Specific versus nonspecific brain activity in a parametric N-back task

Different neural capacity limitations for articulatory and non-articulatory maintenance of verbal information

Many studies have demonstrated attenuated verbal working memory (WM) under articulatory suppression. However, performance is not completely abolished, suggesting a less efficient, non-articulatory mechanism for the maintenance of verbal information. The neural causes for the reduced efficiency of such a putative complementary maintenance system have not yet been addressed. The present study was conducted to fill this gap. Subjects performed a Sternberg task (a) under articulatory maintenance at low, high, and supracapacity set sizes and (b) under non-articulatory maintenance at low and high set sizes. With functional magnetic resonance imaging, set-size related increases in activity were compared between subvocal articulatory rehearsal and non-articulatory maintenance. First, the results replicate previous findings showing different networks underlying these two maintenance strategies. Second, activation of all key nodes of the articulatory maintenance network increased with the amount of memorized information, showing no plateau at high set sizes. In contrast, for non-articulatory maintenance, there was evidence for a plateau at high set sizes in all relevant areas of the network. Third, for articulatory maintenance, the non-articulatory maintenance network was additionally recruited at supracapacity set sizes, presumably to assist processing in this highly demanding condition. This is the first demonstration of differential neural bottlenecks for articulatory and non-articulatory maintenance. This study adds to our understanding of the performance differences between these two strategies supporting verbal WM.

Structural and functional reorganization of working memory system during the first decade in schizophrenia. A cross-sectional study

INTRODUCTION

Progressive atrophy occurs in brain regions involved in the working memory network along the schizophrenia's course, but without parallel evolution of working memory impairment. We investigated the functional organization inside this network at different stages of the disease.

METHODS

Twenty-eight patients with schizophrenia (16 with long disease duration (>60 months) and 12 with short disease duration (<60 months)) and eleven healthy controls underwent structural and functional MRI during an n-back task to determine atrophy and activation patterns.

RESULTS

At similar n-back performances and relative to short disease duration patients, long disease duration patients activated more frontal temporal parietal and frontal network during 0-back and 1-back tasks respectively. n-back scores were correlated to atrophy in the frontal-temporal areas.

DISCUSSION

Functional reorganization in the working memory network may play a compensatory role during the first ten years of schizophrenia.

Modifications of EEG power spectra in mesial temporal lobe during n-back tasks of increasing difficulty. A sLORETA study

The n-back task is widely used to investigate the neural basis of Working Memory (WM) processes. The principal aim of this study was to explore and compare the EEG power spectra during two n-back tests with different levels of difficulty (1-back vs. 3-back). Fourteen healthy subjects were enrolled (seven men and seven women, mean age 31.21 ± 7.05 years, range: 23–48). EEG was recorded while performing the N-back test, by means of 19 surface electrodes referred to joint mastoids. EEG analysis were conducted by means of the standardized Low Resolution brain Electric Tomography (sLORETA) software. The statistical comparison between EEG power spectra in the two conditions was performed using paired t-statistics on the coherence values after Fisher's z transformation available in the LORETA program package. The frequency bands considered were: delta (0.5–4 Hz); theta (4.5–7.5 Hz); alpha (8–12.5 Hz); beta (13–30 Hz); gamma (30.5–100 Hz). Significant changes occurred in the delta band: in the 3-back condition an increased delta power was localized in a brain region corresponding to the Brodmann Area (BA) 28 in the left posterior entorhinal cortex (T = 3.112; p < 0.05) and in the BA 35 in the left perirhinal cortex in the parahippocampal gyrus (T = 2.876; p < 0.05). No significant differences were observed in the right hemisphere and in the alpha, theta, beta, and gamma frequency bands. Our results indicate that the most prominent modification induced by the increased complexity of the task occur in the mesial left temporal lobe structures.

Impaired working memory and normal sustained attention in borderline personality disorder

Lazzaretti M, Morandotti N, Sala M, Isola M, Frangou S, De Vidovich G, Marraffini E, Gambini F, Barale F, Zappoli F, Caverzasi E, Brambilla P. Impaired working memory and normal sustained attention in borderline personality disorder.

Objective: Although reports in the literature describe deficits in working memory in borderline personality disorder (BPD), the evidence is limited and inconsistent. The aim of this study was to evaluate further this cognitive dimension and its clinical correlates in BPD.

Method: We compared the performance of 15 BPD patients to 1:1 matched healthy controls on verbal working memory as determined by the sequential letter N-back test and sustained attention as measured using the continuous performance test (CPT).

Results: BPD patients performed significantly worse on the N-back test compared to healthy controls (p < 0.05), but not on the CPT. The N-back deficit was more pronounced and significant in the 3-back condition and inversely correlated with impulsivity.

Conclusions: These results suggest the presence of working memory deficits in BPD that may be linked to greater impulsivity and sustained by impairment in the dorsolateral prefrontal cortex.

Impact of a microRNA MIR137 susceptibility variant on brain function in people at high genetic risk of schizophrenia or bipolar disorder

A recent 'mega-analysis' combining genome-wide association study data from over 40,000 individuals identified novel genetic loci associated with schizophrenia (SCZ) at genome-wide significance level. The strongest finding was a locus within an intron of a putative primary transcript for microRNA MIR137. In the current study, we examine the impact of variation at this locus (rs1625579, G/T; where T is the common and presumed risk allele) on brain activation during a sentence completion task that differentiates individuals with SCZ, bipolar disorder (BD), and their relatives from controls. We examined three groups of individuals performing a sentence completion paradigm: (i) individuals at high genetic risk of SCZ (n=44), (ii) individuals at high genetic risk of BD (n=90), and (iii) healthy controls (n=81) in order to test the hypothesis that genotype at rs1625579 would influence brain activation. Genotype groups were assigned as 'RISK-' for GT and GG individuals, and 'RISK+' for TT homozygotes. The main effect of genotype was significantly greater activation in the RISK- individuals in the posterior right medial frontal gyrus, BA 6. There was also a significant genotype(*)group interaction in the left amygdala and left pre/postcentral gyrus. This was due to differences between the controls (where individuals with the RISK- genotype showed greater activation than RISK+ subjects) and the SCZ high-risk group, where the opposite genotype effect was seen. These results suggest that the newly identified SCZ locus may influence brain activation in a manner that is partly dependent on the presence of existing genetic susceptibility for SCZ.

