Stop Task After-Effects

In the stop task, response time to the go signal is increased when the immediately preceding trial involves the presentation of a stop signal. A recent explanation suggests that these “after-effects” are due to mechanisms that occur prior to the completion of response selection processes, but it is possible that they instead may reflect a slowed motor response (i.e., deliberate slowing after response selection). The participants completed a novel stop task that allows a differentiation between the time taken to prepare a movement (which incorporates response selection processes) and the time taken to execute a movement (i.e., speed of motor response). If mechanisms underlying stop task after-effects occur prior to the completion of response selection processes, then slowing should only occur during movement preparation. Movement preparation and execution time during go trials were analysed according to the characteristics of the preceding trial. Slowing after a stop trial was found during movement preparation time (regardless of inhibition success on that stop trial), and it further increased during this period when the primary task stimulus was repeated. There was also evidence for general after-effects during movement execution time, but no effect of repetition. These findings support the current theoretical accounts that suggest that repetition-based stop task after-effects are attributable to a mechanism that occurs prior to the completion of response selection processes, and also indicate a possible switch to a more conservative response set (as in signal detection theory terms) that results in deliberate slowing of movement.

Impaired conflict resolution and alerting in children with ADHD: evidence from the Attention Network Task (ANT)

BACKGROUND

An important theory of attention suggests that there are three separate networks that execute discrete cognitive functions. The 'alerting' network acquires and maintains an alert state, the 'orienting' network selects information from sensory input and the 'conflict' network resolves conflict that arises between potential responses. This theory holds promise for dissociating discrete patterns of cognitive impairment in disorders where attentional deficits may often be subtle, such as in attention deficit hyperactivity disorder (ADHD).

METHODS

The Attentional Network Test (ANT), a behavioural assay of the functional integrity of attention networks, was used to examine the performance of 73 children with ADHD and 73 controls.

RESULTS

Performance on the ANT clearly differentiated the children with and without ADHD in terms of mean and standard deviation (SD) of reaction time (RT), the number of incorrect responses made and the number of omission errors made. The ADHD group demonstrated deficits in the conflict network in terms of slower RT and a higher number of incorrect responses. The ADHD group showed deficits in the alerting network in terms of the number of omission errors made. There was no demonstration of a deficit in the orienting network in ADHD on this task.

CONCLUSIONS

The children with ADHD demonstrated deficits in the alerting and conflict attention networks but normal functioning of the orienting network.

Noradrenergic genotype predicts lapses in sustained attention

Sustained attention is modulated by the neurotransmitter noradrenaline. The balance of dopamine and noradrenaline in the cortex is controlled by the DBH gene. The principal variant in this gene is a C/T change at position -1021, and the T allele at this locus is hypothesised to result in a slower rate of dopamine to noradrenaline conversion than the C allele. Two hundred participants who were genotyped for the DBH C-1021T marker performed the Sustained Attention to Response Task (SART). DBH genotype was found to significantly predict performance; participants with more copies of the T allele made more errors of commission, indicative of lapses in sustained attention. A significant negative correlation was also observed for all participants between errors of commission and mean reaction time. The decrease in noradrenaline occasioned by the T allele may impair sustained attention by reducing participants' ability to remain alert throughout the task and by increasing their susceptibility to distractors.

Absence of the 7-repeat variant of the DRD4 VNTR is associated with drifting sustained attention in children with ADHD but not in controls

Many genetic studies have demonstrated an association between the 7-repeat (7r) allele of a 48-base pair variable number of tandem repeats (VNTR) in exon 3 of the DRD4 gene and the phenotype of attention deficit hyperactivity disorder (ADHD). Previous studies have shown inconsistent associations between the 7r allele and neurocognitive performance in children with ADHD. We investigated the performance of 128 children with and without ADHD on the Fixed and Random versions of the Sustained Attention to Response Task (SART). We employed time-series analyses of reaction-time data to allow a fine-grained analysis of reaction time variability, a candidate endophenotype for ADHD. Children were grouped into either the 7r-present group (possessing at least one copy of the 7r allele) or the 7r-absent group. The ADHD group made significantly more commission errors and was significantly more variable in RT in terms of fast moment-to-moment variability than the control group, but no effect of genotype was found on these measures. Children with ADHD without the 7r allele made significantly more omission errors, were significantly more variable in the slow frequency domain and showed less sensitivity to the signal (d') than those children with ADHD the 7r and control children with or without the 7r. These results highlight the utility of time-series analyses of reaction time data for delineating the neuropsychological deficits associated with ADHD and the DRD4 VNTR. Absence of the 7-repeat allele in children with ADHD is associated with a neurocognitive profile of drifting sustained attention that gives rise to variable and inconsistent performance.

