Attention Deficit/Hyperactivity Disorder as a Right Hemisphere Syndrome

Alpha/beta-gamma decoupling in methylphenidate medicated ADHD patients

There is much interest to understand how different neural rhythms function, interact and are regulated. Here, we focus on WM delay gamma to investigate its coupling with alpha/beta rhythms and its neuromodulation by methylphenidate. We address this through the use of human EEG conducted in healthy and ADHD subjects which revealed ADHD-specific electrophysiological deficits and MPH-induced normalization of gamma amplitude and its coupling with alpha/beta rhythms. Decreased alpha/beta-gamma coupling is known to facilitate memory representations via disinhibition of gamma ensembles coding the maintained stimuli. Here, we present EEG evidence which suggests that these dynamics are sensitive to catecholaminergic neuromodulation. MPH decreased alpha/beta-gamma coupling and this was related to the increase in delay-relevant gamma activity evoked by the same drug. These results add further to the neuromodulatory findings that reflect an electrophysiological dimension to the well-known link between WM delay and catecholaminergic transmission.

Abnormal functional asymmetry and its behavioural correlates in adults with ADHD: A TMS-EEG study

OBJECTIVES

Abnormal functional brain asymmetry and deficient response inhibition are two core symptoms of attention deficit hyperactivity disorder (ADHD). We investigated whether these symptoms are inter-related and whether they are underlined by altered frontal excitability and by compromised interhemispheric connectivity.

METHODS

We studied these issues in 52 ADHD and 43 non-clinical adults by comparing: (1) stop-signal reaction time (SSRT); (2) frontal asymmetry of the N200 event-related potential component, which is evoked during response inhibition and is lateralised to the right hemisphere; (3) TMS-evoked potential (TEP) in the right frontal hemisphere, which is indicative of local cortical excitability; and (4) frontal right-to-left interhemispheric TMS signal propagation (ISP), which is reversely indicative of interhemispheric connectivity.

RESULTS

Compared to controls, the ADHD group demonstrated elongated SSRT, reduced N200 right-frontal-asymmetry, weaker TEP, and stronger ISP. Moreover, in the ADHD group, N200 right-frontal-asymmetry correlated with SSRT, with TEP, and with symptoms severity. Conversely, no relationship was observed between ISP and N200 right-frontal-asymmetry, and both TEP and ISP were found to be unrelated to SSRT.

CONCLUSIONS

Our results indicate that abnormal frontal asymmetry is related to a key cognitive symptom in ADHD and suggest that it is underlined by reduced right-frontal excitability.

Links between excessive daytime sleepiness and EEG power and activation in two subtypes of ADHD

OBJECTIVES

This study aimed to replicate previously reported EEG characteristics between typically developing (TD) children and two subtypes of Attention Deficit Hyperactivity Disorder (ADHD) using a frontal, single-channel, dry-sensor portable EEG device, and explore whether differences are moderated by excessive daytime sleepiness (EDS).

METHODS

Children with ADHD Inattentive (ADHD-I) and ADHD Combined presentation (ADHD-C) and typically-developing (TD) children (N = 34 in each group) had frontal EEG recorded during eyes-closed resting, eyes-open resting, and focus tasks. Participants also completed the Children's Self-Report Sleep Patterns - Sleepiness Scale as a measure of EDS.

RESULTS

Consistent with previous literature, there were increases in frontal delta and theta power in the ADHD-C compared to ADHD-I and TD groups, in all conditions. Novel power and activation effects in ADHD subtypes, as well as significant group and EDS interactions for alpha and beta power were also found.

CONCLUSIONS

These findings highlight the importance of considering ADHD subtypes and EDS when exploring EEG characteristics, and have important implications for the diagnosis and treatment of children with ADHD.

Beneficial Effects of Behavioral Parent Training on Inhibitory Control in Children With Attention-Deficit/Hyperactivity Disorder: A Small-Scale Randomized Controlled Trial

The purpose of this study was to examine whether the beneficial effects of behavioral parent training (BPT), as an indirect type of psychosocial treatment, are extended to cognitive manifestations beyond behavioral symptoms of attention-deficit/hyperactivity disorder (ADHD). Although previous studies of community families have shown an association between parenting quality and a child's cognitive functions, little is known about the effects of BPT on cognitive manifestations in children with ADHD. In this study, we focused on inhibitory control among cognitive domains, which is considered to be the most malleable to direct types of psychosocial treatment for ADHD. We hypothesized that inhibitory control is affected by BPT, which uses parents as the primary agents of change to help their children. Thirty school-age children (6-12 years old) with ADHD and their parents (mothers) participated and were randomly assigned to either the standard BPT or waitlist control group. Using two objective laboratory-based tasks of inhibitory control (i.e., go/no-go and single response selection tasks), we assessed baseline and post-treatment response inhibition to suppress task-irrelevant responses and response selection to select task-relevant responses. In addition to decreased ADHD symptoms and negative parenting, the BPT group exhibited significantly improved performance in the single response selection task, but not in the go/no-go task, compared with the waitlist control group. Although tentative, these findings partially support our hypothesis that BPT has beneficial effects on the cognitive inhibitory control of ADHD, highlighting the potential for supportive environmental modifications to advance cognitive development in children with ADHD.

Handedness in ADHD: Meta-Analyses

Meta-analyses have shown that several neurodevelopmental and psychiatric disorders, such as autism spectrum disorder and schizophrenia, are associated with a higher prevalence of atypical (left-, non-right-, or mixed-) handedness. One neurodevelopmental disorder for which this association is unclear is attention deficit hyperactivity disorder (ADHD). Here, some empirical studies have found evidence for a higher prevalence of atypical handedness in individuals with ADHD compared to neurotypical individuals. However, other studies failed to establish such an association. Therefore, meta-analytic integration is critical to estimate whether or not there is an association between handedness and ADHD. We report the results of three meta-analyses (left-, mixed-, and non-right-handedness) comparing handedness in individuals with ADHD to controls (typically developing individuals). The results show evidence of a trend towards elevated levels of atypical handedness when it comes to differences in left- and mixed-handedness (p = 0.09 and p = 0.07, respectively), but do show clear evidence of elevated levels of non-right-handedness between individuals with ADHD and controls (p = 0.02). These findings are discussed in the context of the hypothesis that ADHD is a disorder in which mostly right-hemispheric brain networks are affected. Since right-handedness represents a dominance of the left motor cortex for fine motor behavior, such as writing, as well as a left-hemispheric dominance for language functions, and about 90% of individuals are right-handers, this hypothesis might explain why there is not stronger evidence for an association of left-handedness with ADHD. We suggest that the mechanisms involved in the pathogenesis of ADHD might show an overlap with the mechanisms involved in handedness strength, but not handedness direction.