An examination of the relationship between motor coordination and executive functions in adolescents

AIM

Research suggests important links between motor coordination and executive functions. The current study examined whether motor coordination predicts working memory, inhibition, and switching performance, extending previous research by accounting for attention-deficit-hyperactivity disorder (ADHD) symptomatology and other confounding factors, in an adolescent normative sample.

METHOD

Ninety-three adolescents (38 females, 55 males) aged 12 to 16 years (mean age 4y 2mo, SD 1y 1mo) were assessed on the Movement Assessment Battery for Children-2 (MABC-2), Wechsler Intelligence Scale for Children-IV, N-back task, the inhibition subtest from the NEPSY-II: A Developmental Neuropsychological Assessment, second edition, and the parent-rated Strengths and Weaknesses of ADHD Symptoms and Normal Behaviour Questionnaire.

RESULTS

The MABC-2 total score accounted for a significant proportion of the variance in visuospatial working memory (p=0.041) but not for verbal working memory. The MABC-2 aiming and catching component, however, was found to account for unique variance in both verbal (p=0.019) and visuospatial working memory (p=0.016). The MABC-2 total score was found to account for a significant proportion of the variance in inhibition total completion time (p=0.017). Finally, balance skills accounted for unique variance in a NEPSY-II inhibition total errors variable (p=0.020).

INTERPRETATION

The results provide support for an overlap between motor coordination and executive functions, which has important practical implications. The study also suggests shared mechanisms underpinning the relationship between these areas, including possible cerebellar involvement.

Effects of aging on cerebral oxygenation during working-memory performance: a functional near-infrared spectroscopy study

Working memory is sensitive to aging-related decline. Evidence exists that aging is accompanied by a reorganization of the working-memory circuitry, but the underlying neurocognitive mechanisms are unclear. In this study, we examined aging-related changes in prefrontal activation during working-memory performance using functional Near-Infrared Spectroscopy (fNIRS), a noninvasive neuroimaging technique. Seventeen healthy young (21–32 years) and 17 healthy older adults (64–81 years) performed a verbal working-memory task (n-back). Oxygenated and deoxygenated hemoglobin concentration changes were registered by two fNIRS channels located over the left and right prefrontal cortex. Increased working-memory load resulted in worse performance compared to the control condition in older adults, but not in young participants. In both young and older adults, prefrontal activation increased with rising working-memory load. Young adults showed slight right-hemispheric dominance at low levels of working-memory load, while no hemispheric differences were apparent in older adults. Analysis of the time-activation curve during the high working-memory load condition revealed a continuous increase of the hemodynamic response in the young. In contrast to that, a quadratic pattern of activation was found in the older participants. Based on these results it could be hypothesized that young adults were better able to keep the prefrontal cortex recruited over a prolonged period of time. To conclude, already at low levels of working-memory load do older adults recruit both hemispheres, possibly in an attempt to compensate for the observed aging-related decline in performance. Also, our study shows that aging effects on the time course of the hemodynamic response must be taken into account in the interpretation of the results of neuroimaging studies that rely on blood oxygen levels, such as fMRI.

Brain SCALE: brain structure and cognition: an adolescent longitudinal twin study into the genetic etiology of individual differences

From childhood into adolescence, the child's brain undergoes considerable changes in both structure and function. Twin studies are of great value to explore to what extent genetic and environmental factors explain individual differences in brain development and cognition. In The Netherlands, we initiated a longitudinal study in which twins, their siblings and their parents are assessed at three year intervals. The participants were recruited from The Netherlands Twin Register (NTR) and at baseline consisted of 112 families, with 9-year-old twins and an older sibling. Three years later, 89 families returned for follow-up assessment. Data collection included psychometric IQ tests, a comprehensive neuropsychological testing protocol, and parental and self-ratings of behavioral and emotional problems. Physical maturation was measured through assessment of Tanner stages. Hormonal levels (cortisol, luteinizing hormone, follicle-stimulating hormone, testosterone, and estrogens) were assessed in urine and saliva. Brain scans were acquired using 1.5 Tesla Magnetic Resonance Imaging (MRI), which provided volumetric measures and measures of cortical thickness. Buccal swabs were collected for DNA isolation for future candidate gene and genome-wide analysis studies. This article gives an overview of the study and the main findings. Participants will return for a third assessment when the twins are around 16 years old. Longitudinal twin-sibling studies that map brain development and cognitive function at well-defined ages aid in the understanding of genetic influences on normative brain development.

Long-term and within-day variability of working memory performance and EEG in individuals

OBJECTIVE

Assess individual-subject long-term and within-day variability of a combined behavioral and EEG test of working memory.

METHODS

EEGs were recorded from 16 adults performing n-back working memory tasks, with 10 tested in morning and afternoon sessions over several years. Participants were also tested after ingesting non-prescription medications or recreational substances. Performance and EEG measures were analyzed to derive an Overall score and three constituent sub-scores characterizing changes in performance, cortical activation, and alertness from each individual's baseline. Long-term and within-day variability were determined for each score; medication effects were assessed by reference to each individual's normal day-to-day variability.

RESULTS

Over the several year period, the mean Overall score and sub-scores were approximately zero with standard deviations less than one. Overall scores were lower and their variability higher in afternoon relative to morning sessions. At the group level, alcohol, diphenhydramine and marijuana produced significant effects, but there were large individual differences.

CONCLUSIONS

Objective working memory measures incorporating performance and EEG are stable over time and sensitive at the level of individual subjects to interventions that affect neurocognitive function.

SIGNIFICANCE

With further research these measures may be suitable for use in individualized medical care by providing a sensitive assessment of incipient illness and response to treatment.

Resiliency in adolescents at high risk for substance abuse: flexible adaptation via subthalamic nucleus and linkage to drinking and drug use in early adulthood

INTRODUCTION

The personality trait resiliency is the ability to flexibly adapt impulse control relative to contextual demand. Low resiliency has been linked to later alcohol/drug problems. The underlying psychological and neural mechanisms are unknown, but neurocomputational models suggested relations between resiliency and working memory. Cortical-striatal connectivity has been proposed to underlie adaptive switches between cautious and risky behaviors.