Self-Alert Training: volitional modulation of autonomic arousal improves sustained attention

The present study examines a new alertness training strategy (Self-Alert Training, SAT) designed to explore the relationship between the top-down control processes governing arousal and sustained attention. In order to maximally target frontal control systems SAT combines a previously validated behavioural self-alerting technique [Robertson, I. H., Tegner, R., Tham, K., Lo, A., & Nimmo-Smith, I. (1995). Sustained attention training for unilateral neglect: Theoretical and rehabilitation implications. Journal of Clinical and Experimental Neuropsychology, 17, 416-430] with an autonomic arousal biofeedback protocol in which participants learn to modulate their own arousal levels. The SAT protocol was first validated with a group of 23 neurologically healthy participants and then independently tested in a group of 18 adults with ADHD to determine its clinical utility. Half of the participants in each group were assigned to a placebo condition to control for non-specific effects. All participants performed the sustained attention to response task (SART) during pre- and post-training testing sessions to assess training effects on sustained attention. By the end of SAT all participants were able to modulate their own arousal levels without external prompting. Comparison of pre- and post-training baseline data indicated that, as predicted, SAT was associated with increased levels of autonomic arousal accompanied by improved accuracy on the SART. In contrast, participants in the placebo condition exhibited a gradual reduction in arousal over time and increased reaction time variability indicative of a vigilance decrement. These data demonstrate that the recruitment of top-down control processes during volitional modulation of arousal leads to improved sustained attention. These findings have important implications for the rehabilitation of attention deficits arising from frontal dysfunction.

Dissociable Mechanisms of Cognitive Control in Prefrontal and Premotor Cortex

Intelligent behavior depends on the ability to suppress inappropriate actions and resolve interference between competing responses. Recent clinical and neuroimaging evidence has demonstrated the involvement of prefrontal, parietal, and premotor areas during behaviors that emphasize conflict and inhibition. It remains unclear, however, whether discrete subregions within this network are crucial for overseeing more specific inhibitory demands. Here we probed the functional specialization of human prefrontal cortex by combining repetitive transcranial magnetic stimulation (rTMS) with integrated behavioral measures of response inhibition (stop-signal task) and response competition (flanker task). Participants undertook a combined stop-signal/flanker task after rTMS of the inferior frontal gyrus (IFG) or dorsal premotor cortex (dPM) in each hemisphere. Stimulation of the right IFG impaired stop-signal inhibition under conditions of heightened response competition but did not influence the ability to suppress a competing response. In contrast, stimulation of the right dPM facilitated execution but had no effect on inhibition. Neither of these results was observed during rTMS of corresponding left-hemisphere regions. Overall, our findings are consistent with existing evidence that the right IFG is crucial for inhibitory control. The observed double dissociation of neurodisruptive effects between the right IFG and right dPM further implies that response inhibition and execution rely on distinct neural processes despite activating a common cortical network.

A neurobehavioral examination of individuals with high-functioning autism and Asperger's disorder using a fronto-striatal model of dysfunction

The repetitive, stereotyped, and obsessive behaviors that characterize autism may in part be attributable to disruption of the region of the fronto-striatal system, which mediates executive abilities. Neuropsychological testing has shown that children with autism exhibit set-shifting deficiencies on tests such as the Wisconsin Card Sorting task but show normal inhibitory ability on variants of the Stroop color-word test. According to Minshew and Goldstein's multiple primary deficit theory, the complexity of the executive functioning task is important in determining the performance of individuals with autism. This study employed a visual-spatial task (with a Stroop-type component) to examine the integrity of executive functioning, in particular inhibition, in autism (n = 12) and Asperger's disorder (n = 12) under increasing levels of cognitive complexity. Whereas the Asperger's disorder group performed similarly to age- and IQ-matched control participants, even at the higher levels of cognitive complexity, the high-functioning autism group displayed inhibitory deficits specifically associated with increasing cognitive load.