Asymmetry in the Central Nervous System: A Clinical Neuroscience Perspective

Recent large-scale neuroimaging studies suggest that most parts of the human brain show structural differences between the left and the right hemisphere. Such structural hemispheric asymmetries have been reported for both cortical and subcortical structures. Interestingly, many neurodevelopmental and psychiatric disorders have been associated with altered functional hemispheric asymmetries. However, findings concerning the relation between structural hemispheric asymmetries and disorders have largely been inconsistent, both within specific disorders as well as between disorders. In the present review, we compare structural asymmetries from a clinical neuroscience perspective across different disorders. We focus especially on recent large-scale neuroimaging studies, to concentrate on replicable effects. With the notable exception of major depressive disorder, all reviewed disorders were associated with distinct patterns of alterations in structural hemispheric asymmetries. While autism spectrum disorder was associated with altered structural hemispheric asymmetries in a broader range of brain areas, most other disorders were linked to more specific alterations in brain areas related to cognitive functions that have been associated with the symptomology of these disorders. The implications of these findings are highlighted in the context of transdiagnostic approaches to psychopathology.

Analysis of structural brain asymmetries in attention-deficit/hyperactivity disorder in 39 datasets

OBJECTIVE

Some studies have suggested alterations of structural brain asymmetry in attention-deficit/hyperactivity disorder (ADHD), but findings have been contradictory and based on small samples. Here, we performed the largest ever analysis of brain left-right asymmetry in ADHD, using 39 datasets of the ENIGMA consortium.

METHODS

We analyzed asymmetry of subcortical and cerebral cortical structures in up to 1,933 people with ADHD and 1,829 unaffected controls. Asymmetry Indexes (AIs) were calculated per participant for each bilaterally paired measure, and linear mixed effects modeling was applied separately in children, adolescents, adults, and the total sample, to test exhaustively for potential associations of ADHD with structural brain asymmetries.

RESULTS

There was no evidence for altered caudate nucleus asymmetry in ADHD, in contrast to prior literature. In children, there was less rightward asymmetry of the total hemispheric surface area compared to controls (t = 2.1, p = .04). Lower rightward asymmetry of medial orbitofrontal cortex surface area in ADHD (t = 2.7, p = .01) was similar to a recent finding for autism spectrum disorder. There were also some differences in cortical thickness asymmetry across age groups. In adults with ADHD, globus pallidus asymmetry was altered compared to those without ADHD. However, all effects were small (Cohen's d from -0.18 to 0.18) and would not survive study-wide correction for multiple testing.

CONCLUSION

Prior studies of altered structural brain asymmetry in ADHD were likely underpowered to detect the small effects reported here. Altered structural asymmetry is unlikely to provide a useful biomarker for ADHD, but may provide neurobiological insights into the trait.

Limbic and Executive Meso- and Nigrostriatal Tracts Predict Impulsivity Differences in Attention-Deficit/Hyperactivity Disorder

BACKGROUND

Impulsivity is a defining characteristic of attention-deficit/hyperactivity disorder (ADHD), which has been associated with substance use disorders, higher accident rates, and lower educational and occupational outcomes. The meso- and nigrostriatal pathways of the dopamine system are hypothesized to be functionally heterogeneous, supporting diverse cognitive functions and impairments, including those associated with ADHD. We tested whether human midbrain pathways (where dopaminergic cell bodies originate) between the substantia nigra (SN) and ventral tegmental area (VTA) and the striatum differed between participants with ADHD and typically developing adolescent and young adult participants. We also assessed whether pathway connectivity predicted impulsivity regardless of diagnosis.

METHODS

Diffusion tensor imaging data were used to predict impulsivity (parent and self-report ratings, task-based behavioral measures) from participants with ADHD and typically developing adolescent and young adult participants (n = 155; 86 male, 69 female). Using probabilistic tractography, we mapped these pathways and divided the tracts into limbic, executive, and sensorimotor based on frontostriatal connectivity. ADHD and typically developing participants differed on all behavioral measures of impulsivity. We used correlation and machine learning analyses to test for a relationship between tract probabilities and impulsivity regardless of diagnosis.

RESULTS

Participants with ADHD had stronger structural connectivity between SN/VTA regions and the limbic striatum, weaker connectivity with the executive striatum, and no significant differences in sensorimotor tracts. Increased tract integrity between the limbic striatal and SN/VTA regions predicted greater impulsivity, while increased integrity between executive striatal and SN/VTA regions predicted reduced impulsivity.

CONCLUSIONS

These findings support the theory that functional diversity in the dopamine system is an important consideration for understanding dysfunction in ADHD.

Abnormal hemispheric asymmetry of both brain function and structure in attention deficit/hyperactivity disorder: a meta-analysis of individual participant data

Aberration in the asymmetric nature of the human brain is associated with several mental disorders, including attention deficit/hyperactivity disorder (ADHD). In ADHD, these aberrations are thought to reflect key hemispheric differences in the functioning of attention, although the structural and functional bases of these defects are yet to be fully characterized. In this study, we applied a comprehensive meta-analysis to multimodal imaging datasets from 627 subjects (303 typically developing control [TDCs] and 324 patients with ADHD) with both resting-state functional and structural magnetic resonance imaging (MRI), from seven independent publicly available datasets of the ADHD-200 sample. We performed lateralization analysis and calculated the combined effects of ADHD on each of three cortical regional measures (grey matter volume - GMV, fractional amplitude of low frequency fluctuations at rest -fALFF, and regional homogeneity -ReHo). We found that compared with TDC, 68%,73% and 66% of regions showed statistically significant ADHD disorder effects on the asymmetry of GMV, fALFF, and ReHo, respectively, (false discovery rate corrected, q = 0.05). Forty-one percent (41%) of regions had both structural and functional abnormalities in asymmetry, located in the prefrontal, frontal, and subcortical cortices, and the cerebellum. Furthermore, brain asymmetry indices in these regions were higher in children with more severe ADHD symptoms, indicating a crucial pathoplastic role for asymmetry. Our findings highlight the functional asymmetry in ADHD which has (1) a strong structural basis, and thus is likely to be developmental in nature; and (2) is strongly linked to symptom burden and IQ and may carry a possible prognostic value for grading the severity of ADHD.

