Varieties of attention-deficit/hyperactivity disorder-related intra-individual variability

The restless brain: attention-deficit hyperactivity disorder, resting-state functional connectivity, and intrasubject variability

OBJECTIVES

To highlight recent advances in the conceptualization of attention-deficit hyperactivity disorder (ADHD) emerging from neuroimaging and endophenotypic approaches.

METHODS

We selectively reviewed recent published literature on the phenomena of resting-state functional connectivity, intrasubject variability, and diffusion tensor imaging pertaining to ADHD.

RESULTS

Recent advances based on the novel approach of resting-state functional connectivity appear to be highly promising and likely to link to studies of intrasubject variability.

CONCLUSIONS

Endophenotypic fractionation may offer a means of addressing the complex heterogeneity of ADHD on the path to testable models of pathophysiology. Such models focusing on intrasubject variability, intrinsic brain activity, and reward-related processing are progressing rapidly.

The role of rat dorsomedial prefrontal cortex in spatial working memory

We used an operant delayed spatial alternation task to examine the role of rat dorsomedial prefrontal cortex (dmPFC) in spatial working memory. The task was designed to restrict movements during the delay period to minimize use of motor-mediating strategies. Inactivation of dmPFC (muscimol) resulted in increased errors and increased the temporal variability of responding. Animals did not show perseveration after errors (i.e., responding again at the erroneous location). Under control conditions, the time between spatial responses was greater and more variable before errors as compared to correct responses. These effects were eliminated when muscimol was infused into dmPFC. Trial outcome also affected movement and delay times in the next trial. This effect was diminished with muscimol in dmPFC. By contrast, when muscimol was infused in dorsal agranular insular cortex (AId)-a region that is strongly interconnected with dorsomedial prefrontal regions-there was no effect on delayed spatial alternation performance. These experiments confirm that dmPFC is necessary for successful delayed spatial alternation and establish that there is a relationship between response time variability and trial outcome that depends on dorsomedial prefrontal function.

Attention-deficit-hyperactivity disorder: an update

Attention-deficit-hyperactivity disorder (ADHD) is a common neuropsychiatric disorder that impairs social, academic, and occupational functioning in children, adolescents, and adults. In patients with ADHD, neurobiologic research has shown a lack of connectivity in key brain regions, inhibitory control deficits, delayed brain maturation, and noradrenergic and dopaminergic dysfunction in multiple brain regions. The prevalence of this disorder in the United States is 6-9% in youth (i.e., children and adolescents) and 3-5% in adults. Prevalence rates for youth are similar worldwide. Children with ADHD are at greater risk than children without ADHD for substance abuse and delinquency whether or not they receive drug therapy; however, early treatment with psychoeducation as well as drug therapy and/or behavioral intervention may decrease negative outcomes of ADHD, including the rate of conduct disorder and adult antisocial personality disorder. Drug therapy is effective for all age groups, even preschoolers, and for late-onset ADHD in adults. Stimulants, such as methylphenidate and amphetamine, are the most effective therapy and have a good safety profile; although recent concerns of sudden unexplained death, psychiatric adverse effects, and growth effects have prompted the introduction of other therapies. Atomoxetine, a nonstimulant, has no abuse potential, causes less insomnia than stimulants, and poses minimal risk of growth effects. Other drug options include clonidine and guanfacine, but both can cause bradycardia and sedation. Polyunsaturated fatty acids (fish oil), acetyl-L-carnitine, and iron supplements (for youth with low ferritin levels) show promise in improving ADHD symptoms. As long-term studies show that at least 50% of youth are nonadherent with their drug therapy as prescribed over a 1-year period, long-acting formulations (administered once/day) may improve adherence. Comorbid conditions are common in patients with ADHD, but this patient population can be treated effectively with individualized treatment regimens of stimulants, atomoxetine, or bupropion, along with close monitoring.

Effects of maternal and paternal smoking on attentional control in children with and without ADHD

Maternal smoking during pregnancy is a risk factor for attention-deficit/hyperactivity disorder (ADHD), but data on its adverse effects on cognitive functioning are sparse and inconsistent. Since the effect of maternal smoking during pregnancy may be due to correlated genetic risk factors rather than being a pure environmental effect, we examined the effect of prenatal exposure to smoking on attentional control, taking into account the effects of both maternal and paternal smoking, and examined whether these effects were genetically mediated by parental genotypes. We further examined whether the effect of prenatal exposure to smoking on attentional control interacted with genotypes of the child. Participants were 79 children with ADHD, ascertained for the International Multi-centre ADHD Gene project (IMAGE), and 105 normal controls. Attentional control was assessed by a visual continuous performance task. Three genetic risk factors for ADHD (DRD4 7-repeat allele of the exon 3 variable number of tandem repeats (VNTR), DAT1 10/10 genotype of the VNTR located in the 3' untranslated region, and the DAT1 6/6 genotype of the intron 8 VNTR) were included in the analyses. Paternal smoking had a negative effect on attentional control in children with ADHD and this effect appeared to be mediated by genetic risk factors. The prenatal smoking effect did not interact with genotypes of the child. Maternal smoking had no main effect on attentional control, which may be due to lower smoking rates. This study suggests that the effects of paternal smoking on attentional control in children with ADHD should be considered a proxy for ADHD and/or smoking risk genes. Future studies should examine if the results can be generalized to other cognitive domains.

Oculomotor anomalies in attention-deficit/hyperactivity disorder: evidence for deficits in response preparation and inhibition

OBJECTIVE

To examine patterns of executive and oculomotor control in a group of both boys and girls with attention-deficit/hyperactivity disorder (ADHD).

METHOD

Cross-sectional study of 120 children aged 8 to 12 years, including 60 with ADHD (24 girls) and 60 typically developing controls (29 girls). Oculomotor paradigms included visually guided saccades (VGS), antisaccades, memory-guided saccades, and a go/no-go test, with variables of interest emphasizing response preparation, response inhibition, and working memory.

RESULTS

As a group, children with ADHD demonstrated significant deficits in oculomotor response preparation (VGS latency and variability) and response inhibition but not working memory. Girls, but not boys with ADHD, had significantly longer VGS latencies, even after controlling for differences in ADHD symptom severity. The ADHD subtypes did not differ on response preparation or inhibition measures; however, children with the Inattentive subtype were less accurate on the working memory task than those with the Combined subtype.