Right parietal dysfunction in children with attention deficit hyperactivity disorder, combined type: a functional MRI study

Attention deficit hyperactivity disorder, combined type (ADHD-CT) is associated with spatial working memory deficits. These deficits are known to be subserved by dysfunction of neural circuits involving right prefrontal, striatal and parietal brain regions. This study determines whether decreased right prefrontal, striatal and parietal activation with a mental rotation task shown in adolescents with ADHD-CT is also evident in children with ADHD-CT. A cross-sectional study of 12 pre-pubertal, right-handed, 8-12-year-old boys with ADHD-CT and 12 pre-pubertal, right-handed, performance IQ-matched, 8-12-year-old healthy boys, recruited from local primary schools, was completed. Participants underwent functional magnetic resonance imaging while performing a mental rotation task that requires spatial working memory. The two groups did not differ in their accuracy or response times for the mental rotation task. The ADHD-CT group showed significantly less activation in right parieto-occipital areas (cuneus and precuneus, BA 19), the right inferior parietal lobe (BA 40) and the right caudate nucleus. Our findings with a child cohort confirm previous reports of right striatal-parietal dysfunction in adolescents with ADHD-CT. This dysfunction suggests a widespread maturational deficit that may be developmental stage independent.

Dopaminergic genotype biases spatial attention in healthy children

In everyday life, our sensory system is bombarded with visual input and we rely upon attention to select only those inputs that are relevant to behavioural goals. Typically, humans can shift their attention from one visual field to the other with little cost to perception. In cases of 'unilateral neglect', however, there is a persistent bias of spatial attention towards the same side as the damaged cerebral hemisphere. We used a visual orienting task to examine the influence of functional polymorphisms of the dopamine transporter gene (DAT1) on individual differences in spatial attention in normally developing children. DAT1 genotype significantly influenced spatial bias. Healthy children who were homozygous for alleles that influence the expression of dopamine transporters in the brain displayed inattention for left-sided stimuli, whereas heterozygotes did not. Our data provide the first evidence in healthy individuals of a genetically mediated bias in spatial attention that is related to dopamine signalling.

Visuospatial memory deficits in adolescent onset schizophrenia

Visuospatial memory encoding deficits have been reported in adults with schizophrenia, while adolescents with schizophrenia have not been specifically investigated with visuospatial memory encoding and retrieval paradigms. A cross sectional study of delayed matching-to-sample performance in 19 right handed, male, anti-psychotic medication naïve adolescents with undifferentiated schizophrenia and 28 age, gender, IQ and handedness matched healthy participants was completed. The adolescent-onset schizophrenia group demonstrated significant impairment in visuospatial memory, independent of the degree of delay, consistent with an encoding impairment. The impaired encoding phase of visuospatial memory in the adolescent-onset schizophrenia group is consistent with findings in adult onset schizophrenia samples, suggesting a developmental stage-independent deficit.

The role of cingulate cortex in the detection of errors with and without awareness: a high-density electrical mapping study

Error-processing research has demonstrated that the brain uses a specialized neural network to detect errors during task performance but the brain regions necessary for conscious awareness of an error are poorly understood. In the present study we show that two well-known error-related event-related potential (ERP) components, the error-related negativity (ERN) and error positivity (Pe) have a differential relationship with awareness during performance of a manual response inhibition task optimized to examine error awareness. While the ERN was unaffected by the participants' conscious experience of errors, the Pe was only seen when participants were aware of committing an error. Source localization of these components indicated that the ERN was generated by a caudal region of the anterior cingulate cortex (ACC) while the Pe was associated with contributions from a more anterior ACC region and the posterior cingulate-precuneus. Tonic EEG measures of cortical arousal were correlated with individual rates of error awareness and showed a specific relationship with the amplitude of the Pe. The latter finding is consistent with evidence that the Pe represents a P3-like facilitation of information processing modulated by subcortical arousal systems. Our data suggest that the ACC might participate in both preconscious and conscious error detection and that cortical arousal provides a necessary setting condition for error awareness. These findings may be particularly important in the context of clinical studies in which a proper understanding of self-monitoring deficits requires an explicit measurement of error awareness.