Attention-deficit/hyperactivity disorder is characterized by a delay in subcortical maturation

Although previous studies have found that ADHD is characterized by a delay in cortical maturation, it is not clear whether this phenomenon was secondary to developmental trajectories in subcortical regions (caudate, putamen, pallidum, thalamus, hippocampus and amygdala). Using the ADHD-200 dataset, we estimated subcortical volumes in 339 individuals with ADHD and 568 typically developing controls. We defined the growth trajectory of each subcortical structure, delineating a phase of childhood increase followed by an adolescent decrease in subcortical volumes using a quadratic growth model. From these trajectories, the age of attaining peak subcortical volumes was derived and used as an index of subcortical maturation. We found that subcortical structures (caudate, putamen, pallidum, thalamus, hippocampus and amygdala) followed curvilinear trajectories similar to those reported in previous studies. The volumes of these subcortical structures in ADHD were also delayed in the developmental trajectory, which suggested that ADHD may be characterized by a delay in subcortical maturation. This delay may lead to a shift in which individuals with ADHD go through the process of pruning the nerve connections that is part of the normal maturation process during adolescence. Further, we also found that the asymmetric development of subcortical structures was abnormal in ADHD, which resulted from the imbalance of the maturation delay of bilateral subcortical structures. The subcortical maturation delay may play an important role in the pathophysiology of ADHD. Our findings provide new potential targets to investigate the pathophysiology of ADHD.

Arousal and Executive Alterations in Attention Deficit Hyperactivity Disorder (ADHD)

Attention deficit hyperactivity disorder (ADHD) is the most common neurobehavioral disorder in childhood and can significantly affect a child's personal and social development and academic achievement. Taking into account the model of attentional networks proposed by Posner et al., the aim of the present study was to review the literature regarding two main explicative models of ADHD, i.e., the inhibition model and the cognitive-energetic model, by discussing behavioral and neurological evidence of both models and the limitations of each model. The review highlights evidence that favors the energetic model and points to an unstable arousal as a potential pathogenetic factor in ADHD.

Latent class analysis of attention and white matter correlation in children with attention-deficit/hyperactivity disorder

This study aimed to explore attentional patterns among children with inattentive attention-deficit/hyperactivity disorder (ADHD-I) and children with typical development (TD), using a latent class analysis (LCA). Patterns of brain connectivity were also explored. The sample comprised 29 ADHD-I and 29 TD matched children. An LCA was conducted to reclassify subjects according to their attentional performance, considering cognitive measures of attention and behavioral symptoms, regardless of group of origin. The new clusters were then compared in respect to brain white matter measurements (extracted from diffusion tensor imaging). Participants were rearranged in 2 new latent classes, according to their performance in an attention task and the results of behavioral scales, resulting in groups with more homogeneous attentional profiles. A comparison of the 2 new classes using the white matter measurements revealed increased fractional anisotropy in the left inferior fronto-occipital fasciculus and left inferior longitudinal fasciculus for the class composed by participants with a higher risk of attentional problems. The findings indicated that it was possible to observe variability regarding neuropsychological profile, accompanied by underpinning neurobiological differences, even among individuals with the same disorder subtype - inattentive ADHD. This specific data-driven clustering analysis may help to enhance understanding of the pathophysiology of the disorder's phenotypes.

Genetic imaging study with [Tc-99m] TRODAT-1 SPECT in adolescents with ADHD using OROS-methylphenidate

AIM

To examine theeffects on the brain of 2-month treatment withamethylphenidate extended-release formulation (OROS-MPH) using [Tc-^99m] TRODAT-1SPECT in a sample of treatment-naïve adolescents with Attention Deficit/Hyperactivity Disorder (ADHD). In addition, to assess whether risk alleles (homozygosity for 10-repeat allele at the DAT1 gene were associated with alterations in striatal DAT availability.

METHODS

Twenty adolescents with ADHD underwent brain single-photon emission computed tomography (SPECT) scans with [Tc-^99m] TRODAT-1 at baseline and two months after starting OROS-MPH treatment with dosages up to 1 mg/kg/day. Severity of illness was estimated using the Clinical Global Impression Scale (CGI-S) and DuPaul ADHD Rating Scale-Clinician version (ARS) before treatment,1 month and 2 months after initiating OROS-MPH treatment.

RESULTS

Decreased DAT availability was found in both the right caudate (pretreatment DAT binding: 224.76 ± 33.77, post-treatment DAT binding: 208.86 ± 28.75, p = 0.02) and right putamen (pre-treatment DAT binding: 314.41 ± 55.24, post-treatment DAT binding: 285.66 ± 39.20, p = 0.05) in adolescents with ADHD receiving OROS-MPH treatment. Adolescents with ADHD who showed a robust response to OROS-MPH (n = 7) had significantly greater reduction of DAT density in the right putamen than adolescents who showed less robust response to OROS-MPH (n = 13) (p = 0.02). However, between-group differences by treatment responses were not related with DAT density in the right caudate. Risk alleles (homozygosity for the 10-repeat allele of DAT1 gene) in the DAT1 gene were not associated with alterations in striatal DAT availability.

CONCLUSION

Two months of OROS-MPH treatment decreased DAT availability in both the right caudate and putamen. Adolescents with ADHD who showed a robust response to OROS-MPH had greater reduction of DAT density in the right putamen. However,our findings did not support an association between homozygosity for a 10-repeat allele in the DAT1 gene and DAT density, assessedusing[Tc-^99m] TRODAT-1SPECT.

Hemispheric brain asymmetry differences in youths with attention-deficit/hyperactivity disorder

Introduction

Attention-deficit hyperactive disorder (ADHD) is the most common neurodevelopmental disorder in children. Diagnosis is currently based on behavioral criteria, but magnetic resonance imaging (MRI) of the brain is increasingly used in ADHD research. To date however, MRI studies have provided mixed results in ADHD patients, particularly with respect to the laterality of findings.

Methods

We studied 849 children and adolescents (ages 6-21 y.o.) diagnosed with ADHD (n = 341) and age-matched typically developing (TD) controls with structural brain MRI. We calculated volumetric measures from 34 cortical and 14 non-cortical brain regions per hemisphere, and detailed shape morphometry of subcortical nuclei. Diffusion tensor imaging (DTI) data were collected for a subset of 104 subjects; from these, we calculated mean diffusivity and fractional anisotropy of white matter tracts. Group comparisons were made for within-hemisphere (right/left) and between hemisphere asymmetry indices (AI) for each measure.