CONCLUSIONS

Sex differences in children with ADHD extend beyond symptom presentation to the development of oculomotor control. Saccade latency may represent a specific deficit among girls with ADHD.

Distinct response time distributions in attention deficit hyperactivity disorder subtypes

OBJECTIVE

To address the issue of response time (RT) profiles in hyperactive-impulsive (ADHD-HI), inattentive (ADHD-IA), and combined (ADHD-C) subtypes of ADHD. We hypothesized that children with ADHD-HI should respond more rapidly than children without ADHD and children with ADHD-IA and ADHD-C should respond more slowly than children without ADHD.

METHOD

Four groups (3 ADHD groups and 1 non-ADHD group) each composed of 16 children (7-13 years old) performed a visuospatial choice task.

RESULTS

ANOVA indicated very variable RTs for each ADHD subtype when controlling for individual RT. ANOVA performed on RT distribution showed significant differences between the ADHD and non-ADHD groups: biased to fast responses in ADHD-HI and biased to slow responses in ADHD-IA and ADHD-C.

CONCLUSION

The results suggest that response time profiles were abnormal in all ADHD subtypes and were markedly different between children meeting criteria for ADHD-HI and those meeting criteria for ADHD-IA or ADHD-C.

Why cognitive performance in ADHD may not reveal true potential: findings from a large population-based sample

Focusing on symptoms of attention deficit hyperactivity disorder (ADHD) in a sample obtained from the general population, we aimed to investigate the effects of incentives and event rate on reaction time (RT) performance and response inhibition. We assessed 1156 children, at a mean age of 8 years, on their performance on an inhibition task and a RT task under different experimental conditions that manipulated event rate and incentives. Children with high ADHD (ADHD-H) symptoms showed cognitive performance deficits only under some of the experimental conditions compared to a control group. The fast-incentive condition of the RT task succeeded in normalizing the RT variability, as well as the slow overall speed, in the ADHD-H group. Analyses of ADHD symptom scores as a quantitative trait in the total sample were overall consistent with these findings. The findings suggest that at least some cognitive performance deficits in children with high ADHD symptoms do not reflect stable cognitive deficits. The degree to which cognitive impairments in ADHD can be modulated by energetic or motivational factors has important implications for clinical and educational interventions.

Increased intrasubject variability in response time in youths with bipolar disorder and at-risk family members

OBJECTIVE

Deficits in sustained attention may represent an endophenotype for bipolar disorder (BD). One heritable measure of sustained attention is intrasubject variability in response time (ISV-RT). We tested the hypothesis that, compared with controls, both youths with BD and those at familial risk for the disorder would have increased ISV-RT.

METHOD

Subjects were 28 patients with BD, 26 unaffected youths with a first-degree relative with BD, and 24 control youths without an affected relative, all aged 7 to 17 years. Subjects completed the Flanker Continuous Performance Test.

RESULTS

Bipolar disorder and at-risk youths had increased ISV-RT, compared with the controls. Differences were independent of comorbid psychopathology in youths with BD and present in psychiatrically healthy at-risk youths.

CONCLUSIONS

Increased ISV-RT may be a risk marker for BD. Further research is needed to investigate the neural and genetic underpinnings of this deficit, as well as the specificity of the finding to BD.

Regulation of cognitive resources during an n-back task in youth-onset psychosis and attention-deficit/hyperactivity disorder (ADHD)

The goals of the current study were to use behavioral and pupillary measures to examine working memory on a spatial n-back task in 8-20-year-olds with youth-onset psychosis or ADHD (Combined subtype) and healthy controls to determine the contribution of different attentional factors to spatial working memory impairments, and to examine if age-related changes in performance differed across groups. Although both clinical groups had lower perceptual sensitivity on both 0- and 1-back, there was no evidence of an impairment in spatial working memory or differential order effects on the 0-back. Instead, results suggest that both clinical groups had difficulty encoding the stimuli. They also appeared to have difficulty maintaining attention and/or readiness to respond, and, to a lesser extent, recruiting resources on a trial-to-trial basis. It is likely that these attentional problems prevented the clinical groups from encoding the stimuli effectively and contributed to their general performance deficits.

Variability of attention processes in ADHD: observations from the classroom

OBJECTIVE

Classroom- and laboratory-based efforts to study the attentional problems of children with ADHD are incongruent in elucidating attentional deficits; however, none have explored within- or between-minute variability in the classroom attentional processing in children with ADHD.

METHOD

High and low attention groups of ADHD children defined via cluster analysis, and 36 typically developing children, were observed while completing academic assignments in their general education classrooms.

RESULTS

All children oscillated between attentive and inattentive states; however, children in both ADHD groups switched states more frequently and remained attentive for shorter durations relative to typically developing children.

CONCLUSION

Overall differences in attention and optimal ability to maintain attention among the groups are consistent with laboratory studies of increased ADHD-related interindividual and intergroup variability but inconsistent with laboratory results of increased intra-individual variability and attention decrements over time.

A lack of default network suppression is linked to increased distractibility in ADHD

Heightened distractibility in participants with ADHD as indexed by increased reaction time (RT) variability has been hypothesized to be due to a failure to sufficiently suppress activation in the default attention network during cognitively demanding situations. The present study utilized fMRI to examine the relationship between intra-individual variability (IIV) in task RT and suppression of BOLD response in regions of the default network, using a working memory paradigm and two levels of control tasks. IIV was calculated separately for thirteen healthy control and twelve children with ADHD, Combined Type. Children with ADHD displayed significantly more RT variability than controls. Neural measures showed that although both groups displayed a pattern of increasing deactivation of the medial prefrontal cortex (PFC) with increasing task difficulty, the ADHD group was significantly less deactive than controls. Correlations between IIV and brain activation suggested that greater variability was associated with a failure to deactivate ventromedial PFC with increasing task difficulty. T-tests on brain activation between participants with ADHD with low versus high IIV implicated a similar region so that high variability was associated with greater activity in this region. These data provide support for the theory that increased distractibility in at least some participants with ADHD may be due to an inability to sufficiently suppress activity in the default attention network in response to increasing task difficulty.

