Dissecting the attention deficit hyperactivity disorder (ADHD) phenotype: sustained attention, response variability and spatial attentional asymmetries in relation to dopamine transporter (DAT1) genotype

Reaction Time and Attention: Toward a New Standard in the Assessment of ADHD? A Pilot Study

This pilot study shows results of an experiment comparing reaction times (RTs) and attentional performance between an ADHD group of 30 children and 30 controls, both Spanish speaking. The experiment was carried out using the Seville computerized neuropsychological battery (SNB).

OBJECTIVE

This study had two goals: One was to test sensitivity of SNB for attention deficits in ADHD and the second was to detect differences in RTs between ADHD and controls. Possible explanations and implications of such differences are also discussed.

METHOD

SNB computerized system was used to assess RTs and accuracy, and alternate forms of continuous performance task were used.

RESULTS

Results showed high sensitivity of some of the SNB tests, especially cancellation tests. RTs were significantly different between groups.

CONCLUSION

SNB represents a helpful tool for detection of attention deficits, and RT indices represent the most significant variable in differentiation of both groups studied.

Intratask Variability As a Correlate for DRD4 and SLC6A3 Variants: A Pilot Study in ADHD

Behavioral variability may be an ADHD key feature. Currently used ex-Gaussian/Fast Fourier Transform analyses characterize general distribution and oscillatory/rhythmic components of performance but are unable to demonstrate slow cumulative changes over entire tasks.

OBJECTIVE

To explore how performance of ADHD children and unaffected sibs gradually evolves in relation to genetic variants linked to ADHD.

METHOD

A total of 40 kids (20 ADHD-discordant sib pairs) between 8 and 13 years resolved a visual Go/NoGo with 10% NoGo probability. Variable number tandem repeats (VNTRs) at DRD4 and SLC6A3 were identified following standard protocols. Performance changes were assessed by linear/logistic mixed-effect models.

RESULTS

Models exploring SLC6A3 effects demonstrated less accentuated increments of response time (RT) (p = .046) and cumulative increments in the correct responses to "NoGo" (p = .00027) in 10R/10R participants. Models for DRD4 showed faster decline of correct responses to "Go" (p = .0078) in 2R/7R carriers.

CONCLUSION

Dynamical analysis of attention/inhibition measures may unravel new correlates to DRD4 and SLC6A3 variants.

Mesocorticolimbic dopamine functioning in primary psychopathy: A source of within-group heterogeneity

Despite similar emotional deficiencies, primary psychopathic individuals can be situated on a continuum that spans from controlled to disinhibited. The constructs on which primary psychopaths are found to diverge, such as self-control, cognitive flexibility, and executive functioning, are crucially regulated by dopamine (DA). As such, the goal of this review is to examine which specific alterations in the meso-cortico-limbic DA system and corresponding genes (e.g., TH, DAT, COMT, DRD2, DRD4) might bias development towards a more controlled or disinhibited expression of primary psychopathy. Based on empirical data, it is argued that primary psychopathy is generally related to a higher tonic and population activity of striatal DA neurons and lower levels of D2-type DA receptors in meso-cortico-limbic projections, which may boost motivational drive towards incentive-laden goals, dampen punishment sensitivity, and increase future reward-expectancy. However, increasingly higher levels of DA activity in the striatum (moderate versus pathological elevations), lower levels of DA functionality in the prefrontal cortex, and higher D1-to-D2-type receptor ratios in meso-cortico-limbic projections may lead to increasingly disinhibited and impetuous phenotypes of primary psychopathy. Finally, in order to provide a more coherent view on etiological mechanisms, we discuss interactions between DA and serotonin that are relevant for primary psychopathy.

Genetic contributions to attentional response time slopes across repeated trials

Background

Attention provides vital contribution to everyday functioning, and deficits in attention feature in many psychological disorders. Improved understanding of attention may eventually be critical to early identification and treatment of attentional deficits. One step in that direction is to acquire a better understanding of genetic associations with performance on a task measuring reflexive (exogenous) visual attention. Reflexive attention is an important component of overall attention because (along with voluntary selective attention) it participates in determining where attention is allocated and how susceptible to distractors the subject might be. The task that we used involves the presentation of a target that is preceded by one of several different types of cues (none, double, or single, either ipsilateral or contralateral to where the target subsequently appears). We used several different outcome measures depending on the cue presented. We have previously studied the relationship between selected genes and mean response time (RT). Here we report on the contributions of genetic markers to RT increases or decreases over the course of the task (linear trend in RT slope).

Results

Specifically, we find that RT slope for a variety of reflexive attention outcome measures is dependent on DAT1 genotype. DRD4 was near significant for one outcome measure in the final (best) model. APOE, COMT, and DBH were not significant in any models.