Results

DTI mean diffusivity AI group differences were significant in cingulum, inferior and superior longitudinal fasciculus, and cortico-spinal tracts (p < 0.001) with the effect of stimulant treatment tending to reduce these patterns of asymmetry differences. Gray matter volumes were more asymmetric in medication free ADHD individuals compared to TD in twelve cortical regions and two non-cortical volumes studied (p < 0.05). Morphometric analyses revealed that caudate, hippocampus, thalamus, and amygdala were more asymmetric (p < 0.0001) in ADHD individuals compared to TD, and that asymmetry differences were more significant than lateralized comparisons.

Conclusions

Brain asymmetry measures allow each individual to serve as their own control, diminishing variability between individuals and when pooling data across sites. Asymmetry group differences were more significant than lateralized comparisons between ADHD and TD subjects across morphometric, volumetric, and DTI comparisons.

Frontal alpha asymmetry predicts inhibitory processing in youth with attention deficit/hyperactivity disorder

INTRODUCTION

Atypical asymmetry in brain activity has been implicated in the behavioral and attentional dysregulation observed in ADHD. Specifically, asymmetry in neural activity in the right versus left frontal regions has been linked to ADHD, as well as to symptoms often associated with ADHD such as heightened approach behaviors, impulsivity and difficulties with inhibition. Clarifying the role of frontal asymmetry in ADHD-like traits, such as disinhibition, may provide information on the neurophysiological processes underlying these behaviors.

METHOD

ADHD youth (ADHD: n = 25) and healthy, typically developing controls (TD: n = 25) underwent an electroencephalography (EEG) recording while completing a go/no-go task-a commonly used test measuring behavioral inhibition. In addition, advanced signal processing for source localization estimated the location of signal generators underlying frontal alpha asymmetry (FA) during correct and incorrect trials.

RESULTS

This is the first study in ADHD to demonstrate that the dorsal-lateral prefrontal cortex (DLPFC) may be responsible for generating frontal alpha. During failed inhibition trials, ADHD youth displayed greater FA than TD youth. In addition, within the ADHD group, frontal asymmetry during later processing stages (i.e., 400-800ms after stimulus) predicted a higher number of commission errors throughout the task.

CONCLUSIONS

These results suggest that frontal alpha asymmetry may be a specific biomarker of cognitive disinhibition among youth with ADHD.

fNIRS Studies on Hemispheric Asymmetry in Atypical Neural Function in Developmental Disorders

Functional lateralization is highly replicable trait of human neural system. Many previous studies have indicated the possibility that people with attention-deficits/hyperactivity-disorder (ADHD) and autism spectrum disorder (ASD) show hemispheric asymmetry in atypical neural function. However, despite the abundance of relevant studies, there is still ongoing controversy over this issue. In the present mini-review, we provide an overview of the hemispheric asymmetry in atypical neural function observed in fNIRS studies on people with these conditions. Atypical neural function is defined as group-difference in the task-related concentration change of oxygenated hemoglobin. The existing fNIRS studies give support to the right-lateralized atypicalty in children with ADHD. At the same time, we did not find clear leftward-lateralization in atypical activation in people with ASD. On the basis of these, we discuss the current states and limitation of the existing studies.

1H NMR-based metabolomics reveals neurochemical alterations in the brain of adolescent rats following acute methylphenidate administration

The psychostimulant methylphenidate (MPH) is increasingly used in the treatment of attention deficit hyperactivity disorder (ADHD). While there is little evidence for common brain pathology in ADHD, some studies suggest a right hemisphere dysfunction among people diagnosed with the condition. However, in spite of the high usage of MPH in children and adolescents, its mechanism of action is poorly understood. Given that MPH blocks the neuronal transporters for dopamine and noradrenaline, most research into the effects of MPH on the brain has largely focused on these two monoamine neurotransmitter systems. Interestingly, recent studies have demonstrated metabolic changes in the brain of ADHD patients, but the impact of MPH on endogenous brain metabolites remains unclear. In this study, a proton nuclear magnetic resonance (¹H NMR)-based metabolomics approach was employed to investigate the effects of MPH on brain biomolecules. Adolescent male Sprague Dawley rats were injected intraperitoneally with MPH (5.0 mg/kg) or saline (1.0 ml/kg), and cerebral extracts from the left and right hemispheres were analysed. A total of 22 variables (representing 13 distinct metabolites) were significantly increased in the MPH-treated samples relative to the saline-treated controls. The upregulated metabolites included: amino acid neurotransmitters such as GABA, glutamate and aspartate; large neutral amino acids (LNAA), including the aromatic amino acids (AAA) tyrosine and phenylalanine, both of which are involved in the metabolism of dopamine and noradrenaline; and metabolites associated with energy and cell membrane dynamics, such as creatine and myo-inositol. No significant differences in metabolite concentrations were found between the left and right cerebral hemispheres. These findings provide new insights into the mechanisms of action of the anti-ADHD drug MPH.

A systematic review and meta-analysis of tract-based spatial statistics studies regarding attention-deficit/hyperactivity disorder

Diffusion tensor imaging (DTI) studies that use tract-based spatial statistics (TBSS) have demonstrated the microstructural abnormalities of white matter (WM) in patients with attention-deficit/hyperactivity disorder (ADHD); however, robust conclusions have not yet been drawn. The present study integrated the findings of previous TBSS studies to determine the most consistent WM alterations in ADHD via a narrative review and meta-analysis. The literature search was conducted through October 2015 to identify TBSS studies that compared fractional anisotropy (FA) between ADHD patients and healthy controls. FA reductions were identified in the splenium of the corpus callosum (CC) that extended to the right cingulum, right sagittal stratum, and left tapetum. The first two clusters retained significance in the sensitivity analysis and in all subgroup analyses. The FA reduction in the CC splenium was negatively associated with the mean age of the ADHD group. We hypothesize that, in addition to the fronto-striatal-cerebellar circuit, the disturbed WM matter tracts that integrate the bilateral hemispheres and posterior-brain circuitries play a crucial role in the pathophysiology of ADHD.

Attention-related EEG markers in adult ADHD

ADHD status affects both bottom-up sensory processing and top-down attentional selection, impairing professional and social functioning. The objective of the study was to investigate the functional mechanisms of attention deficits in adult ADHD by examining the electrophysiological activities associated with bottom-up attentional cueing (temporal and spatial orienting of attention) and top-down control (conflict resolution). Continuous EEG was recorded in 21 adult ADHD patients (40.05±9.5 years) and 20 healthy adults (25.5±4 years) during performance of the Attention Network Test (ANT). We examined the cue and target-related P1, N1 and P3 components as well as the contingent negative variation (CNV) developing between cue and target. Oscillatory responses were analyzed in the alpha (8-13Hz) and beta (14-19Hz) frequency bands. ADHD patients performed similarly to controls but showed reduced P3 amplitude, larger early CNV decrementing over time, reduced preparatory activation in both alpha and beta bands, as well as flattened target-related posterior alpha and beta responses. As compared to controls, the inverted CNV pattern suggested peculiar preparatory processing in ADHD patients. The singular pattern of target-related beta response indicated increased inhibitory processes in the case of easier task resolution and more generally, the lack of association between conflict resolution speed and beta activity supported alternative executive processing in ADHD patients. Overall, the reduced activation of the functional networks devoted to bottom-up and top-down attention suggests that adult ADHD patients engage reduced cortical resources in this composite task, compatible with the cortical hypoarousal model.