Origins of altered reinforcement effects in ADHD

Attention-deficit/hyperactivity disorder (ADHD), characterized by hyperactivity, impulsiveness and deficient sustained attention, is one of the most common and persistent behavioral disorders of childhood. ADHD is associated with catecholamine dysfunction. The catecholamines are important for response selection and memory formation, and dopamine in particular is important for reinforcement of successful behavior. The convergence of dopaminergic mesolimbic and glutamatergic corticostriatal synapses upon individual neostriatal neurons provides a favorable substrate for a three-factor synaptic modification rule underlying acquisition of associations between stimuli in a particular context, responses, and reinforcers. The change in associative strength as a function of delay between key stimuli or responses, and reinforcement, is known as the delay of reinforcement gradient. The gradient is altered by vicissitudes of attention, intrusions of irrelevant events, lapses of memory, and fluctuations in dopamine function. Theoretical and experimental analyses of these moderating factors will help to determine just how reinforcement processes are altered in ADHD. Such analyses can only help to improve treatment strategies for ADHD.

Anatomy of an error: a bidirectional state model of task engagement/disengagement and attention-related errors

We present arguments and evidence for a three-state attentional model of task engagement/disengagement. The model postulates three states of mind-wandering: occurrent task inattention, generic task inattention, and response disengagement. We hypothesize that all three states are both causes and consequences of task performance outcomes and apply across a variety of experimental and real-world tasks. We apply this model to the analysis of a widely used GO/NOGO task, the Sustained Attention to Response Task (SART). We identify three performance characteristics of the SART that map onto the three states of the model: RT variability, anticipations, and omissions. Predictions based on the model are tested, and largely corroborated, via regression and lag-sequential analyses of both successful and unsuccessful withholding on NOGO trials as well as self-reported mind-wandering and everyday cognitive errors. The results revealed theoretically consistent temporal associations among the state indicators and between these and SART errors as well as with self-report measures. Lag analysis was consistent with the hypotheses that temporal transitions among states are often extremely abrupt and that the association between mind-wandering and performance is bidirectional. The bidirectional effects suggest that errors constitute important occasions for reactive mind-wandering. The model also enables concrete phenomenological, behavioral, and physiological predictions for future research.

Dopamine vs noradrenaline: inverted-U effects and ADHD theories

The aim of the present study was to review the dopamine theory of attention-deficit-hyperactivity disorder (ADHD), in light of recent use of noradrenergic therapies. A historical review of pharmacological theories of ADHD was conducted, including inverted-U, spatial working memory and neural circuit aspects. Pharmacological advances, including animal and human studies of dopaminergic and noradrenergic mechanisms at the prefrontal cortex (PFC), indicate that alpha-2A adrenoreceptor stimulation results in increased dendritic firing during delay periods for preferred directions, while moderate levels of D1 receptor stimulation result in reduction of delay-related firing to non-preferred directions, allowing representational control in the PFC. Recent studies of the COMT val/met gene and stimulant medication response may help explain variation in inverted-U responses in individuals. Further studies utilizing delay-related firing paradigms should be useful in the investigation of attentional syndromes, and responses to newer pharmacological treatments.

Increased intra-individual reaction time variability in attention-deficit/hyperactivity disorder across response inhibition tasks with different cognitive demands

One of the most consistent findings in children with ADHD is increased moment-to-moment variability in reaction time (RT). The source of increased RT variability can be examined using ex-Gaussian analyses that divide variability into normal and exponential components and Fast Fourier transform (FFT) that allow for detailed examination of the frequency of responses in the exponential distribution. Prior studies of ADHD using these methods have produced variable results, potentially related to differences in task demand. The present study sought to examine the profile of RT variability in ADHD using two Go/No-go tasks with differing levels of cognitive demand. A total of 140 children (57 with ADHD and 83 typically developing controls), ages 8-13 years, completed both a "simple" Go/No-go task and a more "complex" Go/No-go task with increased working memory load. Repeated measures ANOVA of ex-Gaussian functions revealed for both tasks children with ADHD demonstrated increased variability in both the normal/Gaussian (significantly elevated sigma) and the exponential (significantly elevated tau) components. In contrast, FFT analysis of the exponential component revealed a significant task x diagnosis interaction, such that infrequent slow responses in ADHD differed depending on task demand (i.e., for the simple task, increased power in the 0.027-0.074 Hz frequency band; for the complex task, decreased power in the 0.074-0.202 Hz band). The ex-Gaussian findings revealing increased variability in both the normal (sigma) and exponential (tau) components for the ADHD group, suggest that both impaired response preparation and infrequent "lapses in attention" contribute to increased variability in ADHD. FFT analyses reveal that the periodicity of intermittent lapses of attention in ADHD varies with task demand. The findings provide further support for intra-individual variability as a candidate intermediate endophenotype of ADHD.

Error Monitoring in College Students with Attention-Deficit/Hyperactivity Disorder

Recent error-related event-related potential (ERP) studies suggest that children with attention-deficit/hyperactivity disorder (ADHD) display deficits in error monitoring compared to control children. Information regarding error monitoring deficit in adults with ADHD, however, is scarce. We investigated error monitoring in a sample of college students with ADHD and compared them to their control peers. In addition to error (-related) negativity (Ne/ERN) and error positivity (Pe), we examined behavioral performance such as reaction time (RT) as well as self-reported monitoring behaviors in daily-life situations. Thirty-two college students with no known disorders and 36 college students with ADHD between 18 and 30 years of age participated in this study. Results showed that college students with ADHD were slower in their RT and displayed more RT variability. College students with ADHD also demonstrated significantly smaller Ne/ERN amplitude and shorter Ne/ERN latency compared to control students. There were, however, no significant group differences in either Pe amplitude or Pe latency. With self-reported monitoring behaviors, ADHD students reported significantly more difficulties in both the Self-Monitor and Task Monitor scales of the Behavioral Rating Inventory of Executive Function – Adult Version (BRIEF-A) compared to control students. Collectively, these results suggest that college students with ADHD have atypical error monitoring as measured by brain processing, behavioral performance, and self-report.