METHODS

Working memory was probed in sixty-seven 18- to 22-year-olds from a larger community study of alcoholism, using the n-back task during functional magnetic resonance imaging. Functional connectivity between task-related regions was investigated with psychophysiological interaction analysis. Resiliency was measured in early teen years and related to early adulthood measures of drinking/drug use, task activation, and connectivity. Relationships with risk factors, including family history, age of drinking onset, and number of alcohol problems, were also investigated.

RESULTS

Higher resiliency was related to lower levels of substance use, fewer alcohol problems, and better working memory performance. Whole-brain regression revealed resiliency negatively correlated with activation of subthalamic nucleus (STN) and pallidum during the n-back. High and Low resiliency quartile groups (n = 17 each) differed in coupling strength between STN and median cingulate cortex, a region of reduced activation during working memory. The high resiliency group had later onset of drinking, fewer alcohol problems, had used fewer illicit drugs, and were less likely to smoke cigarettes than their low resiliency counterparts.

CONCLUSIONS

These findings suggest that resiliency in early adolescence may protect against alcohol problems and drug use, although the direction of this effect is currently unknown. This protective factor may relate to executive functioning as supported by the finding of a neural link shared between resiliency and working memory in basal ganglia structures. The STN, a key basal ganglia structure, may adaptively link flexible impulse control with cognitive processing, potentially modulating substance use outcomes.

Spatial working memory dysfunction in minimal hepatic encephalopathy: an ethology and BOLD-fMRI study

The term "minimal hepatic encephalopathy" (MHE) refers to a population of individuals who have no recognizable clinical symptoms but perform abnormally on neuropsychological and neurophysiological tests. Research shows that MHE patients have impairments in cognition affecting their daily lives that should be treated. This study explored the neural basis of spatial working memory impairment in MHE patients using behavioral test and BOLD-fMRI. Twelve normal controls, twelve cirrhosis patients without MHE and twelve MHE patients took part. The memory quotient of the MHE group (Wechsler Memory Scale-Chinese revised: WMS-CR) was lower than the normal control group and the cirrhosis-without-MHE group, and primarily concerned short-term memory and transient memory. Performance accuracy was lower for the MHE group than the control group and the cirrhosis-without-MHE group, and mean reaction time was prolonged. The fMRI data highlighted a neural network consisting of: bilateral prefrontal cortex (PFC), bilateral premotor area (PreMA), supplementary motor area (SMA) and bilateral parietal areas (PA), which was activated in the n-back task. The load effect of BOLD-fMRI response appeared in all regions of interest (ROI) for the normal control group, but only appeared in PreMA and PA, and did not vary with n-back load in PFC or SMA for the MHE group. Activation intensities for all ROIs were higher for the normal control group than the MHE group, especially in 2-back load. In conclusion, these results demonstrate that MHE patients have debilitated spatial working memory, and that impairments of bilateral PFC, PMA, SMA, and PA commonly lead to spatial working memory dysfunction. Furthermore, PFC impairment may form the neural basis of spatial working memory impairment.

Neural correlates of the difference between working memory speed and simple sensorimotor speed: an fMRI study

The difference between the speed of simple cognitive processes and the speed of complex cognitive processes has various psychological correlates. However, the neural correlates of this difference have not yet been investigated. In this study, we focused on working memory (WM) for typical complex cognitive processes. Functional magnetic resonance imaging data were acquired during the performance of an N-back task, which is a measure of WM for typical complex cognitive processes. In our N-back task, task speed and memory load were varied to identify the neural correlates responsible for the difference between the speed of simple cognitive processes (estimated from the 0-back task) and the speed of WM. Our findings showed that this difference was characterized by the increased activation in the right dorsolateral prefrontal cortex (DLPFC) and the increased functional interaction between the right DLPFC and right superior parietal lobe. Furthermore, the local gray matter volume of the right DLPFC was correlated with participants' accuracy during fast WM tasks, which in turn correlated with a psychometric measure of participants' intelligence. Our findings indicate that the right DLPFC and its related network are responsible for the execution of the fast cognitive processes involved in WM. Identified neural bases may underlie the psychometric differences between the speed with which subjects perform simple cognitive tasks and the speed with which subjects perform more complex cognitive tasks, and explain the previous traditional psychological findings.

Altered neural activation in ornithine transcarbamylase deficiency during executive cognition: an fMRI study

BACKGROUND

Ornithine transcarbamylase deficiency (OTCD) is an X-linked urea cycle disorder characterized by hyperammonemia resulting in white matter injury and impairments in working memory and executive cognition.

OBJECTIVE

To test for differences in BOLD signal activation between subjects with OTCD and healthy controls during a working memory task.

DESIGN, SETTING AND PATIENTS

Nineteen subjects with OTCD and 21 healthy controls participated in a case-control, IRB-approved study at Georgetown University Medical Center.

INTERVENTION

An N-back working memory task was performed in a block design using 3T functional magnetic resonance imaging.

RESULTS

In subjects with OTCD we observed increased BOLD signal in the right dorsolateral prefrontal cortex (DLPFC) and anterior cingulate cortex (ACC) relative to healthy age matched controls.

CONCLUSIONS

Increased neuronal activation in OTCD subjects despite equivalent task performance points to sub-optimal activation of the working memory network in these subjects, most likely reflecting damage caused by hyperammonemic events. These increases directly relate to our previous finding of reduced frontal white matter integrity in the superior extents of the corpus callosum; key hemispheric connections for these areas. Future studies using higher cognitive load are required to further characterize these effects.

Neurocognitive deficits in children with sickle cell disease are associated with the severity of anemia

BACKGROUND

Although neurocognitive deficits in children with sickle cell disease (SCD) have been well documented, the etiology of these deficits has not been completely clarified. The aim of this study was to investigate the association of laboratory markers of disease severity and radiological parameters with neurocognitive functioning in children with SCD.

DESIGN AND METHODS

Participants were 37 children with SCD ((HbSS or HbS-β(0)-thalassemia) aged 6-18 years. All participants underwent extensive neurocognitive assessment. Further data (TCD values, laboratory test results, and MRI data) were obtained from medical charts. Associations were analyzed by hierarchical regression analysis.

RESULTS

Hemoglobin was associated with a decrease in verbal short-term memory. There was no association between TCD velocities and neurocognitive functioning, when controlled for age. Children with silent infarcts did not differ from children with normal MRI in neurocognitive functioning. Children with right-left asymmetries in cerebral blood flow as measured by continuous arterial spin labelling (CASL) MRI had better sustained attention than children without asymmetries.