Conclusions

It is especially interesting that genotype predicts linear changes in RT across trials (and not just mean differences or moment-to-moment variability). DAT1 is a gene that produces a protein involved in the transport of dopamine from the synapse. To our knowledge, this is the first study that has associated neurotransmitter genotypes with RT slope on a reflexive attention experiment. The direction of these effects is consistent with genetic risk for attention deficit hyperactivity disorder (ADHD). That is, those with two risk alleles for ADHD (6R/6R on the DAT1 intron 8 VNTR) either got slower as the task progressed or had the least improvement. Those with no risk alleles (5R/5R) had the most improvement in RT as the task progressed.

Cortical thickness differences in the prefrontal cortex in children and adolescents with ADHD in relation to dopamine transporter (DAT1) genotype

Several lines of evidence suggest that the dopamine transporter gene (DAT1) plays a crucial role in attention deficit hyperactivity disorder (ADHD). Concretely, recent data indicate that the 10-repeat (10R) DAT1 allele may mediate neuropsychological functioning, response to methylphenidate, and even brain function and structure in children with ADHD. This study aimed to investigate the influence of 10R DAT1 on thickness of the prefrontal cortex in children and adolescents with ADHD. To this end, brain magnetic resonance images were acquired from 33 patients with homozygosity for the 10R allele and 30 patients with a single copy or no copy of the allele. The prefrontal cortex of each MRI scan was automatically parceled into regions of interest (ROIs) based on Brodmann areas (BA). The two groups were matched for age, gender, IQ, ADHD subtype, symptom severity, comorbidity and medication status. However, patients with two copies of the 10R allele exhibited significantly decreased cortical thickness in right BA 46 relative to patients with one or fewer copies of the allele. No other prefrontal ROI differed significantly between the two groups. Present findings suggest that cortical thickness of right lateral prefrontal cortex (BA 46) is influenced by the presence of the DAT1 10 repeat allele in children and adolescents with ADHD.

Altered white matter tract property related to impaired focused attention, sustained attention, cognitive impulsivity and vigilance in attention-deficit/ hyperactivity disorder

BACKGROUND

The neural substrate for clinical symptoms and neuropsychological performance in individuals with attention-deficit/hyperactivity disorder (ADHD) has rarely been studied and has yielded inconsistent results. We sought to compare the microstructural property of fibre tracts associated with the prefrontal cortex and its association with ADHD symptoms and a wide range of attention performance in youth with ADHD and healthy controls.

METHODS

We assessed youths with ADHD and age-, sex-, handedness-, coil- and intelligence-matched controls using the Conners' Continuous Performance Test (CCPT) for attention performance and MRI. The 10 target tracts, including the bilateral frontostriatal tracts (caudate to dorsolateral prefrontal cortex, ventrolateral prefrontal cortex and orbitofrontal cortex), superior longitudinal fasciculus (SLF) and cingulum bundle were reconstructed using diffusion spectrum imaging tractography. We computed generalized fractional anisotropy (GFA) values to indicate tract-specific microstructural property.

RESULTS

We included 50 youths with ADHD and 50 healthy controls in our study. Youths with ADHD had lower GFA in the left frontostriatal tracts, bilateral SLF and right cingulum bundle and performed worse in the CCPT than controls. Furthermore, alteration of the right SLF GFA was most significantly associated with the clinical symptom of inattention in youths with ADHD. Finally, youths with ADHD had differential association patterns of the 10 fibre tract GFA values with attention performance compared with controls.

LIMITATIONS

Ten of the youths with ADHD were treated with methylphenidate, which may have long-term effects on microstructural property.

CONCLUSION

Our study highlights the importance of the SLF, cingulum bundle and frontostriatal tracts for clinical symptoms and attention performance in youths with ADHD and demonstrates the involvement of different fibre tracts in attention performance in these individuals.

The role of age in association analyses of ADHD and related neurocognitive functioning: A proof of concept for dopaminergic and serotonergic genes