The Attenuation of Very Low Frequency Brain Oscillations in Transitions from a Rest State to Active Attention

Background: The default mode interference hypothesis ( Sonuga-Barke & Castellanos, 2007 ) predicts (1) the attenuation of very low frequency oscillations (VLFO; e.g., .05 Hz) in brain activity within the default mode network during the transition from rest to task, and (2) that failures to attenuate in this way will lead to an increased likelihood of periodic attention lapses that are synchronized to the VLFO pattern. Here, we tested these predictions using DC-EEG recordings within and outside of a previously identified network of electrode locations hypothesized to reflect DMN activity (i.e., S3 network; Helps et al., 2008 ). Method: 24 young adults (mean age 22.3 years; 8 male), sampled to include a wide range of ADHD symptoms, took part in a study of rest to task transitions. Two conditions were compared: 5 min of rest (eyes open) and a 10-min simple 2-choice RT task with a relatively high sampling rate (ISI 1 s). DC-EEG was recorded during both conditions, and the low-frequency spectrum was decomposed and measures of the power within specific bands extracted. Results: Shift from rest to task led to an attenuation of VLFO activity within the S3 network which was inversely associated with ADHD symptoms. RT during task also showed a VLFO signature. During task there was a small but significant degree of synchronization between EEG and RT in the VLFO band. Attenuators showed a lower degree of synchrony than nonattenuators. Discussion: The results provide some initial EEG-based support for the default mode interference hypothesis and suggest that failure to attenuate VLFO in the S3 network is associated with higher synchrony between low-frequency brain activity and RT fluctuations during a simple RT task. Although significant, the effects were small and future research should employ tasks with a higher sampling rate to increase the possibility of extracting robust and stable signals.

The predictive brain state: asynchrony in disorders of attention?

It is postulated that a key function of attention in goal-oriented behavior is to reduce performance variability by generating anticipatory neural activity that can be synchronized with expected sensory information. A network encompassing the prefrontal cortex, parietal lobe, and cerebellum may be critical in the maintenance and timing of such predictive neural activity. Dysfunction of this temporal process may constitute a fundamental defect in attention, causing working memory problems, distractibility, and decreased awareness.

SNPs in dopamine D2 receptor gene (DRD2) and norepinephrine transporter gene (NET) are associated with continuous performance task (CPT) phenotypes in ADHD children and their families

Haplotype-tagging SNP analyses were conducted to identify molecular genetic substrates of quantitative phenotypes derived from performance on a Continuous Performance Task (CPT). Three hundred sixty-four individuals were sampled from 152 families ascertained on the basis of at least one child having ADHD. Probands, their affected and unaffected siblings, and parents were administered a CPT. Four different components of performance were analyzed and tested for association with SNPs from 10 candidate genes involved in monoaminergic function. After correcting for multiple comparisons and controlling for multiple individuals from the same family, significant associations were identified between commission errors and SNPs in the DRD2 gene (rs2075654, rs1079596), and between reaction time variability and a SNP in the NET gene (rs3785155). These findings suggest that commission errors and reaction time variability are excellent candidates as ADHD endophenotypes based on previously published criteria. Results also shed light on the molecular genetic basis of specific processes that may underlie the disorder.

Model comparison for automatic characterization and classification of average ERPs using visual oddball paradigm

OBJECTIVE

To determine whether automated classifiers can be used for correctly identifying target categorization responses from averaged event-related potentials (ERPs) along with identifying appropriate features and classification models for computer-assisted investigation of attentional processes.

METHODS

ERPs were recorded during a target categorization task. Automated classification of average target ERPs versus average non-target ERPs was performed by extracting different combinations of features from the P300 and N200 components, which were used to train six classifiers: Euclidean classifier (EC), Mahalanobis discriminant (MD), quadratic classifier (QC), Fisher linear discriminant (FLD), multi-layer perceptron neural network (MLP) and support vector machine (SVM).

RESULTS

The best classification performance (accuracy: 91-92%; sensitivity: 85-86%; specificity: 95-99%) was provided by QC, MLP, SVM on feature vectors extracted from P300 recorded at multiple sites. In general, non-linear and non-parametric classifiers (QC, MLP, SVM) performed better than linear classifiers (EC, MD, FLD). The N200 did not explain variance beyond that of P300 recorded at multiple sites.

CONCLUSIONS

The results suggest that automatic characterization and classification of average target and non-target ERPs is feasible. Features of P300 recorded at multiple sites used to train non-linear classifiers are recommended for optimal classification performance.

SIGNIFICANCE

Automatic characterization of target ERPs can provide an objective approach for detecting and diagnosing abnormalities and evaluating interventions for clinical populations, paving the way for future real-time monitoring of attentional processes.

Symptoms of hyperactivity and inattention can mediate deficits of postural stability in developmental dyslexia

Developmental dyslexia is a reading disorder associated with impaired postural control. However, such deficits are also found in attention deficit hyperactivity disorder (ADHD), which is present in a substantial subset of dyslexia diagnoses. Very few studies of balance in dyslexia have assessed ADHD symptoms, thereby motivating the hypothesis that such measures can account for the group differences observed. In this study, we assessed adults with dyslexia and similarly aged controls on a battery of cognitive, literacy and attention measures, alongside tasks of postural stability. Displacements of centre of mass to perturbations of posture were measured in four experimental conditions using digital optical motion capture. The largest group differences were obtained in conditions where cues to the support surface were reduced. Between-group differences in postural sway and in sway variability were largely accounted for by co-varying hyperactivity and inattention ratings, however. These results therefore suggest that postural instability in dyslexia is more strongly associated with symptoms of ADHD than to those specific to reading impairment.

Decomposing intra-subject variability in children with attention-deficit/hyperactivity disorder

BACKGROUND

Increased intra-subject response time standard deviations (RT-SD) discriminate children with attention-deficit/hyperactivity disorder (ADHD) from healthy control subjects. The RT-SD is averaged over time; thus it does not provide information about the temporal structure of RT variability. We previously hypothesized that such increased variability might be related to slow spontaneous fluctuations in brain activity occurring with periods between 15 sec and 40 sec. Here, we investigated whether these slow RT fluctuations add unique differentiating information beyond the global increase in RT-SD.