CONCLUSIONS

Neurocognitive deficits are associated with the severity of anemia, indicating reduced oxygen delivery to the brain as an etiological mechanism. This implies that children with SCD and normal MRIs may still suffer from neurocognitive impairments, possibly affecting their academic development and full participation in society.

Neurocognitive deficits in children with sickle cell disease: a comprehensive profile

BACKGROUND

Sickle cell disease (SCD) can lead to profound cerebral damage, associated with neurocognitive deficits. The aim of the current study was to evaluate a broad range of neurocognitive functions in children with SCD compared to a SES-matched control group, in order to gain more insight into the specific deficits of these patients.

METHODS

Forty-one children with homozygous SCD (HbSS or HbS-β0-thalassemia) and 38 controls were assessed on a comprehensive set of well-defined and validated measures of neurocognitive functioning. Besides general intelligence, we evaluated executive functioning extensively (including response inhibition, sustained attention, planning, visuo-spatial working memory, and verbal working memory) as well as visuo-motor functioning.

RESULTS

SCD was clearly associated with lower IQ scores. More than one in three children with SCD had a Full-scale IQ below 75. Furthermore, children with SCD showed deficits in visuo-motor functioning. Some evidence was found for executive dysfunction: Children with SCD displayed poor visuo-spatial working memory, as well as subtle deficits in sustained attention and planning. No significant differences were found between children with SCD and controls in terms of response inhibition and verbal working memory.

CONCLUSIONS

Children with SCD are at increased risk of lower intelligence, visuo-motor impairments, and executive dysfunction. These neurocognitive deficits may underlie high rates of scholastic impairments in these children. The present findings further illuminate the importance of regular neurocognitive evaluations and future neurocognitive rehabilitation programs for children with SCD.

A cognitive and neurophysiological test of change from an individual's baseline

OBJECTIVE

An automated cognitive neurophysiological test is presented that characterizes how an individual was affected by a drug or treatment. The test calculates sub-scores for working memory task performance, cortical activation, and alertness, and combines the sub-scores into an overall score.

METHODS

The test was applied in a double-blind, placebo-controlled study of alcohol, caffeine, diphenhydramine, and sleep deprivation in 16 healthy adults.

RESULTS

The between- and within-day variability of the sub-scores and overall scores for placebo were all near zero, suggesting that the scores are stable. All treatments affected the overall score, while differential effects on sub-scores highlighted the added value of EEG measures.

CONCLUSIONS

The test is sensitive to relatively mild alterations in cognitive function. Its automation makes it suitable for use in large-scale clinical trials.

SIGNIFICANCE

By combining task performance with EEG brain function measures, the test may prove to have better sensitivity and specificity in detecting changes due to drugs or other treatments than comparable neuropsychological test batteries that do not directly measure brain function signals.

Working memory load modulates the auditory "What" and "Where" neural networks

Working memory for sound identity (What) and sound location (Where) has been associated with increased neural activity in ventral and dorsal brain regions, respectively. To further ascertain this domain specificity, we measured fMRI signals during an n-back (n=1, 2) working memory task for sound identity or location, where stimuli selected randomly from three semantic categories (human, animal, and music) were presented at three possible virtual locations. Accuracy and reaction times were comparable in both "What" and "Where" tasks, albeit worse for the 2-back than for the 1-back condition. The analysis of fMRI data revealed greater activity in ventral and dorsal brain regions during sound identity and sound location, respectively. More importantly, there was an interaction between task and working memory load in the inferior parietal lobule (IPL). Within the right IPL, there were two sub-regions modulated differentially by working memory load: an anterior ventromedial region modulated by location load and a posterior dorsolateral region modulated by category load. These specific changes in neural activity as a function of working memory load reveal domain-specificity within the parietal cortex.

Between- and within-scanner variability in the CaliBrain study n-back cognitive task

Psychiatric neuroimaging techniques are likely to improve understanding of the brain in health and disease, but studies tend to be small, based in one imaging centre and of unclear generalisability. Multicentre studies have great appeal but face problems if functional magnetic resonance imaging (fMRI) data from different centres are to be combined. Fourteen healthy volunteers had two brain scans on different days at three scanners. Considerable effort was first made to use similar scanning sequences and standardise task implementation across centres. The n-back cognitive task was used to investigate between- and within-scanner reproducibility and reliability. Both the functional imaging and behavioural results were in good accord with the existing literature. We found no significant differences in the activation/deactivation maps between scanners, or between repeat visits to the same scanners. Between- and within-scanner reproducibility and reliability was very similar. However, the smoothness of images from the scanners differed, suggesting that smoothness equalization might further reduce inter-scanner variability. Our results for the n-back task suggest it is possible to acquire fMRI data from different scanners which allows pooling across centres, when the same field strength scanners are used and scanning sequences and paradigm implementations are standardised.

Neurocognitive impairments in MDMA and other drug users: MDMA alone may not be a cognitive risk factor

BACKGROUND

MDMA (3,4-methylenedioxymethamphetamine; "Ecstasy") is an amphetamine derivative with mild hallucinogenic and stimulant qualities. MDMA leads to serotonin (5-hydroxytryptamine; 5-HT) neurotoxicity and has been linked to cognitive impairments. It remains unclear whether these impairments are due to MDMA versus other drug use.

METHOD

Neurocognitive functioning was measured in a sample of abstinent polydrug users (n = 52) with a range of MDMA use and healthy nondrug controls (n = 29). Participants completed a comprehensive neuropsychological battery and self-report measures of drug use.

RESULTS

Polydrug users performed worse than controls on spatial span and spatial working memory (ps < .05). Among polydrug users, lifetime marijuana use significantly predicted verbal learning and memory performance (p < .01), while MDMA use was not predictive of cognitive impairment.

CONCLUSIONS

This study and our previous report (Hanson, Luciana, & Sullwold, 2008) suggest that moderate MDMA use does not lead to persistent impairments above and beyond that associated with generally heavy drug use, but polydrug use may lead to dose-related temporal and frontoparietal dysfunction. Marijuana use may be particularly problematic. Cause-effect relations are unclear.