Loss of insight in frontotemporal dementia, corticobasal degeneration and progressive supranuclear palsy

Loss of insight is one of the core features of frontal/behavioural variant frontotemporal dementia (FTD). FTD shares many clinical and pathological features with corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP). The aim of this study was to investigate awareness of cognitive deficits in FTD, CBD and PSP using a multidimensional approach to assessment, which examines metacognitive knowledge of the disorders, online monitoring of errors (emergent awareness) and ability to accurately predict performance on future tasks (anticipatory awareness). Thirty-five patients (14 FTD, 11 CBD and 10 PSP) and 20 controls were recruited. Results indicated that loss of insight was a feature of each of the three patient groups. FTD patients were most impaired on online monitoring of errors compared to the other two patient groups. Linear regression analysis demonstrated that different patterns of neuropsychological performance and behavioural rating scores predicted insight deficits across the three putative awareness categories. Furthermore, higher levels of depression were associated with poor anticipatory awareness, reduced empathy was related to impaired metacognitive awareness and impaired recognition of emotional expression in faces was associated with both metacognitive and anticipatory awareness deficits. The results are discussed in terms of neurocognitive models of awareness and different patterns of neurobiological decline in the separate patient groups.

Dissociation in performance of children with ADHD and high-functioning autism on a task of sustained attention

Attention deficit hyperactivity disorder (ADHD) and autism are two neurodevelopmental disorders associated with prominent executive dysfunction, which may be underpinned by disruption within fronto-striatal and fronto-parietal circuits. We probed executive function in these disorders using a sustained attention task with a validated brain-behaviour basis. Twenty-three children with ADHD, 21 children with high-functioning autism (HFA) and 18 control children were tested on the Sustained Attention to Response Task (SART). In a fixed sequence version of the task, children were required to withhold their response to a predictably occurring no-go target (3) in a 1–9 digit sequence; in the random version the sequence was unpredictable. The ADHD group showed clear deficits in response inhibition and sustained attention, through higher errors of commission and omission on both SART versions. The HFA group showed no sustained attention deficits, through a normal number of omission errors on both SART versions. The HFA group showed dissociation in response inhibition performance, as indexed by commission errors. On the Fixed SART, a normal number of errors was made, however when the stimuli were randomised, the HFA group made as many commission errors as the ADHD group. Greater slow-frequency variability in response time and a slowing in mean response time by the ADHD group suggested impaired arousal processes. The ADHD group showed greater fast-frequency variability in response time, indicative of impaired top-down control, relative to the HFA and control groups. These data imply involvement of fronto-parietal attentional networks and sub-cortical arousal systems in the pathology of ADHD and prefrontal cortex dysfunction in children with HFA.

Cognitive remediation in ADHD: effects of periodic non-contingent alerts on sustained attention to response

Few studies have attempted direct cognitive remediation of attention deficits in attention-deficit hyperactivity disorder (ADHD). The present study investigated the efficacy of periodic non-informative alerting cues for improving sustaining attention in ADHD. This technique is known to improve sustained attention in right frontal injury patients and may be effective in ADHD, given that this disorder has also been linked with right frontal dysfunction. Fifteen children with ADHD and 15 matched controls completed four blocks of a modified version of the Sustained Attention to Response Task (SART). Eight random non-contingent alerts were introduced on two of these blocks as a cue for participants to adopt a more supervisory stance to their performance. While the alerting cues did not alter the total number of commission errors committed by ADHD children over a task block, they did produce a significant short-term reduction in commission errors in the period immediately following an alerting cue. Our data demonstrate that sustained attention performance can be enhanced in children with ADHD using a simple cognitive training strategy. Methods from the field of cognitive rehabilitation may be viably applied to the remediation of attention deficits in ADHD.