Elucidating genetic mechanisms involved in Attention-Deficit/Hyperactivity Disorder (ADHD) has been challenging. Relatively unexplored is the fact that genetic mechanisms can differ with age. The current study explored the association between dopaminergic and serotonergic genes, ADHD symptoms, and neurocognitive functioning in relation to age. Associations of three genetic ADHD risk factors, DAT1, DRD4, and 5-HTT with symptoms and six neurocognitive measures were explored in two samples of the NeuroIMAGE study: 756 children, adolescents, and young adults with ADHD, their siblings, and controls (M age 17 years, SD 3.2), and 393 parents with and without ADHD (M age 48 years, SD 4.8). Association analyses were performed in both samples, and effects were compared to address dichotomous age effects. Gene*age interactions were examined to address continuous age effects. Moderating effects of age were found for DRD4-7R carriership and ADHD symptoms in the adult group only; in the adolescents the 5-HTT LL genotype was differentially associated with inhibition and with motor timing at different ages, and to inhibition in adults; DAT1 10-6 haplotype carriership showed differential working memory performance depending on age. None of our effects survived correction for multiple comparisons. Our results are preliminary, but may point to differential genotype-phenotype associations at different ages. This can be seen as a proof of concept for the importance of age in dopaminergic and serotonergic genetic association analyses. Our findings are consistent with the idea that genetic and neurocognitive mechanisms underlying ADHD may change throughout life. © 2015 Wiley Periodicals, Inc.

Left anterior cingulate activity predicts intra-individual reaction time variability in healthy adults

Within-subject, or intra-individual, variability in reaction time (RT) is increasingly recognised as an important indicator of the efficiency of attentional control, yet there have been few investigations of the neural correlates of trial-to-trial RT variability in healthy adults. We sought to determine the neural correlates of intra-individual RT variability during a go/no-go response inhibition task in 27 healthy, male participants. We found that reduced trial-to-trial RT variability (i.e. greater response stability) was significantly associated with greater activation in the left pregenual anterior cingulate. These results support the role of the left anterior cingulate in the dynamic control of attention and efficient response selection. Greater understanding of intra-individual RT variability and top-down attentional control in healthy adults may help to inform disorders that impact executive/attentional control, such as attention deficit hyperactivity disorder and schizophrenia.

An association between a dopamine transporter gene (SLC6A3) haplotype and ADHD symptom measures in nonclinical adults

Previous genetic studies have postulated that attention deficit hyperactivity disorder (ADHD) should be regarded as the extreme end of a set of behavioural traits that can be continuously measured in the general population. The current study adopted a quantitative trait approach to examine the relationship between dopamine gene variants and self-reported ADHD symptoms in 517 nonclinical adults. Although genetic associations with variants of both the dopamine transporter (DAT1; SLC6A3) and D4 receptor (DRD4) genes have been reliably reported in children, results in adults are less consistent. We probed two potentially functional variable number of tandem repeat (VNTR) polymorphisms in the 3'UTR and intron 8 of DAT1, the 10-repeat and 6-repeat alleles of which respectively form a haplotype (10/6 DAT1 haplotype) that is associated with childhood ADHD. We also genotyped the exon 3 VNTR of DRD4, the 7-repeat allele of which is also an established risk factor for childhood ADHD. Permutation analysis showed an influence of the 10/6 DAT1 haplotype on both CAARS-G and CAARS-H (DSM-IV ADHD Symptoms Total and ADHD Index respectively), such that ADHD symptom scores increased with each additional copy of the 10/6 DAT1 haplotype. This result survived corrections for multiple comparisons both at the level of genotype and phenotype. A nominal association with CAARS-G was also found for the 7-repeat allele of the DRD4 VNTR however this did not survive multiple comparison correction. Our results provide further support for the influence of variation in the 10/6 DAT1 haplotype and individual differences in ADHD symptoms in adults.

The molecular genetic architecture of attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD) is a common childhood behavioral condition which affects 2-10% of school age children worldwide. Although the underlying molecular mechanism for the disorder is poorly understood, familial, twin and adoption studies suggest a strong genetic component. Here we provide a state-of-the-art review of the molecular genetics of ADHD incorporating evidence from candidate gene and linkage designs, as well as genome-wide association (GWA) studies of common single-nucleotide polymorphisms (SNPs) and rare copy number variations (CNVs). Bioinformatic methods such as functional enrichment analysis and protein-protein network analysis are used to highlight biological processes of likely relevance to the aetiology of ADHD. Candidate gene associations of minor effect size have been replicated across a number of genes including SLC6A3, DRD5, DRD4, SLC6A4, LPHN3, SNAP-25, HTR1B, NOS1 and GIT1. Although case-control SNP-GWAS have had limited success in identifying common genetic variants for ADHD that surpass critical significance thresholds, quantitative trait designs suggest promising associations with Cadherin13 and glucose-fructose oxidoreductase domain 1 genes. Further, CNVs mapped to glutamate receptor genes (GRM1, GRM5, GRM7 and GRM8) have been implicated in the aetiology of the disorder and overlap with bioinformatic predictions based on ADHD GWAS SNP data regarding enriched pathways. Although increases in sample size across multi-center cohorts will likely yield important new results, we advocate that this must occur in parallel with a shift away from categorical case-control approaches that view ADHD as a unitary construct, towards dimensional approaches that incorporate endophenotypes and statistical classification methods.