Altered working memory process in the manganese-exposed brain

Chronic manganese (Mn) exposure often leads to impairments in fine motor and cognitive functions, particularly memory. However, the neural correlates of Mn-induced alterations in memory remain unclear. In the present study, we performed functional MRI (fMRI) with 2-back memory tests to assess the neural correlates of Mn-induced memory impairment in response to subclinical dysfunction in the working memory networks in welders exposed to Mn for extended periods of time. Within-group and between-group analyses revealed that brain activity in working memory networks was increased in welders with chronic Mn exposure during the 2-back verbal working memory task compared to healthy control individuals. Therefore, our fMRI findings indicate that welders might require more neural resources in working memory networks to compensate for subtle deficits in working memory and altered working memory processes, even if they performed the tasks at the same level as healthy control individuals.

Spatial working memory deficits in GluA1 AMPA receptor subunit knockout mice reflect impaired short-term habituation: evidence for Wagner's dual-process memory model

Genetically modified mice, lacking the GluA1 AMPA receptor subunit, are impaired on spatial working memory tasks, but display normal acquisition of spatial reference memory tasks. One explanation for this dissociation is that working memory, win-shift performance engages a GluA1-dependent, non-associative, short-term memory process through which animals choose relatively novel arms in preference to relatively familiar options. In contrast, spatial reference memory, as exemplified by the Morris water maze task, reflects a GluA1-independent, associative, long-term memory mechanism. These results can be accommodated by Wagner's dual-process model of memory in which short and long-term memory mechanisms exist in parallel and, under certain circumstances, compete with each other. According to our analysis, GluA1^−/− mice lack short-term memory for recently experienced spatial stimuli. One consequence of this impairment is that these stimuli should remain surprising and thus be better able to form long-term associative representations. Consistent with this hypothesis, we have recently shown that long-term spatial memory for recently visited locations is enhanced in GluA1^−/− mice, despite impairments in hippocampal synaptic plasticity. Taken together, these results support a role for GluA1-containing AMPA receptors in short-term habituation, and in modulating the intensity or perceived salience of stimuli.

Performance level modulates adult age differences in brain activation during spatial working memory

Working memory (WM) shows pronounced age-related decline. Functional magnetic resonance imaging (fMRI) studies have revealed age differences in task-related brain activation. Evidence based primarily on episodic memory studies suggests that brain activation patterns can be modulated by task difficulty in both younger and older adults. In most fMRI aging studies on WM, however, performance level has not been considered, so that age differences in activation patterns are confounded with age differences in performance level. Here, we address this issue by comparing younger and older low and high performers in an event-related fMRI study. Thirty younger (20-30 years) and 30 older (60-70 years) healthy adults were tested with a spatial WM task with three load levels. A region-of-interest analysis revealed marked differences in the activation patterns between high and low performers in both age groups. Critically, among the older adults, a more "youth-like" load-dependent modulation of the blood oxygen level-dependent signal was associated with higher levels of spatial WM performance. These findings underscore the need of taking performance level into account when studying changes in functional brain activation patterns from early to late adulthood.

Working memory representation in atypical language dominance

One of the most important factors controlling material specific processing in the human brain is language dominance, i.e. hemispheric specialization in semantic processes. Although previous studies have shown that lateralized long-term memory processes in the medial temporal lobes are modified in subjects with atypical (right) language dominance, the effect of language dominance on the neural basis of working memory (WM) has remained unknown. Here, we used functional MRI (fMRI) to study the impact of language dominance on the neural representation of WM. We conducted an n-back task in three different load conditions and with both verbal and nonverbal (spatial) material in matched groups of left and right language dominant subjects. This approach allowed us to investigate regions showing significant interactions between language dominance and material. Overall, right dominant subjects showed an increased inter-individual variability of WM-related activations. Verbal WM involved more pronounced activation of the left fusiform cortex in left dominant subjects and of the right inferior parietal lobule in the right dominant group. Spatial WM, on the other hand, induced activation of right hemispheric regions in left dominant subjects, but no specific activations in right dominant subjects. Taken together, these findings indicate that the neural basis of verbal WM processes depends on language dominance and is more mutable in right dominant subjects. The increased variability in right dominant subjects strongly suggests that a standard network of material-dependent WM processes exists in left dominant subjects, and that right dominant subjects use variable alternative networks.

FUNCTIONAL NEUROANATOMY OF SUBCOMPONENT COGNITIVE PROCESSES INVOLVED IN VERBAL WORKING MEMORY

Recent research has used functional magnetic resonance imaging (fMRI) to examine brain regions related to specific subcomponent cognitive processes of verbal working memory, which include initial encoding of material, maintenance of the information over a brief delay interval, and later retrieval of the information. The present study examined each of these subcomponents in 14 healthy adults using a Sternberg verbal working memory task and fMRI. Group analysis revealed several brain regions active during all subcomponent processes, which included dorsolateral and ventrolateral prefrontal, parietal, hippocampal, and premotor cortex. Several other brain regions showed activation limited to specific subcomponent processes.

Background MR gradient noise and non-auditory BOLD activations: a data-driven perspective

The effect of echoplanar imaging (EPI) acoustic background noise on blood oxygenation level dependent (BOLD) activations was investigated. Two EPI pulse sequences were compared: (i) conventional EPI with a pulsating sound component of typically 8-10 Hz, which is a potent physiological stimulus, and (ii) the more recently developed continuous-sound EPI, which is perceived as less distractive despite equivalent peak sound pressure levels. Sixteen healthy subjects performed an established demanding visual n-back working memory task. Using an exploratory data analysis technique (tensorial probabilistic independent component analysis; tensor-PICA), we studied the inter-session/within-subject response variability introduced by continuous-sound versus conventional EPI acoustic background noise in addition to temporal and spatial signal characteristics. The analysis revealed a task-related component associated with the established higher-level working memory and motor feedback response network, which exhibited a significant 19% increase in its average effect size for the continuous-sound as opposed to conventional EPI. Stimulus-related lower-level activations, such as primary visual areas, were not modified. EPI acoustic background noise influences much more than the auditory system per se. This analysis provides additional evidence for an enhancement of task-related, extra-auditory BOLD activations by continuous-sound EPI due to less distractive acoustic background gradient noise.