Developmental Learning Disorders: From Generic Interventions to Individualized Remediation

Developmental learning disorders affect many children, impairing their experience in the classroom and hindering many aspects of their life. Once a bleak sentence associated with life-long difficulties, several learning disorders can now be successfully alleviated, directly benefiting from promising interventions. In this review, we focus on two of the most prevalent learning disorders, dyslexia and attention-deficit/hyperactivity disorder (ADHD). Recent advances have refined our understanding of the specific neural networks that are altered in these disorders, yet questions remain regarding causal links between neural changes and behavioral improvements. After briefly reviewing the theoretical foundations of dyslexia and ADHD, we explore their distinct and shared characteristics, and discuss the comorbidity of the two disorders. We then examine current interventions, and consider the benefits of approaches that integrate remediation within other activities to encourage sustained motivation and improvements. Finally, we conclude with a reflection on the potential for remediation programs to be personalized by taking into account the specificities and demands of each individual. The effective remediation of learning disorders is critical to modern societies, especially considering the far-reaching ramifications of successful early interventions.

Latent profiles of executive functioning in healthy young adults: evidence of individual differences in hemispheric asymmetry

Two competing theoretical models of individual differences in executive functioning (EF) were examined: the Prefrontal Convexity Model and the Hemispheric Asymmetry Model. Neurologically healthy individuals (N = 315; mean age 20.8) completed a modified switching task (MST) and the Attention Network Test (ANT) in a single testing session. Data analysis was conducted in two phases. In the first phase (model identification), latent profile analysis was applied to MST variables measuring the abilities to form, switch, and maintain mental sets under conditions designed to tax left or right hemisphere resources. In the second phase (model validation), participant clusters obtained from the first phase were compared on the ANT. The Model Identification phase yielded a 3-profile solution consistent with the Hemispheric Asymmetry Model. Profile 1 (N = 203) was characterized by average EF performances. Profile 2 (N = 43) revealed a set maintenance weakness under non-verbal conditions. Profile 3 (N = 38) demonstrated weaknesses in cognitive flexibility combined with poor executive performances under verbal conditions. The Model Validation phase confirmed group differences. Profile 1 demonstrated average EF performances. Profile 2 demonstrated distractibility and decreased alertness, consistent with a right hemisphere weakness. Profile 3 demonstrated cognitive rigidity in the absence of external cues, consistent with a left hemisphere weakness. Individual differences in EF appear to follow a Hemispheric Asymmetry Model of EF among neurologically healthy adults. Investigating the relationship between hemispherically mediated executive functions and other individual difference factors known to confer health risk or resilience could inform numerous disciplines within the field of psychology.

Brain lateralization and self-reported symptoms of ADHD in a population sample of adults: a dimensional approach

Many clinical studies reported a compromised brain lateralization in patients with Attention-Deficit/Hyperactivity Disorder (ADHD) without being conclusive about whether the deficit existed in the left or right hemisphere. It is well-recognized that studying ADHD dimensionally is more controlled for comorbid problems and medication effects, and provides more accurate assessment of the symptoms. Therefore, the present study applied the dimensional approach to test the relationship between brain lateralization and self-reported ADHD symptoms in a population sample. Eighty-five right-handed university students filled in the Conners' Adult ADHD Rating Scales and performed a lateralization reaction time task. The task consists of two matching conditions: one condition requires nominal identification for letters tapping left hemisphere specialization (Letter Name-Identity condition) and the other one requires physical and visuospatial identification for shapes tapping right hemisphere specialization (Shape Physical-Identity condition). The letters or shapes to be matched are presented in left or right visual field of a fixation cross. For both task conditions, brain lateralization was indexed as the difference in mean reaction time between left and right visual field. Linear regression analyses, controlled for mood symptoms reported by a depression, anxiety, and stress scale, showed no relationship between the variables. These findings from a population sample of adults do not support the dimensionality of lateralized information processing deficit in ADHD symptomatology. However, group comparison analyses showed that subjects with high level of inattention symptoms close to or above the clinical cut-off had a reduced right hemisphere processing in the Shape Physical-Identity condition.

Processing of affective prosody in boys suffering from attention deficit hyperactivity disorder: A near-infrared spectroscopy study

Neurobiological studies on facial affect recognition have demonstrated reduced response amplitudes to anger cues in patients suffering from attention deficit hyperactivity disorder (ADHD). It is still unclear whether a similar deficit exists in the auditory domain. Therefore, this near-infrared spectroscopy study focused on neuronal correlates of affective prosody processing. Fourteen boys suffering from ADHD and fourteen healthy boys were exposed to emotionally intoned, standardized sentences of the categories anger, sadness, happiness, and to affectively neutral sentences. Relative to controls, the patients displayed a diminished activation of the right superior temporal gyrus (STG) when processing anger prosody, which was correlated with aggressive behavior. There were no group differences for the other emotions. Additionally, the ADHD group showed increased supramarginal gyrus (SMG) activation in the anger condition. This might mirror compensatory attention allocation. In summary, we identified a selectively lowered STG activation to auditory anger cues in ADHD patients. Consequently, STG recruitment during anger exposure might be used for evaluation of psychotherapy effects.

Caudate asymmetry is related to attentional impulsivity and an objective measure of ADHD-like attentional problems in healthy adults

Case-control studies comparing ADHD with typically developing individuals suggest that anatomical asymmetry of the caudate nucleus is a marker of attention deficit hyperactivity disorder (ADHD). However, there is no consensus on whether the asymmetry favors the right or left caudate nucleus in ADHD, or whether the asymmetry is increased or decreased in ADHD. The current study aimed to clarify this relationship by applying a dimensional approach to assessing ADHD symptoms that, instead of relying on clinical classification, utilizes the natural behavioral continuum of traits related to ADHD. Structural T1-weighted MRI was collected from 71 adults between 18 and 35 years and analyzed for caudate asymmetry. ADHD-like attentional symptoms were assessed with an objective measure of attentional problems, the ADHD score from the Test of Variables of Attention (TOVA). Impulsivity, a core feature in ADHD, was measured using the Barratt Impulsiveness Scale, a self-report measure that assesses attentional, non-planning, and motor features of impulsivity. We found that larger right relative to left caudate volumes correlated with both higher attentional impulsiveness and worse ADHD scores on the TOVA. Higher attentional impulsiveness also correlated with worse ADHD scores, establishing coherence between the objective measure and the self-report measure of attentional problems. These results suggest that a differential passage of information through frontal-striatal networks may produce instability leading to attentional problems. The findings also demonstrate the utility of a dimensional approach to understanding structural correlates of ADHD symptoms.