An examination of movement kinematics in young people with high-functioning autism and Asperger's disorder: further evidence for a motor planning deficit

This paper examines upper-body movement kinematics in individuals with high-functioning autism (HFA) and Asperger's disorder (AD). In general, the results indicate that HFA is more consistently associated with impaired motoric preparation/initiation than AD. The data further suggest that this quantitative difference in motor impairment is not necessarily underpinned by greater executive dysfunction vulnerability in autism relative to AD. Quantitative motoric dissociation between autism and AD may have down-stream effects on later stages of movement resulting in qualitative differences between these disorder groups, e.g. "motor clumsiness" in AD versus "abnormal posturing" in autism. It will be important for future research to map the developmental trajectory of motor abnormalities in these disorder groups.

The cognitive genetics of attention deficit hyperactivity disorder (ADHD): sustained attention as a candidate phenotype

Here we describe the application of cognitive genetics to the study of attention deficit hyperactivity disorder (ADHD). Cognitive genetics owes much to the pioneering work of cognitive neuropsychologists such as John Marshall, whose careful observations of cognitive dissociations between brain-lesioned patients greatly advanced the theoretical understanding of normal cognitive function. These theories have in turn helped to constrain linkages between candidate genes and cognitive processes and thus help to drive the relatively new field of cognitive genetics in a hypothesis-driven fashion. We examined the relationship between sustained attention deficits in ADHD and genetic variation in a catecholamine-related gene, dopamine beta hydroxylase (DbetaH). DBH encodes the enzyme that converts dopamine to noradrenaline and is crucial to catecholamine regulation. A polymorphism with the DBH gene has been associated with ADHD. In fifty-two children with ADHD, we examined whether variation in the Taq I DBH gene polymorphism was related to sustained attention performance. Participants performed the Sustained Attention to Response Test (SART). Performance on the SART discriminates ADHD from control children, and in imaging work, is associated with right frontoparietal activation. A significant effect of DBH genotype was found on SART performance measures. Children possessing two copies of the ADHD-associated risk allele (A2) had significantly poorer sustained attention than those ADHD children who did not possess this allele or a non-genotyped control group. The DBH gene may contribute to the susceptibility for ADHD, in part because of its varying effects on the development of brain mechanisms mediating sustained attention.

Response variability in attention deficit hyperactivity disorder: evidence for neuropsychological heterogeneity

Response time (RT) variability is a common finding in ADHD research. RT variability may reflect frontal cortex function and may be related to deficits in sustained attention. The existence of a sustained attention deficit in ADHD has been debated, largely because of inconsistent evidence of time-on-task effects. A fixed-sequence Sustained Attention to Response Task (SART) was given to 29 control, 39 unimpaired and 24 impaired-ADHD children (impairment defined by the number of commission errors). The response time data were analysed using the Fast Fourier Transform, to define the fast-frequency and slow-frequency contributions to overall response variability. The impaired-ADHD group progressively slowed in RT over the course of the 5.5min task, as reflected in this group's greater slow-frequency variability. The fast-frequency trial-to-trial variability was also significantly greater, but did not differentially worsen over the course of the task. The higher error rates of the impaired-ADHD group did not become differentially greater over the length of the task. The progressive slowing in mean RT over the course of the task may relate to a deficit in arousal in the impaired-ADHD group. The consistently poor performance in fast-frequency variability and error rates may be due to difficulties in sustained attention that fluctuate on a trial-to-trial basis.

Impaired temporal resolution of visual attention and dopamine beta hydroxylase genotype in attention-deficit/hyperactivity disorder

BACKGROUND

Dopamine beta hydroxylase (DbetaH) catalyzes the conversion of dopamine to noradrenaline. Attention-deficit/hyperactivity disorder (ADHD) has been associated with the A2 allele of a Taq I polymorphism of the DBH gene. Since catecholamines regulate visual attention, we examined whether participants with ADHD were impaired on a task requiring temporal attention and how DBH genotype influenced temporal attention in ADHD.