Intra-Individual Response Variability Assessed by Ex-Gaussian Analysis may be a New Endophenotype for Attention-Deficit/Hyperactivity Disorder

Intra-individual variability of response times (RTisv) is considered as potential endophenotype for attentional deficit/hyperactivity disorder (ADHD). Traditional methods for estimating RTisv lose information regarding response times (RTs) distribution along the task, with eventual effects on statistical power. Ex-Gaussian analysis captures the dynamic nature of RTisv, estimating normal and exponential components for RT distribution, with specific phenomenological correlates. Here, we applied ex-Gaussian analysis to explore whether intra-individual variability of RTs agrees with criteria proposed by Gottesman and Gould for endophenotypes. Specifically, we evaluated if normal and/or exponential components of RTs may (a) present the stair-like distribution expected for endophenotypes (ADHD > siblings > typically developing children (TD) without familiar history of ADHD) and (b) represent a phenotypic correlate for previously described genetic risk variants. This is a pilot study including 55 subjects (20 ADHD-discordant sibling-pairs and 15 TD children), all aged between 8 and 13 years. Participants resolved a visual Go/Nogo with 10% Nogo probability. Ex-Gaussian distributions were fitted to individual RT data and compared among the three samples. In order to test whether intra-individual variability may represent a correlate for previously described genetic risk variants, VNTRs at DRD4 and SLC6A3 were identified in all sibling-pairs following standard protocols. Groups were compared adjusting independent general linear models for the exponential and normal components from the ex-Gaussian analysis. Identified trends were confirmed by the non-parametric Jonckheere-Terpstra test. Stair-like distributions were observed for μ (p = 0.036) and σ (p = 0.009). An additional "DRD4-genotype" × "clinical status" interaction was present for τ (p = 0.014) reflecting a possible severity factor. Thus, normal and exponential RTisv components are suitable as ADHD endophenotypes.

Dopamine receptor D4 (DRD4) gene modulates the influence of informational masking on speech recognition

Listeners vary substantially in their ability to recognize speech in noisy environments. Here we examined the role of genetic variation on individual differences in speech recognition in various noise backgrounds. Background noise typically varies in the levels of energetic masking (EM) and informational masking (IM) imposed on target speech. Relative to EM, release from IM is hypothesized to place greater demand on executive function to selectively attend to target speech while ignoring competing noises. Recent evidence suggests that the long allele variant in exon III of the DRD4 gene, primarily expressed in the prefrontal cortex, may be associated with enhanced selective attention to goal-relevant high-priority information even in the face of interference. We investigated the extent to which this polymorphism is associated with speech recognition in IM and EM conditions. In an unscreened adult sample (Experiment 1) and a larger screened replication sample (Experiment 2), we demonstrate that individuals with the DRD4 long variant show better recognition performance in noise conditions involving significant IM, but not in EM conditions. In Experiment 2, we also obtained neuropsychological measures to assess the underlying mechanisms. Mediation analysis revealed that this listening condition-specific advantage was mediated by enhanced executive attention/working memory capacity in individuals with the long allele variant. These findings suggest that DRD4 may contribute specifically to individual differences in speech recognition ability in noise conditions that place demands on executive function.

Applications of pharmacogenetics in children with attention-deficit/hyperactivity disorder

This review examines molecular genetic studies shown to be of importance in the etiology of attention-deficit/hyperactivity disorder (ADHD) and contrasts prefrontal versus sub-cortical mechanisms. Although these mechanisms are not completely dissociated, an understanding of prefrontal dopaminergic/noradrenergic versus subcortical D1/D2 receptor mechanisms is useful for studies of diagnosis versus potential adverse effects. Dopamine physiology, dopamine receptor studies, alpha-2 agonist studies, and dopamine transporter and potential new therapies are reviewed. Further understandings of molecular mechanisms involved in etiology versus treatment and adverse effects should help personalize the treatment of ADHD.

Biomarkers in the diagnosis of ADHD--promising directions

The etiology and pathogenesis of attention-deficit/hyperactivity disorder (ADHD) are unclear and a more valid diagnosis would certainly be welcomed. Starting from the literature, we built an hypothetical pyramid representing a putative set of biomarkers where, at the top, variants in DAT1 and DRD4 genes are the best candidates for their associations to neuropsychological tasks, activation in specific brain areas, methylphenidate response and gene expression levels. Interesting data come from the noradrenergic system (norepinephrine transporter, norepinephrine, 3-methoxy-4-hydroxyphenylglycol, monoamine oxidase, neuropeptide Y) for their altered peripheral levels, their association with neuropsychological tasks, symptomatology, drugs effect and brain function. Other minor putative genetic biomarkers could be dopamine beta hydroxylase and catechol-O-methyltransferase. In the bottom, we placed endophenotype biomarkers. A more deep integration of "omics" sciences along with more accurate clinical profiles and new high-throughput computational methods will allow us to identify a better list of biomarkers useful for diagnosis and therapies.