METHODS

We recorded RT at 3-sec intervals for 15 min during an Eriksen flanker task for 29 children with ADHD and 26 age-matched typically developing control subjects (TDC) (mean ages 12.5 +/- 2.4 and 11.6 +/- 2.5; 26 and 12 boys, respectively). The primary outcome was the magnitude of the spectral component in the frequency range between .027 and .073 Hz measured with continuous Morlet wavelet transform.

RESULTS

The magnitude of the low-frequency fluctuation was greater for children with ADHD compared with TDC (p = .02, d = .69). After modeling ADHD diagnosis as a function of RT-SD, adding this specific frequency range significantly improved the model fit (p = .03; odds ratio = 2.58).

CONCLUSIONS

Fluctuations in low-frequency RT variability predict the diagnosis of ADHD beyond the effect associated with global differences in variability. Future studies will examine whether such spectrally specific fluctuations in behavioral responses are linked to intrinsic regional cerebral hemodynamic oscillations that occur at similar frequencies.

Possible association of the alpha-2A-adrenergic receptor gene with response time variability in attention deficit hyperactivity disorder

Previous studies have demonstrated that the MspI and DraI polymorphisms at the alpha-2A-adrenergic receptor gene (ADRA2A) are associated with ADHD. However, few studies have been designed to ascertain the association between the ADRA2A genotypes and the performance on neurocognitive measures in ADHD. The aims of this study were to examine the association of the ADRA2A MspI and DraI polymorphisms with ADHD in Korean subjects, and to determine the relationship between the genotypes of these two polymorphisms and the candidate endophenotypes, as measured by the continuous performance test (CPT). In a case-control study, we assessed 186 ADHD probands and 150 normal controls. One hundred eight trios were studied in a family based association analysis. The transmission disequilibrium test (TDT) analysis showed preferential transmission of the C allele of the DraI polymorphism (chi(2) = 5.88, P = 0.015). In the haplotype analyses, a trend of over-transmission of haplotype C/C was observed (chi(2) = 3.80, P = 0.051). The homozygous subjects for the C allele (C/C genotype) at the DraI polymorphism showed a trend toward a higher mean T-score with respect to the response time variability profiles of the CPT than did those with the other genotypes (C/T + T/T genotypes; P = 0.042). The homozygous subjects for the G allele (G/G genotype) at the MspI polymorphism showed a tendency to have a lower mean T-score with respect to the response time variability profiles of the CPT (P = 0.068). The results of this study provide important evidence for the involvement of the ADRA2A MspI and DraI polymorphisms in the etiology of ADHD in Korean subjects. In addition, our results provide evidence for the possible role of these two polymorphisms in ADHD symptom expression, such as increased response time variability.

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.

The predictive brain state: timing deficiency in traumatic brain injury?

Attention and memory deficits observed in traumatic brain injury (TBI) are postulated to result from the shearing of white matter connections between the prefrontal cortex, parietal lobe, and cerebellum that are critical in the generation, maintenance, and precise timing of anticipatory neural activity. These fiber tracts are part of a neural network that generates predictions of future states and events, processes that are required for optimal performance on attention and working memory tasks. The authors discuss the role of this anticipatory neural system for understanding the varied symptoms and potential rehabilitation interventions for TBI. Preparatory neural activity normally allows the efficient integration of sensory information with goal-based representations. It is postulated that an impairment in the generation of this activity in traumatic brain injury (TBI) leads to performance variability as the brain shifts from a predictive to reactive mode. This dysfunction may constitute a fundamental defect in TBI as well as other attention disorders, causing working memory deficits, distractibility, a loss of goal-oriented behavior, and decreased awareness.

Placing neuroanatomical models of executive function in a developmental context: imaging and imaging--genetic strategies

Children show gradual and protracted improvement in an array of behaviors involved in the conscious control of thought and emotion. Behavioral research has shown that these abilities, collectively referred to as executive functions (EF), can be dissociated into separable processes, such as inhibition and working memory. Furthermore, noninvasive neuroimaging shows that these component processes often rely on separable neural circuits involving areas of the frontal cortex and nuclei of the basal ganglia. As additional noninvasive methodologies become available, it is increasingly possible to continue to dissect and dissociate components of EF and also test predictions made by a number of theoretical neuroanatomical models. One method of late is genetics, which is noninvasive and readily used in concert with neuroimaging. The biological data obtained with neuroimaging and genetics is particularly able to inform neuroanatomical models that link specific brain systems with higher more abstract process models derived from purely behavioral work. As much progress in this area continues to occur, we seek to evaluate the age dependency and manner in which certain aspects of EF and certain anatomical circuits show changes and interactions as children develop. Some examples are taken from research on children with the developmental disability attention deficit hyperactivity disorder. A review of selected developmental research shows that current cognitive and neuroanatomical models of EF offer a great many system- and synaptic-level hypotheses that can be tested using imaging and imaging genetics in longitudinal and cross-sectional study designs. Here, we focus on age-related changes in inhibition and working memory.

Functional magnetic resonance imaging evidence for abnormalities in response selection in attention deficit hyperactivity disorder: differences in activation associated with response inhibition but not habitual motor response

Impaired response inhibition is thought to be a core deficit in attention deficit hyperactivity disorder (ADHD). Prior imaging studies investigating response inhibition in children with ADHD have used tasks involving different cognitive resources, thereby complicating the interpretation of their findings. In this study, a classical go/no-go task with a well-ingrained stimulus-response association (green = go; red = no-go) was used in order to minimize extraneous cognitive demands. Twenty-five children with ADHD and 25 typically developing (TD) children between the ages of 8 and 13 years and group-matched for IQ and performance on the go/no-go task were studied using event-related functional magnetic resonance imaging (fMRI). Analyses were used to examine differences in activation between the ADHD and TD groups for "go" (habitual motor response) and "no-go" (requiring inhibition of the motor response) events. Region-of-interest analyses revealed no between-group difference in activation in association with "go" events. For "no-go" events, the children with ADHD demonstrated significantly less activation than did TD controls within a network important for inhibiting a motor response to a visual stimulus, with frontal differences localized to the pre-supplementary motor area. Although blood oxygenation level-dependent fMRI data show no differences between children with ADHD and TD children in association with a habituated motor "go" response, during "no-go" events, which require selecting not to respond, children with ADHD show diminished recruitment of networks important for response inhibition. The findings suggest that abnormalities in circuits important for motor response selection contribute to deficits in response inhibition in children with ADHD and lend support to the growing awareness of ADHD-associated anomalies in medial frontal regions important for the control of voluntary actions.