Characterization of regional heterogeneity in cerebrovascular reactivity dynamics using novel hypocapnia task and BOLD fMRI

We offer a new method for characterizing the magnitude and dynamics of the vascular response to changes in arterial gas tensions using non-invasive blood oxygenation level-dependent functional magnetic resonance imaging (BOLD fMRI) and paradigms appropriate for clinical settings. A novel respiratory task, "Cued Deep Breathing" (CDB), consisting of two consecutive cycles of cued breaths, has been developed to cause transient hypocapnia, and consequently a strong, short-lived BOLD signal decrease. Data from CDB hypocapnia paradigms and traditional breath-holding hypercapnia paradigms were analyzed on a voxel-wise basis to map regional heterogeneity in magnitude and timing parameters. The tasks caused comparable absolute BOLD percent signal changes (approximately 0.5-3.0% in gray matter) and both datasets suggested consistent regional heterogeneity in the response timing: parts of the basal ganglia, particularly the putamen, and bilateral areas of medial cortex reached their maximum signal change several seconds earlier than remaining cortical gray matter voxels. This phenomenon and a slightly delayed response in posterior cortical regions were present in group-maps of ten healthy subjects. An auxiliary experiment in different subjects measured end-tidal CO2 changes associated with the new CDB task and quantitatively compared the resulting reactivity maps with those acquired using a traditional hypercapnia challenge of 4% CO2 gas inspiration. The CDB task caused average end-tidal CO2 decreases between 6.0+/-1.1 and 10.5+/-2.6 mm Hg, with levels returning to baseline after approximately three breaths, giving evidence that the task indeed causes transient mild hypocapnia. Similarity between resulting reactivity maps suggest CDB offers an alternative method for mapping cerebrovascular reactivity.

Hyperfrontality in patients with schizophrenia during saccade and antisaccade tasks: a study with fMRI

AIMS

Antisaccadic eye movements, requiring inhibition of a saccade toward a briefly appearing peripheral target, are known to be impaired in schizophrenia. Previous neuroimaging studies have indicated that patients with schizophrenia show diminished activations in the frontal cortex and basal ganglia. These studies used target fixation as a baseline condition. However, if the levels of brain activities at baseline are not compatible between patients and healthy subjects, between-group comparison on antisaccade-related activations is consequently invalidated. One possibility is that patients with schizophrenia may present with greater activation during fixation than healthy subjects. In order to examine this possibility, here we investigated brain activities associated with antisaccade in the two groups without using target fixation at baseline.

METHODS

Functional brain images were acquired during prosaccades and antisaccades in 18 healthy subjects and 18 schizophrenia patients using a box-car functional magnetic resonance imaging design. Eye movements were measured during scanning.

RESULTS

In the patient group, the elevated activities in the dorsolateral prefrontal cortex (DLPFC) and thalamus, normally seen in antisaccade tasks relative to saccade tasks, were no longer observed. Moreover, in normal subjects, activities in the DLPFC and thalamus were greater during the antisaccade task than during the saccade task. In patients, no such difference was observed between the two tasks, suggesting that these brain regions are likely to be highly activated even by a simple task such as fixation. In particular, the DLPFC and thalamus in patients were not activated at a level commensurate with the difficulty of the tasks presented.

CONCLUSIONS

From these results, it is suggested that schizophrenia entails dysfunctions in the fronto-striato-thalamo-cortical network associated with motor function control.

Development of spatial and verbal working memory capacity in the human brain

A core aspect of working memory (WM) is the capacity to maintain goal-relevant information in mind, but little is known about how this capacity develops in the human brain. We compared brain activation, via fMRI, between children (ages 7-12 years) and adults (ages 20-29 years) performing tests of verbal and spatial WM with varying amounts (loads) of information to be maintained in WM. Children made disproportionately more errors than adults as WM load increased. Children and adults exhibited similar hemispheric asymmetry in activation, greater on the right for spatial WM and on the left for verbal WM. Children, however, failed to exhibit the same degree of increasing activation across WM loads as was exhibited by adults in multiple frontal and parietal cortical regions. Thus, children exhibited adult-like hemispheric specialization, but appeared immature in their ability to marshal the neural resources necessary to maintain large amounts of verbal or spatial information in WM.

Validity of the CogState brief battery: relationship to standardized tests and sensitivity to cognitive impairment in mild traumatic brain injury, schizophrenia, and AIDS dementia complex

This study examined the validity of the four standard psychological paradigms that have been operationally defined within the CogState brief computerized cognitive assessment battery. Construct validity was determined in a large group of healthy adults. CogState measures of processing speed, attention, working memory, and learning showed strong correlations with conventional neuropsychological measures of these same constructs (r's = .49 to .83). Criterion validity was determined by examining patterns of performance on the CogState tasks in groups of individuals with mild head injury, schizophrenia, and AIDS dementia complex. Each of these groups was impaired on the CogState performance measures (Cohen's d's = -.60 to -1.80) and the magnitude and nature of this impairment was qualitatively and quantitatively similar in each group. Taken together, the results suggest that the cognitive paradigms operationally defined in the CogState brief battery have acceptable construct and criterion validity in a neuropsychological context.

Interference of working memory load with long-term memory formation

Traditionally, it has been assumed that the medial temporal lobe (MTL) is indispensable for long-term memory (LTM) encoding, but only plays a minor role for working memory (WM) maintenance. Recently, however, an increasing number of studies questioned this seemingly clear distinction by showing that the MTL does participate in some WM processes, especially if multiple items are being maintained. This would predict that WM maintenance of multiple items interferes with simultaneous LTM encoding. Here, we tested this idea in a functional magnetic resonance imaging paradigm that required subjects to encode stimuli into LTM during simultaneous WM maintenance of either single or multiple items. Indeed, we found that maintenance of multiple items deteriorates simultaneous LTM encoding as compared with maintenance of single items. WM-related activation of the hippocampus was more pronounced in the condition with high WM load; in contrast, hippocampal activation related to LTM encoding was stronger in the low WM load condition. Successful LTM encoding was associated with a high level of activity in the adjacent parahippocampal cortex (PHC), leading to pronounced parahippocampal subsequent memory effects in the high load condition. This suggests that the PHC is a locus of WM-LTM interaction. Functional connectivity analysis with a seed in the PHC confirmed this result by revealing strong connectivity with the medial frontal cortex, which was only active in the high WM load condition. Taken together, these findings suggest that high WM demands interfere with LTM encoding and thus support the idea that WM and LTM processes interact in the MTL.