Disrupted response inhibition toward facial anger cues in children with attention-deficit hyperactivity disorder (ADHD): an event-related potential study

This event-related potential study focused on neural correlates of inhibitory affective control in attention-deficit hyperactivity disorder (ADHD). Sixteen boys with ADHD and 16 healthy boys underwent an emotional Go/NoGo task with pictures of facial expressions from the categories anger, sadness, happiness, and neutral. The participants were instructed to execute or withhold a motor response to specific emotions. Patients relative to controls displayed a severe impairment in response inhibition toward anger cues, which was accompanied by a reduced P300 amplitude (positive voltage deflection about 300 ms after picture onset). The control group showed a P300 differentiation of the affective categories that was absent in the ADHD group. The pronounced anger-processing deficit in ADHD patients might be linked to their interpersonal difficulties and should be addressed in psychotherapy.

Links between infant temperament and neurophysiological measures of attention to happy and fearful faces

BACKGROUND

Developing control of attention helps infants to regulate their emotions, and individual differences in attention skills may shape how infants perceive and respond to their socio-emotional environments. This study examined whether the temperamental dimensions of self-regulation and negative emotionality relate to infants' attention skills and whether the emotional content of the attended stimulus affects this relation.

METHODS

Event-related potentials provided a neurophysiological index of attention (Nc) while 3 to 13-month-old infants viewed images of happy and fearful facial expressions. Temperament was measured via parent report using the Infant Behavior Questionnaire-Revised.

RESULTS

The peak latency of the Nc was slower for infants with lower regulatory capacity, independent of facial expression. The amplitude of the Nc over right fronto-central electrodes was related to both self-regulation and negative emotionality, but the effects differed by emotion: infants with better self-regulation had larger Nc responses to fearful faces, and infants scoring higher on negative emotionality had larger Nc responses to happy faces. These results are discussed in relation to the development of executive attention networks and their modulation by the amygdala.

A verbal planning impairment in adult ADHD indexed by script generation tasks

OBJECTIVE

Though juvenile and adult ADHD cases are well known to have a nonverbal planning impairment, a verbal-planning impairment has been demonstrated only in juvenile ADHD. The purpose of this investigation is to determine whether a verbal planning impairment also characterizes adult ADHD.

METHODS

A cohort of 30 adult ADHD clients of a university psychological clinic are compared to 30 age-, education-, gender-, and IQ-matched persons recruited from the general population who did not have ADHD. The dependent measure is a set of 6 paper/pencil 10-item script generation tasks.

RESULTS

The findings reveal that the ADHD cohort was significantly impaired on the script task and the script task correlated significantly with severity of ADHD (CAARS index + WURS), whereas several neuropsychological measures of executive function (Stroop, COWA, Rey's Complex Figure, D2, CVLT, CPT-II) did not. Findings further showed that the script measure was weakly correlated with the other established neuropsychological measures of executive function (r < .46, shared variance of less than 21%).

CONCLUSIONS

On the basis of the study findings, it is concluded that verbal planning measured with script generation tasks is distinctly impaired in clinically referred adult ADHD.

EEG alpha asymmetry in schizophrenia, depression, PTSD, panic disorder, ADHD and conduct disorder

Models of laterality infer distinct aspects of EEG alpha asymmetry in clinical disorders, which has been replicated for over three decades. This biomarker now requires a more fine-grained assessment of its clinical utility as a diagnostic and treatment predictive marker. Here, within the same study we assessed resting brain laterality across six clinical disorders, for which deviant laterality has been implicated as core dysfunction. These disorders were evaluated in comparison to a large normative dataset (approximately 1,900) from the Brain Resource International Database. EEG alpha asymmetry was assessed in the frontocentral region, for resting Eyes Closed and Eyes Open conditions. Schizophrenia was characterized by significantly greater left lateralized alpha power than controls, indicating a deficit in left frontal activity at rest, which may relate to "disconnections" across wider fronto-temporal networks. The depression group showed a trend-level tendency towards the opposite pattern of greater right-lateralized activity than controls. The remaining anxiety and behavioral disorders did not show any significant deviance in alpha asymmetry from the normative control group. However, at a non-significant level laterality for these groups was generally consistent with expected directions, suggesting a propensity towards a particular lateralization but still remaining within the normative range. Overall, the results of the current study indicate that EEG alpha asymmetry may show the most clinical utility as a biomarker for schizophrenia and depression in comparison to other clinical disorders.

ADHD familial loading and abnormal EEG alpha asymmetry in children with ADHD

OBJECTIVE

Abnormal brain laterality (ABL) is indicated in ADHD. ADHD and brain laterality are heritable. Genetic factors contributing to lateralization of brain function may contribute to ADHD. If so, increased ADHD family loading should be associated with greater ABL. Previous studies have shown increased rightward alpha asymmetry in ADHD. We tested whether this was more pronounced in ADHD children with increased ADHD family loading.

METHODS

We compared EEG alpha asymmetry at rest and during the Conner's Continuous Performance Test (CPT) in ADHD children with and without ADHD affected parents, and replicated our findings in a second larger sample. The replication study additionally stratified the parent-affected sample by parental persistent versus non-persistent ADHD status, increased spatial resolution of EEG measures, and assessed low versus high-alpha.

RESULTS

Study-1: the parent-affected group showed increased rightward asymmetry across frontal and central regions and reduced rightward parietal asymmetry during an eyes closed (EC) condition, as well as increasing rightward parietal asymmetry with advancing age during the CPT. Study-2 replicated these findings and further delineated influences of low versus high-alpha, recording site, and effects of parental persistent versus non-persistent ADHD status.

CONCLUSION

Increased ADHD familial loading was associated with increased rightward frontal asymmetry. In contrast, increased rightward parietal asymmetry was associated with reduced ADHD family loading. Frontal results are consistent with an ADHD endophenotype. Parietal results suggest an ADHD adaptive trait prevalent with less ADHD family loading. Age effects indicate a unique developmental course among ADHD children whose parents have non-persistent ADHD.