Inhibitory performance, response speed, intraindividual variability, and response accuracy in ADHD

OBJECTIVE

To determine the potential of inhibitory performance, response speed, and response accuracy and variability, measures central to the conceptualization of attention-deficit/hyperactivity disorder (ADHD), in distinguishing children with ADHD from healthy controls (HCs).

METHOD

The stop signal paradigm was administered to 38 children with ADHD and 31 NCs. The stop signal reaction time (SSRT), mean reaction time (MRT), intraindividual coefficient of variation (ICV), and number of errors were used to predict diagnostic status.

RESULTS

Univariate tests showed that the ADHD group performed worse than NCs on all of the dependent variables. Exploratory univariate analyses showed that oppositional defiant disorder comorbidity and ADHD type did not influence results except for the ICV, the effect for this variable (more variability in the ADHD group) being less pronounced for the Predominantly Inattentive type than for the Hyperactive-Impulsive and Combined types. A logistic regression model of the MRT, ICV, and number of errors combined showed best predictive performance, with the MRT contributing the most to group classification (56% of the variance). The final model (MRT, ICV, and number of errors) predicted 87% of the sample in the correct diagnostic category. Operating characteristics showed excellent sensitivity and specificity of 89.5% and 83.9%, respectively.

CONCLUSIONS

Our results contrast with theoretical accounts emphasizing inhibitory control as the pivotal measure characterizing cognitive performance in ADHD. Results are discussed in the context of a delay aversion perspective of ADHD.

Intra-individual variability in ADHD, autism spectrum disorders and Tourette's syndrome

The potential for response variability to serve as an endophenotype for attention deficit hyperactivity disorders (ADHD) rests, in part, upon the development of reliable and valid methods to decompose variability. This study investigated the specificity of intra-individual variability (IIV) in 53 children with ADHD by comparing them with 25 children with high functioning autism (HFA), 32 children with autism spectrum disorders (ASD), who also were comorbid for ADHD (ASD+ADHD), 21 children with Tourette's syndrome (TS), and 85 typically developing controls (TD). In order to decompose the variability of the reaction times, we applied three distinct techniques: ex-Gaussian modeling, intra-individual variability analysis, and spectral analysis. Our data revealed that children with HFA and children with ASD+ADHD exhibited substantial IIV compared with ADHD and TD children. We argue that: (1) all three methods lead to a single consistent conclusion; (2) careful documentation of the analytic steps used in spectral analysis is mandatory for comparison between studies; (3) the presence of comorbidities may constitute an important factor in the observed response variability in previous studies of ADHD.

Response inhibition and response selection: two sides of the same coin

Response inhibition refers to the suppression of actions that are inappropriate in a given context and that interfere with goal-driven behavior. Studies using a range of methodological approaches have implicated executive control processes mediated by frontal-subcortical circuits as being critical to response inhibition; however, localization within the frontal lobe has been inconsistent. In this review, we present evidence from behavioral, lesion, neuroimaging, electrophysiology, and neurological population studies. The findings lay the foundation for a construct in which response inhibition is akin to response selection, such that pre-SMA circuits are critical to selection of appropriate behavior, including both selecting to engage appropriate motor responses and selecting to withhold (inhibit) inappropriate motor responses. Recruitment of additional prefrontal and posterior cortical circuits, necessary to guide response selection, varies depending on the cognitive and behavioral demands of the task.

Utility of objective measures of activity and attention in the assessment of therapeutic response to stimulants in children with attention-deficit/hyperactivity disorder

BACKGROUND AND PURPOSE

Attention-deficit/hyperactivity disorder (ADHD) is a highly prevalent disorder that can respond dramatically to medication, if dose is appropriately titrated. Studies suggest that computer measures of attention cannot be used for titration as they show improvement on doses too low to produce clinical benefits. We assessed whether measures of motor activity and attention using the McLean Motion Attention Test (M-MAT) could identify doses associated with optimal clinical response.

METHODS

Eleven boys (9.6 +/- 1.8 years), receiving treatment with methylphenidate, and meeting DSM-IV criteria for ADHD, participated in this triple-blind (parent, child, rater), within-subject, efficacy study. Subjects received 1 week each of placebo, low (0.4 mg/kg), medium (0.8 mg/kg), and high (1.5 mg/kg) daily doses of methylphenidate. Parents rated response using an index of clinical global improvement.

RESULTS

In 9/11 subjects, the dose that produced the best improvement on M-MAT measures was also the dose that produced the best clinical outcome (p < 10(5)). Parents rated response to this dose significantly better than response to previously prescribed treatment. Objective measures of primarily activity and secondarily attention responded to treatment in a manner concordant with clinical ratings, suggesting that these measures have ecological validity, and the potential to facilitate medication management and titration.

Neuroimaging of attention deficit hyperactivity disorder: can new imaging findings be integrated in clinical practice?

Recent advances in neuroimaging research have helped elucidate the neurobiology of attention deficit hyperactivity disorder (ADHD) and the mechanisms by which medications used to treat ADHD exert their effects. The complex nature and array of imaging techniques, however, present challenges for the busy clinician in assessing possible clinical uses of brain imaging. Even though currently there are no accepted uses for imaging in diagnosing ADHD (other than ruling out identifiable medical or neurologic conditions that may mimic ADHD), this review introduces the main imaging techniques used to study ADHD, identifies relevant complexities facing psychiatric researchers in implementing neuroimaging techniques for clinical purposes, and provides benchmarks to help determine when imaging modalities have advanced to a point that they are deemed clinically useful.