Greater working memory load results in greater medial temporal activity at retrieval

Most functional magnetic resonance imaging (fMRI) studies examining working memory (WM) load have focused on the prefrontal cortex (PFC) and have demonstrated increased prefrontal activity with increased load. Here we examined WM load effects in the medial temporal lobe (MTL) using an fMRI Sternberg task with novel complex visual scenes. Trials consisted of 3 sequential events: 1) sample presentation (encoding), 2) delay period (maintenance), and 3) probe period (retrieval). During sample encoding, subjects saw either 2 or 4 pictures consecutively. During retrieval, subjects indicated whether the probe picture matched one of the sample pictures. Results revealed that activity in the left anterior hippocampal formation, bilateral retrosplenial area, and left amygdala was greater at retrieval for trials with larger memory load, whereas activity in the PFC was greater at encoding for trials with larger memory load. There was no load effect during the delay. When encoding, maintenance, and retrieval periods were compared with fixation, activity was present in the hippocampal body/tail and fusiform gyrus bilaterally during encoding and retrieval, but not maintenance. Bilateral dorsolateral prefrontal activity was present during maintenance, but not during encoding or retrieval. The results support models of WM predicting that activity in the MTL should be modulated by WM load.

Magnetic resonance imaging studies of cigarette smoking

This chapter reviews studies that have applied magnetic resonance imaging (MRI) toward a better understanding of the neurobiological correlates and consequences of cigarette smoking and nicotine dependence. The findings demonstrate that smokers differ from nonsmokers in regional brain structure and neurochemistry, as well as in activation in response to smoking-related stimuli and during the execution of cognitive tasks. We also review functional neuroimaging studies on the effects of nicotine administration on brain activity, both at rest and during the execution of cognitive tasks, independent of issues related to nicotine withdrawal and craving. Although chronic cigarette smoking is associated with poor cognitive performance, acute nicotine administration appears to enhance cognitive performance and increase neural efficiency in smokers.

Bilateral subthalamic stimulation impairs cognitive-motor performance in Parkinson's disease patients

Deep brain stimulation (DBS) is a surgical procedure that has been shown effective in improving the cardinal motor signs of advanced Parkinson's disease, however, declines in cognitive function have been associated with bilateral subthalamic nucleus (STN) DBS. Despite the fact that most activities of daily living clearly have motor and cognitive components performed simultaneously, postoperative assessments of cognitive and motor function occur, in general, in isolation of one another. The primary aim of this study was to determine the effects of unilateral and bilateral STN DBS on upper extremity motor function and cognitive performance under single- and dual-task conditions in advanced Parkinson's disease patients. Data were collected from eight advanced Parkinson's disease patients between the ages of 48 and 70 years (mean 56.5) who had bilaterally placed STN stimulators. Stimulation parameters for DBS devices were optimized clinically and were stable for at least 6 months prior to study participation. Data were collected while patients were Off anti-parkinsonian medications under three stimulation conditions: Off stimulation, unilateral DBS and bilateral DBS. In each stimulation condition patients performed a cognitive (n-back task) and motor (force tracking) task under single- and dual-task conditions. During dual-task conditions, patients performed the n-back and force-maintenance task simultaneously. Under relatively simple dual-task conditions there were no differences in cognitive or motor performance under unilateral and bilateral stimulation. As dual-task complexity increased, cognitive and motor performance was significantly worse with bilateral compared with unilateral stimulation. In the most complex dual-task condition (i.e. 2-back + force tracking), bilateral stimulation resulted in a level of motor performance that was similar to the Off stimulation condition. Significant declines in cognitive and motor function under modest dual-task conditions with bilateral but not with unilateral STN DBS suggest that unilateral procedures may be an alternative to bilateral DBS for some patients, in particular, those with asymmetric symptomology. From a clinical perspective, these results underscore the need to assess cognitive and motor function simultaneously during DBS programming as these conditions may better reflect the context in which daily activities are performed.

Quantifying the heritability of task-related brain activation and performance during the N-back working memory task: a twin fMRI study

Working memory-related brain activation has been widely studied, and impaired activation patterns have been reported for several psychiatric disorders. We investigated whether variation in N-back working memory brain activation is genetically influenced in 60 pairs of twins, (29 monozygotic (MZ), 31 dizygotic (DZ); mean age 24.4+/-1.7S.D.). Task-related brain response (BOLD percent signal difference of 2 minus 0-back) was measured in three regions of interest. Although statistical power was low due to the small sample size, for middle frontal gyrus, angular gyrus, and supramarginal gyrus, the MZ correlations were, in general, approximately twice those of the DZ pairs, with non-significant heritability estimates (14-30%) in the low-moderate range. Task performance was strongly influenced by genes (57-73%) and highly correlated with cognitive ability (0.44-0.55). This study, which will be expanded over the next 3 years, provides the first support that individual variation in working memory-related brain activation is to some extent influenced by genes.

Spatial working memory performance and fMRI activation interaction in abstinent adolescent marijuana users

Previous studies have suggested neural disruption and reorganization in adult marijuana users. However, it remains unclear whether these effects persist in adolescents after 28 days of abstinence and, if they do, what Performance x Brain Response interactions occur. Adolescent marijuana users (n=17) and controls (n=17) aged 16-18 years were recruited from local schools. Functional magnetic resonance imaging data were collected after 28 days' monitored abstinence as participants performed a spatial working memory task. Marijuana users show Performance x Brain Response interactions in the bilateral temporal lobes, left anterior cingulate, left parahippocampal gyrus, and right thalamus (clusters >/=1358 microl; p<.05), although groups do not differ on behavioral measures of task performance. Marijuana users show differences in brain response to a spatial working memory task despite adequate performance, suggesting a different approach to the task via altered neural pathways.