The rare DAT coding variant Val559 perturbs DA neuron function, changes behavior, and alters in vivo responses to psychostimulants

Despite the critical role of the presynaptic dopamine (DA) transporter (DAT, SLC6A3) in DA clearance and psychostimulant responses, evidence that DAT dysfunction supports risk for mental illness is indirect. Recently, we identified a rare, nonsynonymous Slc6a3 variant that produces the DAT substitution Ala559Val in two male siblings who share a diagnosis of attention-deficit hyperactivity disorder (ADHD), with other studies identifying the variant in subjects with bipolar disorder (BPD) and autism spectrum disorder (ASD). Previously, using transfected cell studies, we observed that although DAT Val559 displays normal total and surface DAT protein levels, and normal DA recognition and uptake, the variant transporter exhibits anomalous DA efflux (ADE) and lacks capacity for amphetamine (AMPH)-stimulated DA release. To pursue the significance of these findings in vivo, we engineered DAT Val559 knock-in mice, and here we demonstrate in this model the presence of elevated extracellular DA levels, altered somatodendritic and presynaptic D2 DA receptor (D2R) function, a blunted ability of DA terminals to support depolarization and AMPH-evoked DA release, and disruptions in basal and psychostimulant-evoked locomotor behavior. Together, our studies demonstrate an in vivo functional impact of the DAT Val559 variant, providing support for the ability of DAT dysfunction to impact risk for mental illness.

Symptoms of attention-deficit hyperactivity disorder, nonsyndromic orofacial cleft children, and dopamine polymorphisms: a pilot study

PURPOSE

Attention-deficit hyperactivity disorder (ADHD) is a common childhood neurobehavioral disorder characterized by inattention, poor impulse control, and motor restlessness. Risk factors include familial stressors, anxiety disorders, learning disabilities, abnormal brain development, heritability, and dopamine polymorphisms. Children with an orofacial clefting (OFC) history are at increased risk of familial stressors, anxiety disorders, learning disabilities, and abnormal brain development. Given this overlap, we present a conceptual model proposing that children with OFC may be more likely to exhibit ADHD symptoms than children without and explore this relationship using pilot data.

DESIGN

This cross-sectional pilot study included 29 children with OFC or a first-degree relative with OFC recruited through a cleft research registry.

METHODS

The Disruptive Behavior Disorder Scale was used to collect data on children's ADHD symptoms. Saliva or whole blood samples were collected from children and parents for DNA analyses. ADHD-associated dopamine polymorphisms within the DRD4, DRD2, and DAT1 genes were genotyped. We tested for associations between presence of OFC and dopamine polymorphisms. Mixed-effects models tested whether children with OFC and dopamine polymorphisms had more ADHD symptoms.

RESULTS

The DRD4 4-repeat allele was associated with increased inattentive ADHD symptoms (p = .03). Having the DRD2 Taq1A1 allele and OFC predicted fewer (p = .02) inattentive ADHD symptoms. Children with OFC were significantly less likely to have the DAT1 10-repeat allele (p = .04).

CONCLUSIONS

Results indicate that further investigation among a larger sample of children with OFC is warranted, particularly for relationships with inattentive ADHD.

Alpha-2A adrenergic receptor gene variants are associated with increased intra-individual variability in response time

Intra-individual variability in response time has been proposed as an important endophenotype for attention deficit hyperactivity disorder (ADHD). Here we asked whether intra-individual variability is predicted by common variation in catecholamine genes and whether it mediates the relationship between these gene variants and self-reported ADHD symptoms. A total of 402 non-clinical Australian adults of European descent completed a battery of five cognitive tasks and the Conners' Adult ADHD Rating Scale. Exclusion criteria included the presence of major psychiatric or neurologic illnesses and substance dependency. A total of 21 subjects were excluded due to incomplete data or poor quality cognitive or genotyping data. The final sample comprised 381 subjects (201 males; mean age=21.2 years, s.d.=5.1 years). Principal components analysis on variability measures yielded two factors (response selection variability vs selective attention variability). Association of these factors with catecholamine gene variants was tested using single-step linear regressions, with multiple comparisons controlled using permutation analysis. The response selection variability factor was associated with two ADRA2A single-nucleotide polymorphisms (SNPs) (rs1800544, rs602618), p corrected=0.004, 0.012, respectively, whereas the selective attention variability factor was associated with a TH SNP (rs3842727), p corrected=0.024. A bootstrapping analysis indicated that the response selection variability factor mediated the relationship between the ADRA2A SNP rs1800544 and self-reported ADHD symptoms. Thus this study finds evidence that DNA variation in the ADRA2A gene may be causally related to ADHD-like behaviors, in part through its influence on intra-individual variability. Evidence was also found for a novel association between a TH gene variant and intra-individual variability.

