persons with Parkinson's disease (PD). Three hypotheses were tested: a limited storage capacity, an impaired executive component, and a reduction of psychomotor speed. Verbal working memory was assessed in 14 PD patients without dementia and 14 matched control participants. Participants were administered a classical verbal span test, working memory tasks that required either updating or manipulation capacities, and motor and psychomotor speed tasks. Patients' performance was comparable to that of control participants on the verbal span test. However, results on the working memory tasks indicated a deficit in manipulation with normal updating capacities. Motor and psychomotor slowing were found in the patient group, but slowing could not fully account for the impairment observed in the manipulation task. Results indicated that there is a genuine but selective working memory impairment in patients with PD.

From sensory processes to conscious perception

In recent years, cognitive neuroscientists have began to explore the process of how sensory information gains access to awareness. To further probe this process, event-related functional magnetic resonance imaging (fMRI) was used while testing subjects with a paradigm known as the "attentional blink." In this paradigm, visually presented information sporadically fails to reach awareness. It was found that the magnitude and time course of activation within the anterior cingulate (BA 32), medial prefrontal cortex (BA 9), and frontopolar cortex (BA 10) predicted whether or not information was consciously perceived during the critical period for the attentional blink. These results are discussed in light of a neural framework for conscious processing.

Neuroimaging studies of working memory: a meta-analysis

We performed meta-analyses on 60 neuroimaging (PET and fMRI) studies of working memory (WM), considering three types of storage material (spatial, verbal, and object), three types of executive function (continuous updating of WM, memory for temporal order, and manipulation of information in WM), and interactions between material and executive function. Analyses of material type showed the expected dorsal-ventral dissociation between spatial and nonspatial storage in the posterior cortex, but not in the frontal cortex. Some support was found for left frontal dominance in verbal WM, but only for tasks with low executive demand. Executive demand increased right lateralization in the frontal cortex for spatial WM. Tasks requiring executive processing generally produce more dorsal frontal activations than do storage-only tasks, but not all executive processes show this pattern. Brodmann's areas (BAs) 6, 8, and 9, in the superior frontal cortex, respond most when WM must be continuously updated and when memory for temporal order must be maintained. Right BAs 10 and 47, in the ventral frontal cortex, respond more frequently with demand for manipulation (including dual-task requirements or mental operations). BA 7, in the posterior parietal cortex, is involved in all types of executive function. Finally, we consider a potential fourth executive function: selective attention to features of a stimulus to be stored in WM, which leads to increased probability of activating the medial prefrontal cortex (BA 32) in storage tasks.

Near-infrared spectroscopy analysis of frontal lobe dysfunction in schizophrenia

BACKGROUND

Previous studies have shown that near-infrared spectroscopy (NIRS) has high temporal resolution, requires little restraint, and is suitable for examining the effect of psychological tasks on brain circulation. In the present study, frontal function in schizophrenic patients was analyzed by NIRS during random number generation (RNG), ruler-catching (RC), and sequential finger-to-thumb (SFT) tasks.

METHODS

Two sets of NIRS probes were attached to the foreheads of 13 schizophrenic patients and 10 control subjects approximately at Fp1-F7 and Fp2-F8. Near-infrared spectroscopy was conducted at a sampling rate of 1 Hz, with the pathlength being determined by time-resolved spectroscopy with differential pathlength factor measurements. The absolute changes in oxygenated (oxy-Hb) and deoxygenated (deoxy-Hb) hemoglobin concentrations in response to each task were measured, and total hemoglobin (total-Hb) concentration was calculated as the sum of the two.

RESULTS

During RNG task, total- and oxy-Hb concentrations increased, and deoxy-Hb decreased, but the responses were significantly smaller in schizophrenic patients. During RC task, oxy-Hb in schizophrenic patients tended to decrease, in contrast to the mostly increasing response in control subjects. No group difference was observed during SFT task.

CONCLUSIONS

Task-dependent profile of functional abnormalities was observed in schizophrenic frontal brain metabolism. These results support the usefulness of NIRS data in investigating frontal lobe dysfunction and evaluating psychopathologic condition in schizophrenic patients.