Tyrosine depletion alters cortical and limbic blood flow but does not modulate spatial working memory performance or task-related blood flow in humans

Dopamine appears critical in regulating spatial working memory (SWM) within the PFC of non-human primates; however findings in humans are less clear. Recent studies of the effects of global depletion of dopamine via acute tyrosine/phenylalanine depletion (TPD) on SWM task performance have yielded inconsistent results, which may be partly related to task differences. These previous studies do not address whether TPD can directly impair PFC functioning. The current study investigated the effects of TPD on (1) regional cerebral blood flow (rCBF) during a SWM n-back task using H(2) (15)O Positron Emission Tomography (PET), and (2) behavioural performance on three different SWM tasks. Ten healthy males were scanned twice: once following a placebo (balanced) amino acid mixture and once following an equivalent mixture deficient in tyrosine/phenylalanine (TPD condition). Participants completed two additional delayed-response tasks to examine whether differences in response demands influenced TPD effects on performance. TPD resulted in widespread increases in rCBF, with maximum increases in the region of the parahippocampal gyrus bilaterally, left inferior frontal gyrus, and the putamen. TPD related rCBF reductions were observed in the medial frontal gyrus bilaterally, right inferior temporal gyrus and the pons. Despite widespread changes in blood flow following TPD, no specific effects on SWM neural networks or task performance were observed. The use of three different SWM tasks suggests that task differences are unlikely to account for the lack of effects observed. These findings question the capacity of TPD to consistently modulate dopamine function and SWM neural networks in humans.

Functional imaging studies of cognition using 99mTc-HMPAO SPECT: empirical validation using the n-back working memory paradigm

Purpose

Functional activation protocols are widely applied for the study of brain-cognition relations. Only few take advantage of the intrinsic characteristics of SPECT, particularly those allowing cognitive assessment outside of the camera, in settings close to the standard clinical or laboratory ones. The purpose of the study was to assess the feasibility of a split-dose activation protocol with ^99mTc-HMPAO using low irradiation dose.

Materials and methods

A two-scans protocol was applied to 12 healthy young volunteers using 270 MBq of ^99mTc-HMPAO per scan, with each image associated to a particular experimental condition of the verbal n-back working memory task (0-back, 2-back). Subtraction method was used to identify regional brain activity related to the task.

Results

Voxel-wise statistical analysis showed left lateralized activity associated with the 2-back task, compared to the 0-back task. Activated regions, mainly prefrontal and parietal, were similar to those observed in previous fMRI and ¹⁵O-PET studies.

Conclusion

The results support the use of ^99mTc-HMPAO SPECT for the investigation of brain-cognition relations and demonstrate the feasibility of optimal quality images despite low radiopharmaceutical doses. The findings also acknowledge the use of HMPAO as a radioligand to capture neuro-energetic modulations linked to cognitive activity. They encourage extending the application of the described activation protocol to clinical populations.

Nonlinear changes in brain activity during continuous word repetition: an event-related multiparametric functional MR imaging study

BACKGROUND AND PURPOSE

Changes in brain activation as a function of continuous multiparametric word recognition have not been studied before by using functional MR imaging (fMRI), to our knowledge. Our aim was to identify linear changes in brain activation and, what is more interesting, nonlinear changes in brain activation as a function of extended word repetition.

MATERIALS AND METHODS

Fifteen healthy young right-handed individuals participated in this study. An event-related extended continuous word-recognition task with 30 target words was used to study the parametric effect of word recognition on brain activation. Word-recognition-related brain activation was studied as a function of 9 word repetitions. fMRI data were analyzed with a general linear model with regressors for linearly changing signal intensity and nonlinearly changing signal intensity, according to group average reaction time (RT) and individual RTs.

RESULTS

A network generally associated with episodic memory recognition showed either constant or linearly decreasing brain activation as a function of word repetition. Furthermore, both anterior and posterior cingulate cortices and the left middle frontal gyrus followed the nonlinear curve of the group RT, whereas the anterior cingulate cortex was also associated with individual RT.

CONCLUSION

Linear alteration in brain activation as a function of word repetition explained most changes in blood oxygen level-dependent signal intensity. Using a hierarchically orthogonalized model, we found evidence for nonlinear activation associated with both group and individual RTs.

Post-error behavior in active cocaine users: poor awareness of errors in the presence of intact performance adjustments

Active cocaine abusers have a diminished neural response to errors, particularly in the anterior cingulate cortex thought critical to error processing. The inability to detect, or adjust performance following errors has been linked to clinical symptoms including the loss of insight and perseverative behavior. We investigated the cognitive implications of this diminished error-related activity, using response inhibition tasks that required error awareness and performance adaptation. Twenty-one active cocaine users (six female subjects, mean age=40.3) and 22 non-drug using adults (six female subjects, mean=39.9) participated. The results indicated that cocaine users consistently demonstrated poorer inhibitory control, a deficit accompanied by reduced awareness of errors. Adaptation of post-error reaction times did not differ between groups, although a different measure of adaptive behavior: exerting inhibitory control on the trial immediately after failing to inhibit, was significantly poorer in the cocaine using sample. In summary, cocaine users demonstrated a diminished capacity for monitoring their behavior, but were able to perform post-error adjustment to processes not already suffering an underlying deficit. These difficulties are consistent with previous reports of cocaine-related hypoactivity in the neural system underlying cognitive control, and highlight the potential for cognitive dysfunction to manifest as behavioral deficits that likely contribute to the maintenance of drug dependence.

fMRI delineation of working memory for emotional prosody in the brain: commonalities with the lexico-semantic emotion network

Decoding emotional prosody is crucial for successful social interactions, and continuous monitoring of emotional intent via prosody requires working memory. It has been proposed by Ross and others that emotional prosody cognitions in the right hemisphere are organized in an analogous fashion to propositional language functions in the left hemisphere. This study aimed to test the applicability of this model in the context of prefrontal cortex working memory functions. BOLD response data were therefore collected during performance of two emotional working memory tasks by participants undergoing fMRI. In the prosody task, participants identified the emotion conveyed in pre-recorded sentences, and working memory load was manipulated in the style of an N-back task. In the matched lexico-semantic task, participants identified the emotion conveyed by sentence content. Block-design neuroimaging data were analyzed parametrically with SPM5. At first, working memory for emotional prosody appeared to be right-lateralized in the PFC, however, further analyses revealed that it shared much bilateral prefrontal functional neuroanatomy with working memory for lexico-semantic emotion. Supplementary separate analyses of males and females suggested that these language functions were less bilateral in females, but their inclusion did not alter the direction of laterality. It is concluded that Ross et al.'s model is not applicable to prefrontal cortex working memory functions, that evidence that working memory cannot be subdivided in prefrontal cortex according to material type is increased, and that incidental working memory demands may explain the frontal lobe involvement in emotional prosody comprehension as revealed by neuroimaging studies.