Dopamine system genes are associated with orienting bias among healthy individuals

Healthy individuals display subtle orienting bias, manifested as a tendency to direct greater attention toward one hemispace, and evidence suggests that this bias reflects an individual trait, which may be modulated by asymmetric dopamine signaling in striatal and frontal regions. The current study examined the hypothesis that functional genetic variants within dopaminergic genes (DAT1 3' VNTR, dopamine D2 receptor Taq1A (rs1800497) and COMT Val158Met (rs4680)) contribute to individual differences in orienting bias, as measured by the greyscales paradigm, in a sample of 197 young healthy Israeli Jewish participants. For the Taq1A variant, homozygous carriers of the A2 allele displayed significantly increased leftward orienting bias compared to the carriers of the A1 allele. Additionally, and as previously reported by others, we found that bias towards leftward orienting of attention was significantly greater among carriers of the 9-repeat allele of the DAT1 3' VNTR as compared to the individuals who were homozygous for the 10-repeat allele. No significant effect of the COMT Val158Met on orienting bias was found. Taken together, our findings support the potential influence of genetic variants on inter-individual differences in orienting bias, a phenotype relevant to both normal and impaired cognitive processes.

Genetic targeting of the amphetamine and methylphenidate-sensitive dopamine transporter: on the path to an animal model of attention-deficit hyperactivity disorder

Alterations in dopamine (DA) signaling underlie the most widely held theories of molecular and circuit level perturbations that lead to risk for attention-deficit hyperactivity disorder (ADHD). The DA transporter (DAT), a presynaptic reuptake protein whose activity provides critical support for DA signaling by limiting DA action at pre- and postsynaptic receptors, has been consistently associated with ADHD through pharmacological, behavioral, brain imaging and genetic studies. Currently, the animal models of ADHD exhibit significant limitations, stemming in large part from their lack of construct validity. To remedy this situation, we have pursued the creation of a mouse model derived from a functional nonsynonymous variant in the DAT gene (SLC6A3) of ADHD probands. We trace our path from the identification of these variants to in vitro biochemical and physiological studies to the production of the DAT Val559 mouse model. We discuss our initial findings with these animals and their promise in the context of existing rodent models of ADHD.

Increased response-time variability across different cognitive tasks in children with ADHD

OBJECTIVE

Increased response-time (RT) fluctuations below 0.2 Hz have been reported as characteristic of ADHD in some but not all studies, possibly due to methodological differences. Accordingly, We contrasted two tasks and two analytical approaches in the same sample of children with ADHD.

METHOD

Fifty-two children with ADHD and 49 typically developing children completed an Eriksen Flanker Task and a fixed-sequence version of the sustained attention to response task. RT fluctuations with two different frequency analyses were examined.

RESULTS

Robust ADHD-related increases of slow RT fluctuations within all frequencies were found in both tasks. Tasks were significantly correlated in both groups for frequencies above 0.07 Hz. RT fluctuations across all frequencies were greatest in children with ADHD with abnormally elevated omissions.

CONCLUSION

We observed significantly increased fluctuations of RT in children with ADHD across two different tasks and methods supporting the hypothesis that slow frequency RT fluctuations reflect neurophysiological processes underlying ADHD.

Methylphenidate blocks effort-induced depletion of regulatory control in healthy volunteers

A recent wave of studies--more than 100 conducted over the last decade--has shown that exerting effort at controlling impulses or behavioral tendencies leaves a person depleted and less able to engage in subsequent rounds of regulation. Regulatory depletion is thought to play an important role in everyday problems (e.g., excessive spending, overeating) as well as psychiatric conditions, but its neurophysiological basis is poorly understood. Using a placebo-controlled, double-blind design, we demonstrated that the psychostimulant methylphenidate (commonly known as Ritalin), a catecholamine reuptake blocker that increases dopamine and norepinephrine at the synaptic cleft, fully blocks effort-induced depletion of regulatory control. Spectral analysis of trial-by-trial reaction times revealed specificity of methylphenidate effects on regulatory depletion in the slow-4 frequency band. This band is associated with the operation of resting-state brain networks that produce mind wandering, which raises potential connections between our results and recent brain-network-based models of control over attention.