Going AWOL in the Brain: Mind Wandering Reduces Cortical Analysis of External Events

Converging evidence from neuroscience suggests that our attention to the outside world waxes and wanes over time. We examined whether these periods of “mind wandering” are associated with reduced cortical analysis of the external environment. Participants performed a sustained attention to response task in which they responded to frequent “nontargets” (digits 0–9) and withheld responses for infrequent “targets” (the letter X). Mind wandering was defined both behaviorally, indicated by a failure to withhold a response to a target, and subjectively, via self-report at a thought probe. The P300 event-related potential component for nontargets was reduced prior to both the behavioral and subjective reports of mind wandering, relative to periods of being “on-task.” Regression analysis of P300 amplitude revealed significant common variance between behavioral and subjective markers of mind wandering, suggesting that both markers reflect a common underlying mental state. Finally, control analysis revealed that the effect of mind wandering on the P300 could not be ascribed to changes in motor activity nor was it associated with general arousal. Our data suggest that when trying to engage attention in a sustained manner, the mind will naturally ebb and flow in the depth of cognitive analysis it applies to events in the external environment.

Cingulate-precuneus interactions: a new locus of dysfunction in adult attention-deficit/hyperactivity disorder

BACKGROUND

Pathophysiologic models of attention-deficit/hyperactivity disorder (ADHD) have focused on frontal-striatal circuitry with alternative hypotheses relatively unexplored. On the basis of evidence that negative interactions between frontal foci involved in cognitive control and the non-goal-directed "default-mode" network prevent attentional lapses, we hypothesized abnormalities in functional connectivity of these circuits in ADHD.

METHODS

Resting-state blood oxygen level-dependent functional magnetic resonance imaging (fMRI) scans were obtained at 3.0-Tesla in 20 adults with ADHD and 20 age- and sex-matched healthy volunteers.

RESULTS

Examination of healthy control subjects verified presence of an antiphasic or negative relationship between activity in dorsal anterior cingulate cortex (centered at x = 8, y = 7, z = 38) and in default-mode network components. Group analyses revealed ADHD-related compromises in this relationship, with decreases in the functional connectivity between the anterior cingulate and precuneus/posterior cingulate cortex regions (p < .0004, corrected). Secondary analyses revealed an extensive pattern of ADHD-related decreases in connectivity between precuneus and other default-mode network components, including ventromedial prefrontal cortex (p < 3 x 10(-11), corrected) and portions of posterior cingulate (p < .02, corrected).

CONCLUSIONS

Together with prior unbiased anatomic evidence of posterior volumetric abnormalities, our findings suggest that the long-range connections linking dorsal anterior cingulate to posterior cingulate and precuneus should be considered as a candidate locus of dysfunction in ADHD.

Segmenting the stream of consciousness: The psychological correlates of temporal structures in the time series data of a continuous performance task

Using principal component analysis, we examined whether structural properties in the time series of response time would identify different mental states during a continuous performance task. We examined whether it was possible to identify regular patterns which were present in blocks classified as lacking controlled processing, either behaviourally (as a failure to withhold a response to a target) or subjectively (as an off task report at a thought probe). Principal component analysis identified three components present in response times accounting for 58.8% of the variance in the data. Of these components, the second largest factor showed two features that implied it was a marker for mind wandering. First, it was stronger under slow relative to fast stimulus presentation conditions, and so paralleled the distribution of mind wandering reports. Second, it was more powerful prior to behaviour markers of mind wandering (failures in response inhibition) and less powerful prior to reports of task focused thinking (on task reports). Taken together, the use of principal components analysis on response times seem a viable tool for differentiating different mental states and so could help identify the neural substrates which underpin mind wandering and other subjective states.

Intra-individual reaction time variability in schizophrenia, depression and borderline personality disorder

Intra-individual reaction time variability (IIV) in neuropsychological task performance reflects short term fluctuations in performance. Increased IIV has been reported in patients with schizophrenia and could be related to a deficient neural timing mechanism, but the role of IIV in adult patients with other psychiatric disorders has not been established. Therefore, we compared IIV measures obtained in a Go/Nogo task from patients with schizophrenia, major depression and borderline personality disorder. IIV was increased for patients with schizophrenia. When correcting for differences in mean reaction time, depressive and borderline patients also showed increased IIV. Importantly, all groups showed a strong association between IIV and accuracy of task performance. This suggests that increased IIV might be a sensitive marker for the efficiency of top-down attentional control in all diagnostic groups. Aside from these similarities, the complete results including measures of IIV, mean reaction time and accuracy show differential patterns for patients with schizophrenia compared to those with borderline personality disorder or depression. These results are discussed with respect to common versus disorder-specific neural mechanisms underlying increased IIV.

Attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a behavioral diagnosis based on the presence of developmentally inappropriate levels of impulsivity, overactivity, and inattentiveness. It is a familial condition with a complex pattern of inheritance. Variation of several genes involved in the regulation of dopamine, norepinephrine, and serotonin neurotransmission is associated with ADHD. We highlight the two most prominent findings with the dopamine D4 receptor (DRD4) gene and the dopamine transporter (DAT1) gene, and their implications for the understanding of the cellular and neurobiological basis for ADHD. Cognitive and functional studies using electrophysiology and brain imaging frequently indicate altered processing in ADHD during performance on cognitive tasks hypothesized to measure a "core" deficit, such as response inhibition. Yet, children with ADHD appear to suffer from a more general deficit, including impairment in attentional alerting, orienting, response preparation, and control. Reward processes are also altered and, further, a strong association emerges with intraindividual variability, with several causal hypotheses being proposed. Task performance correlates with underactivation of, especially, frontostriatal areas of the brain, but an extended network of brain regions is also implicated. Electroencephalography studies indicate abnormalities in ADHD in relation to slow-wave activity, linked to underarousal. These advances in the areas of genetics, cognitive function, neurophysiology, and neuroanatomy of ADHD give important leads for interdisciplinary research that aims to delineate the causal pathways. Such research is only at its beginning, but is illustrated by recent findings of an association between DAT1 and increased response variability in ADHD.

The neurobiological profile of girls with ADHD

Since boys are more commonly diagnosed with Attention Deficit Hyperactivity Disorder (ADHD) than girls, the majority of theories and published research studies of ADHD have been based on samples comprised primarily (or exclusively) of boys. While psychosocial impairment in girls with ADHD is well established, the neuropsychological and neurobiological basis of these deficits is less consistently observed. There is growing evidence that boys' and girls' brains develop and mature at different rates, suggesting that the trajectory of early anomalous brain development in ADHD may also be sex-specific. It remains unclear, however, whether earlier brain maturation observed in girls with ADHD is protective. In this review, we outline the current theory and research findings that seek to establish a unique neurobiological profile of girls with ADHD, highlighting sex differences in typical brain development and among children with ADHD. The review highlights findings from neurological, neurocognitive, and behavioral studies. Future research directions are suggested, including the need for longitudinal neuroimaging and neurobehavioral investigation beginning as early as the preschool years, and continuing through adolescence and adulthood, with consideration of identified sex differences in the development of ADHD.