METHODS

Thirty-seven children and adolescents with ADHD and 52 matched, normal control subjects participated. Participants were presented with two visual stimuli, separated in time by either 50, 100, or 200 milliseconds, and were asked to judge the temporal order of their onset. Genotypes for the Taq 1 polymorphism were available for 33 of the ADHD participants.

RESULTS

Attention-deficit/hyperactivity disorder participants were more error prone than control subjects, particularly when stimuli were presented close together in time (i.e., at the 50 milliseconds asynchrony). Moreover, ADHD individuals homozygous for the A2 allele performed more poorly than those without this allele, and this difference was accentuated at the 50 milliseconds asynchrony.

CONCLUSIONS

Attention-deficit/hyperactivity disorder participants have an impaired rate of perceptual processing for rapidly presented visual events. Deficits in the temporal resolution of visual attention in ADHD are associated with the A2 allele of the Taq I DBH polymorphism or another variant with which it is in linkage disequilibrium.

Visuospatial working memory deficits in adolescent onset schizophrenia

This study determines that visuospatial working memory (VSWM) deficits are evident in adolescent-onset schizophrenia, while the spatial strategy and spatial span components of VSWM are spared. These findings imply that frontal-striatal-parietal neural networks are dysfunctional in adolescent-onset schizophrenia, while mid-dorsolateral and ventrolateral PFC functions remain intact: the current conceptualisation of schizophrenia as a progressive neurodevelopmental disorder is consistent with these results.

Executive "brake failure" following deactivation of human frontal lobe

In the course of daily living, humans frequently encounter situations in which a motor activity, once initiated, becomes unnecessary or inappropriate. Under such circumstances, the ability to inhibit motor responses can be of vital importance. Although the nature of response inhibition has been studied in psychology for several decades, its neural basis remains unclear. Using transcranial magnetic stimulation, we found that temporary deactivation of the pars opercularis in the right inferior frontal gyrus selectively impairs the ability to stop an initiated action. Critically, deactivation of the same region did not affect the ability to execute responses, nor did it influence physiological arousal. These findings confirm and extend recent reports that the inferior frontal gyrus is vital for mediating response inhibition.

Lateralized deficit of response inhibition in early-onset schizophrenia

BACKGROUND

The ability to inhibit inappropriate or unwanted actions is a key element of executive control. The existence of executive function deficits in schizophrenia is consistent with frontal lobe theories of the disorder. Relatively few studies have examined response inhibition in schizophrenia, and none in adolescent patients with early-onset schizophrenia (EOS).

METHODS

Twenty-one adolescents with the onset of clinically impairing psychosis before 19 years of age and 16 matched controls performed a stop-signal task to assess response inhibition. The patients with EOS were categorized as paranoid (n = 10) and undifferentiated subtypes (n = 11). The undifferentiated group had higher levels of negative symptomatology. Stop-signal reaction time (SSRT) and go-signal reaction time (Go-RT) were analysed with respect to hand of response.

RESULTS

The undifferentiated early-onset patients had significantly longer SSRTs, indicative of poor response inhibition, for the left hand compared to the paranoid early-onset patients and control participants. No differences existed for inhibitory control with the right hand. The three groups did not differ in Go-RT.

CONCLUSIONS

Our results indicate a specific lateralized impairment of response inhibition in patients with undifferentiated, but not paranoid, EOS. These findings are consistent with reports of immature frontostriatal networks in EOS and implicate areas such as the pre-motor cortex and supplementary motor area (SMA) that are thought to play a role in both voluntary initiation and inhibition of movement.

Executive “Brake Failure” following Deactivation of Human Frontal Lobe

In the course of daily living, humans frequently encounter situations in which a motor activity, once initiated, becomes unnecessary or inappropriate. Under such circumstances, the ability to inhibit motor responses can be of vital importance. Although the nature of response inhibition has been studied in psychology for several decades, its neural basis remains unclear. Using transcranial magnetic stimulation, we found that temporary deactivation of the pars opercularis in the right inferior frontal gyrus selectively impairs the ability to stop an initiated action. Critically, deactivation of the same region did not affect the ability to execute responses, nor did it influence physiological arousal. These findings confirm and extend recent reports that the inferior frontal gyrus is vital for mediating response inhibition.