Potential contribution of dopaminergic gene variants in ADHD core traits and co-morbidity: a study on eastern Indian probands

Association of dopaminergic genes, mainly receptors and transporters, with Attention Deficit Hyperactivity Disorder (ADHD) has been investigated throughout the world due to the importance of dopamine (DA) in various physiological functions including attention, cognition and motor activity, traits. However, till date, etiology of ADHD remains unknown. We explored association of functional variants in the DA receptor 2 (rs1799732 and rs6278), receptor 4 (exon 3 VNTR and rs914655), and transporter (rs28363170 and rs3836790) with hyperactivity, cognitive deficit, and co-morbid disorders in eastern Indian probands. Diagnostic and Statistical Manual for Mental Disorders-IV was followed for recruitment of nuclear families with ADHD probands (N = 160) and ethnically matched controls (N = 160). Cognitive deficit and hyperactive traits were measured using Conner's parents/teachers rating scale. Peripheral blood was collected after obtaining informed written consent and used for genomic DNA isolation. Genetic polymorphisms were analyzed by PCR-based methods followed by population- as well as family-based statistical analyses. Association between genotypes and cognitive/hyperactivity traits and co-morbidities was analyzed by the Multifactor dimensionality reduction (MDR) software. Case-control analysis showed statistically significant difference for rs6278 and rs28363170 (P = 0.004 and 1.332e-007 respectively) while family-based analysis exhibited preferential paternal transmission of rs28363170 '9R' allele (P = 0.04). MDR analyses revealed independent effects of rs1799732, rs6278, rs914655, and rs3836790 in ADHD. Significant independent effects of different sites on cognitive/hyperactivity traits and co-morbid disorders were also noticed. It can be summarized from the present investigation that these gene variants may influence cognitive/hyperactive traits, thereby affecting the disease etiology and associated co-morbid features.

Response time intra-subject variability: commonalities between children with autism spectrum disorders and children with ADHD

Despite the common co-occurrence of symptoms of attention deficit hyperactivity disorder (ADHD) in individuals with autism spectrum disorders (ASD), the underlying mechanisms are under-explored. A potential candidate for investigation is response time intra-subject variability (RT-ISV), a hypothesized marker of attentional lapses. Direct comparisons of RT-ISV in ASD versus ADHD are limited and contradictory. We aimed to examine whether distinct fluctuations in RT-ISV characterize children with ASD and with ADHD relative to typically developing children (TDC). We applied both a priori-based and data-driven strategies to RT performance of 46 children with ASD, 46 with ADHD, and 36 TDC (aged 7-11.9 years). Specifically, we contrasted groups relative to the amplitude of four preselected frequency bands as well as to 400 frequency bins from 0.006 to 0.345 Hz. In secondary analyses, we divided the ASD group into children with and without substantial ADHD symptoms (ASD(+) and ASD(-), respectively). Regardless of the strategy employed, RT-ISV fluctuations at frequencies between 0.20 and 0.345 Hz distinguished children with ADHD, but not children with ASD, from TDC. Children with ASD(+) and those with ADHD shared elevated amplitudes of RT-ISV fluctuations in frequencies between 0.18 and 0.345 Hz relative to TDC. In contrast, the ASD(-) subgroup did not differ from TDC in RT-ISV frequency fluctuations. RT-ISV fluctuations in frequencies 0.18-0.345 Hz (i.e., periods between 3 and 5 s) are associated with ADHD symptoms regardless of categorical diagnosis and may represent a biomarker. These results suggest that children with ADHD and those with ASD(+) share common underlying pathophysiological mechanisms of RT-ISV.

Epigenetic histone modification regulates developmental lead exposure induced hyperactivity in rats

Lead (Pb) exposure was commonly considered as a high environmental risk factor for the development of attention-deficit/hyperactivity disorder (ADHD). However, the molecular basis of this pathological process still remains elusive. In light of the role of epigenetics in modulating the neurological disease and the causative environment, the alterations of histone modifications in the hippocampus of rats exposed by various doses of lead, along with concomitant behavioral deficits, were investigated in this study. According to the free and forced open field test, there showed that in a dosage-dependent manner, lead exposure could result in the increased locomotor activity of rats, that is, hyperactivity: a subtype of ADHD. Western blotting assays revealed that the levels of histone acetylation increased significantly in the hippocampus by chronic lead exposure, while no dramatic changes were detected in terms of expression yields of ADHD-related dopaminergic proteins, indicating that histone acetylation plays essential roles in this toxicant-involved pathogenesis. In addition, the increased level of histone acetylation might be attributed to the enzymatic activity of p300, a typical histone acetyltransferase, as the transcriptional level of p300 was significantly increased upon higher-dose Pb exposure. In summary, this study first discovered the epigenetic mechanism bridging the environmental influence (Pb) and the disease itself (ADHD) in the histone modification level, paving the way for the comprehensive understanding of ADHD's etiology and in further steps, establishing the therapy strategy of this widespread neurological disorder.