Quantifying ADHD classroom inattentiveness, its moderators, and variability: a meta-analytic review

BACKGROUND

Most classroom observation studies have documented significant deficiencies in the classroom attention of children with attention-deficit/hyperactivity disorder (ADHD) compared to their typically developing peers. The magnitude of these differences, however, varies considerably and may be influenced by contextual, sampling, diagnostic, and observational differences.

METHODS

Meta-analysis of 23 between-group classroom observation studies using weighted regression, publication bias, goodness of fit, best case, and original metric analyses.

RESULTS

Across studies, a large effect size (ES = .73) was found prior to consideration of potential moderators. Weighted regression, best case, and original metric estimation indicate that this effect may be an underestimation of the classroom visual attention deficits of children with ADHD. Several methodological factors-classroom environment, sample characteristics, diagnostic procedures, and observational coding schema-differentially affect observed rates of classroom attentive behavior for children with ADHD and typically developing children. After accounting for these factors, children with ADHD were on-task approximately 75% of the time compared to 88% for their classroom peers (ES = 1.40). Children with ADHD were also more variable in their attentive behavior across studies.

CONCLUSIONS

The present study confirmed that children with ADHD exhibit deficient and more variable visual attending to required stimuli in classroom settings and provided an aggregate estimation of the magnitude of these deficits at the group level. It also demonstrated the impact of situational, sampling, diagnostic, and observational variables on observed rates of on-task behavior.

Reaction time performance in ADHD: improvement under fast-incentive condition and familial effects

BACKGROUND

Reaction time (RT) variability is one of the strongest findings to emerge in cognitive-experimental research of attention deficit hyperactivity disorder (ADHD). We set out to confirm the association between ADHD and slow and variable RTs and investigate the degree to which RT performance improves under fast event rate and incentives. Using a group familial correlation approach, we tested the hypothesis that there are shared familial effects on RT performance and ADHD.

METHOD

A total of 144 ADHD combined-type probands, 125 siblings of the ADHD probands and 60 control participants, ages 6-18, performed a four-choice RT task with baseline and fast-incentive conditions.

RESULTS

ADHD was associated with slow and variable RTs, and with greater improvement in speed and RT variability from baseline to fast-incentive condition. RT performance showed shared familial influences with ADHD. Under the assumption that the familial effects represent genetic influences, the proportion of the phenotypic correlation due to shared familial influences was estimated as 60-70%.

CONCLUSIONS

The data are inconsistent with models that consider RT variability as reflecting a stable cognitive deficit in ADHD, but instead emphasize the extent to which energetic or motivational factors can have a greater effect on RT performance in ADHD. The findings support the role of RT variability as an endophenotype mediating the link between genes and ADHD.

Response variability in attention-deficit disorders

Reaction times in a mental rotation task were measured across a diverse population that sorted into two groupings based on overall variability. Although both the low- and the high-variance groups produced data that displayed the trends typical of mental rotation, the two groups' reaction time sequences had very different autocorrelation functions. Power spectra derived from the two groups' data showed the presence of distinctive noise processes with long memory. Normal levels of variance were associated with 1/f noise, whereas high-variance data had substantial traces of random walk contour. These findings provide new perspectives on cognitive assessments of attention dysfunction.

Understanding decision-making deficits in neurological conditions: insights from models of natural action selection

Models of natural action selection implicate fronto-striatal circuits in both motor and cognitive 'actions'. Dysfunction of these circuits leads to decision-making deficits in various populations. We review how computational models provide insights into the mechanistic basis for these deficits in Parkinson's patients and those with ventromedial frontal damage. We then consider implications of the models for understanding behaviour and cognition in attention-deficit/hyperactivity disorder (ADHD). Incorporation of cortical noradrenaline function into the model improves action selection in noisy environments and accounts for response variability in ADHD. We close with more general clinical implications.

Testing for neuropsychological endophenotypes in siblings discordant for attention-deficit/hyperactivity disorder

BACKGROUND

Neurocognitive deficits associated with attention-deficit/hyperactivity disorder (ADHD) might be useful intermediate endophenotypes for determining specific genetic pathways that contribute to ADHD.

METHODS

This study administered 17 measures from prominent neuropsychological theories of ADHD (executive function, processing speed, arousal regulation and, motivation/delay aversion) in dizygotic (DZ) twin pairs discordant for ADHD and control twin pairs (ages 8-18 years) to compare performance between twins affected with ADHD (n = 266), their unaffected co-twins (n = 228), and control children from twin pairs without ADHD or learning difficulties (n = 332).

RESULTS

The ADHD subjects show significant impairment on executive function, processing speed, and response variability measures compared with control subjects. Unaffected co-twins of ADHD subjects are significantly impaired on nearly all the same measures as their ADHD siblings, even when subclinical symptoms of ADHD are controlled.

CONCLUSIONS

Executive function, processing speed, and response variability deficits might be useful endophenotypes for genetic studies of ADHD.

Heart rate and reinforcement sensitivity in ADHD

BACKGROUND

Both theoretical and clinical accounts of attention-deficit/hyperactivity disorder (ADHD) implicate a dysfunctional reinforcement system. This study investigated heart rate parameters in response to feedback associated with reward and response cost in ADHD children and controls aged 8 to 12.

METHODS

Heart rate responses (HRRs) following feedback and heart rate variability (HRV) in the low frequency band (.04-.08 Hz), a measure of mental effort, were calculated during a time production paradigm. Performance was coupled to monetary gain, loss or feedback-only in a cross-over design.

RESULTS

Children with ADHD exhibited smaller HRRs to feedback compared to controls. HRV of children with ADHD decreased when performance was coupled to reward or response cost compared to feedback-only. HRV of controls was similar across conditions.

CONCLUSIONS

Children with ADHD were characterised by (a) possible abnormalities in feedback monitoring and (b) motivational deficits, when no external reinforcement is present.