The prefrontal cortex: insights from functional neuroimaging using cognitive activation tasks

This review presents neuroimaging studies which have explored the functional anatomy of a variety of cognitive processes represented by the prefrontal cortex (PFC). Overall, these studies have demonstrated that standard prefrontal neuroactivation tasks recruit a widely distributed network within the brain of which the PFC consistently forms a part. As such, these results are in keeping with the notion that executive functions within the PFC rely not only on anterior (mainly prefrontal) brain areas, but also on posterior (mainly parietal) brain regions. Moreover, intervention of similar brain regions in a large number of different executive tasks suggests that higher-level cognitive functions may best be understood in terms of an interactive network of specialised anterior as well as posterior brain regions.

Neural correlates of "hot" and "cold" emotional processing: a multilevel approach to the functional anatomy of emotion

The neural correlates of two hypothesized emotional processing modes, i.e., schematic and propositional modes, were investigated with positron emission tomography. Nineteen subjects performed an emotional mental imagery task while mentally repeating sentences linked to the meaning of the imagery script. In the schematic conditions, participants repeated metaphoric sentences, whereas in the propositional conditions, the sentences were explicit questions about specific emotional appraisals of the imagery scenario. Five types of emotional scripts were proposed to the subjects (happiness, anger, affection, sadness, and a neutral scenario). The results supported the hypothesized distinction between schematic and propositional emotional processing modes. Specifically, schematic mode was associated with increased activity in the ventromedial prefrontal cortex whereas propositional mode was associated with activation of the anterolateral prefrontal cortex. In addition, interaction analyses showed that schematic versus propositional processing of happiness (compared with the neutral scenario) was associated with increased activity in the ventral striatum whereas "schematic anger" was tentatively associated with activation of the ventral pallidum.

Brain imaging of the central executive component of working memory

This review presents neuroimaging studies which have explored the cerebral substrates of the central executive component of the working memory model proposed by Baddeley and Hitch [Working memory (1986); Recent advances in learning and motivation (1974)]. These studies have demonstrated that different executive functions (manipulating and updating of information, dual-task coordination, inhibition and shifting processes) not only recruit various frontal areas, but also depend upon posterior (mainly parietal) regions. Such results are in agreement with the hypothesis that executive functions rely on a distributed cerebral network not restricted to anterior cerebral areas. Moreover, the intervention of similar prefrontal regions in a large number of executive tasks suggests that the central executive functioning must be understood in terms of different interactions between a network of regions rather than in terms of a specific association between one region and one higher-level cognitive process.

Honig's model of working memory and brain activation: an fMRI study

The present study investigated changes in neuronal activation with fMRI related to Honig's model of working memory, which is much less studied compared with other working memory models. In contrast to other studies which have applied recognition procedures, the primary aim with the present study was to examine brain activation when subjects had to continuously recall and forget items held in working memory. The results showed that the mid-ventrolateral frontal cortex was particularly activated in the left hemisphere, whereas the mid-dorsolateral frontal cortex was particularly activated in the right hemisphere during execution of the working memory task. The findings are discussed in relation to process- and domain-specific accounts of working memory.

Frontal lobe function, sleep loss and fragmented sleep

Recent experimental studies involving total sleep loss, sleep reduction and clinically related sleep fragmentation report impaired performance on tasks of frontal lobe or executive function, including measures of verbal fluency, creativity and planning skills. Severity of sleep disturbance in obstructive sleep apnoea syndrome (OSAS) is correlated with level of executive impairment, with some residual impairment despite treatment (continuous positive airway pressure - CPAP). Executive impairment appears to be more closely related to hypoxaemic events rather than daytime sleepiness. Studies of electroencephalographic (EEG) changes throughout the course of sleep and following sleep deprivation as well as functional neuroimaging and psychophysiological changes (event-related potentials - ERPs) following sleep deprivation provide further indication of the relative importance of the frontal regions of the brain to sleep. However, neurocognitive studies present many inconsistencies, task classification is often ambiguous and, in the absence of any unifying explanation at the level of cognitive mechanisms, the overall picture is one of a disparate range of impairment following sleep loss and sleep fragmentation. Poorly defined concepts of frontal lobe function, executive function, memory and attention, using tasks largely developed with more severe deficit levels in mind, create further difficulties in interpreting current findings.