Rethinking a right hemisphere deficit in ADHD

INTRODUCTION

Early observations from lesion studies suggested right hemisphere (RH) dysfunction in ADHD. However, a strictly right-lateralized deficit has not been well supported. An alternatively view suggests increased R > L asymmetry of brain function and abnormal interhemispheric interaction. If true, RH pathology in ADHD should reflect interhemispherically networked and overactivated functioning. The authors evaluated these assertions.

METHOD

Four elements of lateralized brain function were measured: LH specialized, RH specialized, LH with interhemispheric processing (LH/IH), and RH with interhemispheric processing (RH/IH). Next, the authors tested their association with cognitive ability, psychiatric comorbidity, and sibling correlations in 79 children with ADHD.

RESULTS

RH/IH processing was uniquely associated with other outcome measures. There were no associations for independent RH or LH function alone.

CONCLUSION

Interhemispherically networked RH processing is critical in ADHD. In addition, lack of association between LH specialized processing and cognitive ability (especially for verbal cognitive tasks) supports increased RH mediation of task processing.

Facial affect interpretation in boys with attention deficit/hyperactivity disorder

Recent studies have produced mixed evidence of impairments in facial affect interpretation for children with attention deficit/hyperactivity disorder (ADHD). This study investigated the presence and nature of such impairments across different stimulus formats. Twenty-four boys with ADHD and 24 age-matched comparison boys completed a 72-trial task that included facial expressions of happiness, sadness, fear, anger, surprise, and disgust. Three versions of each expression were used: a static version, a dynamic version, and a dynamic version presented within a relevant situational context. Expressions were also presented in one of two portrayal modes (cartoon versus real-life). Results indicated significant impairments for boys with ADHD on two of the six emotions (fear and disgust), which were consistent across stimulus formats. Directions for further research to identify mediating factors in the expression of such impairments in children with ADHD are discussed.

Motor control in children with ADHD and non-affected siblings: deficits most pronounced using the left hand

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is strongly influenced by heritability. Identifying heritable vulnerability traits (endophenotypes) that mark a relatively high risk of developing the disorder can contribute to the identification of risk genes. A fruitful area for the search for such endophenotypes may be motor control in children with ADHD, since the disorder is frequently accompanied by motor problems.

METHOD

The current study used a large sample of 350 children with ADHD, 195 non-affected siblings and 271 normal controls aged 5-19 years. Children were administered two computerised motor control tasks in which they had to trace a path between two circles (Tracking task) and follow a randomly moving target (Pursuit task). Both tasks were performed with both the right and the left hand.

RESULTS

Children with ADHD were less precise and stable than controls. Non-affected siblings also deviated from controls, but only on the Tracking task. Group differences were modulated by the use of the right versus the left hand: no group differences emerged when the right hand was used, yet group differences did emerge when the left hand was used. Performance on both tasks was significantly familial.

CONCLUSIONS

Imprecision and instability of movements in children with ADHD and in their non-affected siblings as measured by the Tracking task might be suitable endophenotypic candidates: these deficits are familially present in children having ADHD as well as in their non-affected siblings. Motor performance might be best assessed in children using their left hand, because motor control deficits are most pronounced using the left hand. This might relate to right hemispheric brain pathology in children with ADHD (and possibly in their non-affected siblings) that is related to the control of the left hand and/or relate to differential effects of daily life practice on both hands, which may be smaller on the left hand.

Attention-deficit/hyperactivity disorder: a neuropsychological perspective towards DSM-V

Neuropsychological methods and techniques have much to offer in the evaluation of the individual suspected as having Attention-Deficit/Hyperactivity Disorder (ADHD). After a review of the historical evolution of the ADHD concept, incidence and prevalence, and DSM-IV criteria for diagnosis, especially as regards omission related to gender differences, and other associated cultural, familial, socioenvironmental, and subject influences, this paper describes a number of dilemmas and obstacles encountered in clinical practice. Included are the confounds associated with the wide range of possible comorbidities, the insufficiency of current DSM-IV criteria, the emergence of subtype differentiation and its impact on diagnosis and treatment. The complex relationship between neuropsychological constructs and ADHD, and obstacles to valid assessment are also addressed. The complexities associated with a thorough ADHD evaluation are viewed within an impressive and expansive existing scientific framework and recommendations are made for future directions.

The role of simple emotion recognition skills among school aged boys at risk of ADHD

Poor social skills and behavioral problems are a major component of ADHD. The different explanations offered so far, such as cognitive deficits and deficient self regulation, have not been able fully to account for the various aspects of the social dysfunction, suggesting that other mechanisms might underlay this impairment. Our study sought to assess the emotion recognition of Israeli boys at risk of ADHD, and to evaluate its associations with their social skills. A group of 111 boys (grades 4 and 5) were assigned to an At-risk (n=50) and a control (n=61) group based on their scores in an ADHD symptoms questionnaire. The two groups were matched on age, socio-economic status and class and school environment. Group comparisons revealed that compared to their non-At-risk counterparts, At-risk boys have impaired emotion recognition. Finally, multiple groups Structural Equation Modeling analyses (SEM) demonstrated that emotion misrecognition plays a significant role in the At-risk children's social functioning and behavioral problems compared to its role in the social competence and behavioral problems among the comparison group. Implications for the understanding and treatment of social skills problems among children at risk of ADHD are proposed.

Hemispheric functioning in children with subtypes of attention-deficit/hyperactivity disorder

OBJECTIVE

The authors investigated line bisection performance in children with Attention-Deficit/Hyperactivity Disorder (AD/HD) subtypes. Previous research with neurotypical children found a rightward bias with right-hand use and a leftward bias with left-hand use; however, research with AD/HD participants has failed to similarly measure the effects of hand use, which was the focus of this study.

METHOD

Line bisection was used to measure differences in right hemisphere functioning in children (7 to 12 years) with AD/HD-I and AD/HD-C.

RESULTS

Initial AD/HD group findings (without subtype differentiation) replicated previous research. However, further subtype analyses showed that the ADHD-I and ADHD-C groups perform significantly differently. Specifically, the ADHD-I group showed a leftward bias, irrespective of hand use, and the ADHD-C group showed a rightward bias, irrespective of hand use.

CONCLUSION

These findings suggest that the subtypes represent two distinct disorders and that, unlike ADHD-C, ADHD-I may not be the result of right hemisphere dysfunction.