Relationship between regional cerebral blood flow and verbal fluency in Alzheimer's disease

Category and letter verbal fluency tests are widely used for dementia detection and severity measure. Performances of these tasks have been regarded to be mainly associated with the left frontal lobe function. However, some recent studies suggest that there are different neuropsychological bases between these two tasks, and the brain region which contributes to these performances still remains unclear in Alzheimer's disease (AD). To clarify the neural basis of verbal fluency in AD, we examined the relationship between performances of these tasks and regional cerebral blood flow (rCBF). Twenty-five AD patients were administered verbal fluency tasks and single photon emission computed tomography (SPECT) scans. Thirteen cortical regions of interest were symmetrically defined in each hemisphere. Letter fluency scores were correlated significantly only with the left prefrontal (Brodmann's area (BA) 10-46) regional cerebral blood flow (rCBF). In contrast, category fluency scores were correlated most strongly with the left temporal rCBF and also with the left prefrontal (BA 10-46) rCBF. In conclusion, the present study suggests that left prefrontal (BA 10-46) dysfunction contributes to decline in both letter and category fluency scores in AD, while typical posterior dysfunction of AD has a closer relationship with decline in category fluency scores.

The neural substrate of orientation working memory

We have used positron emission tomography (PET) to identify the neural substrate of two major cognitive components of working memory (WM), maintenance and manipulation of a single elementary visual attribute, i.e., the orientation of a grating presented in central vision. This approach allowed us to equate difficulty across tasks and prevented subjects from using verbal strategies or vestibular cues. Maintenance of orientations involved a distributed fronto-parietal network, that is, left and right lateral superior frontal sulcus (SFSl), bilateral ventrolateral prefrontal cortex (VLPFC), bilateral precuneus, and right superior parietal lobe (SPL). A more medial superior frontal sulcus region (SFSm) was identified as being instrumental in the manipulative operation of updating orientations retained in the WM. Functional connectivity analysis revealed that orientation WM relies on a coordinated interaction between frontal and parietal regions. In general, the current findings confirm the distinction between maintenance and manipulative processes, highlight the functional heterogeneity in the prefrontal cortex (PFC), and suggest a more dynamic view of WM as a process requiring the coordinated interaction of anatomically distinct brain areas.

The functional anatomy of inhibition processes investigated with the Hayling task

The cortical areas involved in inhibition processes were examined with positron emission tomography (PET). The tasks administered to subjects were an adaptation of the Hayling test. In the first condition (response initiation), subjects had to complete sentences with a word clearly suggested by the context, whereas in the second condition (response inhibition), subjects had to produce a word that made no sense in the context of the sentence. Results indicated that the response initiation processes were associated to increases of activity in the left inferior frontal gyrus (BA 45/47), whereas response inhibition processes led to increases in a network of left prefrontal areas, including the middle (BA 9 and BA 10) and inferior (BA 45) frontal areas.

The unity and diversity of executive functions and their contributions to complex "Frontal Lobe" tasks: a latent variable analysis

This individual differences study examined the separability of three often postulated executive functions-mental set shifting ("Shifting"), information updating and monitoring ("Updating"), and inhibition of prepotent responses ("Inhibition")-and their roles in complex "frontal lobe" or "executive" tasks. One hundred thirty-seven college students performed a set of relatively simple experimental tasks that are considered to predominantly tap each target executive function as well as a set of frequently used executive tasks: the Wisconsin Card Sorting Test (WCST), Tower of Hanoi (TOH), random number generation (RNG), operation span, and dual tasking. Confirmatory factor analysis indicated that the three target executive functions are moderately correlated with one another, but are clearly separable. Moreover, structural equation modeling suggested that the three functions contribute differentially to performance on complex executive tasks. Specifically, WCST performance was related most strongly to Shifting, TOH to Inhibition, RNG to Inhibition and Updating, and operation span to Updating. Dual task performance was not related to any of the three target functions. These results suggest that it is important to recognize both the unity and diversity of executive functions and that latent variable analysis is a useful approach to studying the organization and roles of executive functions.