Mood changes correlate to changes in brain serotonin precursor trapping in women with premenstrual dysphoria

The cardinal mood symptoms of premenstrual dysphoria can be effectively treated by serotonin-augmenting drugs. The aim of the study was to test the serotonin hypothesis of this disorder, i.e. of an association between premenstrual decline in brain serotonin function and concomitant worsening of self-rated cardinal mood symptoms. Positron emission tomography was used to assess changes in brain trapping of 11C-labeled 5-hydroxytryptophan, the immediate precursor of serotonin, in the follicular and premenstrual phases of the menstrual cycle in eight women with premenstrual dysphoria. Changes in mood and physical symptoms were assessed from daily visual analog scale ratings. Worsening of cardinal mood symptoms showed significant inverse associations with changes in brain serotonin precursor trapping; for the symptom "irritable", r(s)=-0.83, and for "depressed mood" r(s)=-0.81. Positive mood variables showed positive associations, whereas physical symptoms generally displayed weak or no associations. The data indicate strong inverse associations between worsening of cardinal symptoms of premenstrual dysphoria and brain serotonin precursor (11C-labeled 5-hydroxytryptophan) trapping. The results may in part support a role for serotonin in premenstrual dysphoria and may provide a clue to the effectiveness of serotonin-augmenting drugs in this disorder but should, due to small sample size and methodological shortcomings, be considered preliminary.

Moment-to-moment dynamics of ADHD behaviour in South African children

Background

The behaviour of children with Attention-Deficit/Hyperactivity Disorder is characterized by low predictability of responding. Low behavioural predictability is one way of operationalizing intra-individual ADHD-related variability. ADHD-related variability may be caused by inefficient behavioural selection mechanisms linked to reinforcement and extinction, as suggested by the recently published dynamic developmental theory (DDT) of ADHD. DDT argues that ADHD is a basic neurobehavioural disorder, caused by dysfunctioning dopamine systems. For establishing ADHD as a neurobehavioural disorder, findings from studies conducted in Western countries should be replicated in other cultural populations. The present study replicated the study conducted in Norway, with children from the Limpopo province in the Republic of South Africa.

Methods

Boys and girls, aged 6–9 yr, from seven ethnic groups participated. Scores by teachers on the Disruptive Behavior Disorders rating scale defined participation in either ADHD-hyperactive/impulsive (-HI), ADHD-predominantly inattentive (-PI), or ADHD-combined (-C) groups. Children below the 86^th percentile were matched on gender and age and comprised the non-ADHD group. The children completed a computerized game-like task where mouse clicks on one of two squares on the screen resulted in delivery of a reinforcer according to a variable interval schedule of reinforcement. Reinforcers were cartoon pictures presented on the screen together with a sound. Predictability of response location and timing were measured in terms of explained variance.

Results

Overall, the results replicated findings from Norway. Specifically, the ADHD-C group showed significantly lower predictability of responding than the non-ADHD group, while the ADHD-HI and the ADHD-PI groups were in-between. In accordance with the previous study, response location, but not response timing, was a sensitive behavioural measure. There were no significant gender differences. Cartoon pictures were effective reinforcers as the non-ADHD group showed learning of the task. There was no relation between behavioural predictability and motor functions.

Conclusion

The present study makes a strong case for ADHD as a basic, neurobehavioural disorder, not a cultural phenomenon, by replicating findings from a wealthy Western country in a poor province of a developing country. The results were, generally, in line with predictions from the dynamic developmental theory of ADHD by indicating that reinforcers were less efficient in the ADHD group than in the non-ADHD group. Finally, the results substantiated ADHD-related variability as an etiologically important characteristic of ADHD behaviour.

Latent inhibition and asymmetrical visual-spatial attention in children with ADHD

The research was designed to determine whether the purported hemispheric asymmetries that are associated with attention deficit/hyperactivity disorder (ADHD) affect performance on a selective attention visual search task, and whether any obtained asymmetry will be modulated by methylphenidate. Two groups of children (8-15 years) with ADHD, one with methylphenidate treatment (ADHD+) and one without (ADHD+), were compared to matched controls on a two-stage visual search task. The task assessed right-left visual field asymmetries and the effects of changing a previous distractor into a target. Such a procedure, related to latent inhibition (LI; poorer performance to a previously irrelevant stimulus than to a novel one), can provide evidence for dysfunctional processing of irrelevant stimuli. All three groups exhibited the LI effect. The ADHD group, however, exhibited less LI for left- than right-side targets, an effect absent in the control and ADHD+ groups, suggesting a lateralized attentional deficit for ADHD+ that was normalized by methylphenidate.

Atypical brain laterality in adults with ADHD during dichotic listening for emotional intonation and words

Few studies directly examined the nature of hemispheric specialization and interaction in ADHD. The present experiment investigated left/right brain dynamics in unmedicated right handed adults with ADHD (n = 19) and in controls (n = 19), using a dichotic listening task to assess hemispheric differences in word and emotion recognition. We also assessed how focusing attention on a single ear modulated lateralized performance and affected cross-callosal interference effects. Analysis of variance indicated that ADHD subjects showed reduced left hemisphere specialization, were better at processing emotions, and worse at processing words compared to controls. These differences were eliminated during focused attention. Finally, during presumed right hemisphere processing of linguistic stimuli, subjects with ADHD showed reduced left hemisphere interference. We concluded that ADHD subjects demonstrated greater right hemisphere and reduced left hemisphere contribution during this task relative to controls. We posit that these hemispheric differences were due to management or use of available cognitive resources rather than inherent capacity.

Norepinephrine and acetylcholine mediation of the components of reflexive attention: implications for attention deficit disorders

Attention deficit hyperactivity disorders (ADHD) create major learning barriers for children and significant social and legal handicaps for adults worldwide. Important advances in the genetic basis of the disease have been made, but reliable, biological, diagnostic markers remain elusive. This review takes the position that future progress in treating the core symptom of attention deficits requires a clearer understanding of the neuroscience of attention in normal individuals. Two important achievements in this direction have been the development of tasks that identify activity in the orienting, alerting and conflict networks, and the identification of neurotransmitters that mediate these components. The proven ability of these tasks to identify and characterize response components of "normal" attention argues that they could be used advantageously with patient populations. The categorization of neurotransmitter abnormalities in those with ADHD could clarify whether attention deficits occur within or across attention networks. To realize these goals, we evaluate laboratory studies of attention in humans and animals that address the underlying neurotransmitter systems, primarily norepinephrine and acetylcholine. We propose that key facts about deficits in reflexive and voluntary attention may be understood by a model that includes deficits in brain norepinephrine release and its effects on cholinergic activity in the parietal cortex.