Increased intra-individual variability of cognitive processing in subjects at risk mental state and schizophrenia patients

Intra-individual variability (IIV) has received recent attention as an indicator of the stability of cognitive functioning that may outperform mean performance in reflecting putative neurobiological abnormalities. Increased IIV is regarded as a core deficit in schizophrenia patients; however, whether this deficit is present in the prodromal phase before the onset of schizophrenia has not been well established. In the present study, we investigated IIV using the stop-signal paradigm in at-risk mental state (ARMS) individuals and in schizophrenia patients. The study included 27 ARMS subjects, 37 schizophrenia patients, and 38 normal controls. The stop-signal task was administered to assess IIV and response inhibition. IIV was estimated by calculating the standard deviation across sub-blocks for the three groups. We observed increased IIV in ARMS subjects and schizophrenia patients compared with normal controls in both the “stop” and the “go” processes even though the mean response inhibition performances were not impaired in the ARMS group. Schizophrenia patients showed impaired response inhibition that was associated with the severity of negative symptoms. Our findings suggest that the analysis of IIV may identify cognitive and clinical features of ARMS that are not detectable by conventional mean performance analysis. The unstable response patterns associated with ARMS may originate from abnormal processing in neural systems caused by alterations in the integrity of functional brain networks and dopamine neuromodulation.

The effect of different stimulus attributes on the attentional performance of children with attention deficit/hyperactivity disorder and dyslexia

While teachers have traditionally used the interesting objects to increase student attention in the classroom, evidence supporting the effectiveness of this method is lacking. The present study investigated the influence of different stimulus attributes for typical developing students and for students with attention deficit/hyperactivity disorder (ADHD) and dyslexia. Thirty children with ADHD, 30 children with dyslexia, and 30 typical developing students were tested using a measuring tool that was constructed by the authors to assess their sustained attention and selective attention on the geometric-figure assessment and the interesting-figure assessment. The geometric-figure assessment included a square, circle, trapezium, and triangle; and the interesting-figure assessment included a house, cat, hand, and tree. While the typical developing group showed better selective attention on the geometric-figure assessment, there was no difference between the dyslexic group and the ADHD group with respect to selective attention. Furthermore, the typical developing and dyslexic groups did not differ in the geometric-figure assessment in sustained attention and were both better in this area than the ADHD group. In the interesting-figure assessment, the typical developing and dyslexic groups performed similarly in sustained attention, but selective attention of the dyslexic group improved more than the ADHD group, similar to the typical developing group. Both selective attention of the dyslexic group and sustained attention of the ADHD group showed positive significant differences in the interesting-figure assessment, but sustained attention of the dyslexic group and selective attention of the ADHD group showed little difference in the interesting-figure assessment. Surprisingly, the typical developing group did not show any significant difference in the interesting-figure assessment, possibly because they had previously demonstrated a ceiling effect in the geometric-figure assessment.

Monoaminergic dysfunction in recreational users of dexamphetamine

Preclinical studies suggest that dexamphetamine (dAMPH) can lead to monoaminergic neurotoxicity. This exploratory study aimed to investigate effects of recreational dAMPH use on the dopamine (DA) and noradrenaline (NA) systems in humans. To that purpose, eight male abstinent dAMPH (26.0 ± 4.0 years) users and 10 age- and IQ-matched male healthy control subjects (23.0 ± 3.8) underwent neuropsychological testing sensitive to DAergic function and single photon emission computed tomography (SPECT) scanning with [(123)I]FP-CIT to determine striatal DA transporter (DAT) binding. In addition, changes in cerebral blood flow (CBF) induced by the DA/NA reuptake inhibitor methylphenidate (MPH) were measured using pharmacological magnetic resonance imaging (phMRI). Performance of dAMPH users was significantly worse on executive function and verbal memory tasks. Striatal DAT binding ratios were on average lower in dAMPH users (near-significant, p=0.05). In addition, CBF in control subjects decreased significantly in response to MPH in gray matter and basal ganglia, among which the striatum, thalamus and hippocampus by 10% to 29%. However, in dAMPH users the CBF response was blunted in most brain areas studied, only decreasing in the hippocampus and orbitofrontal cortex. When comparing groups, CBF response was found to be significantly different in the thalamus with a decrease for healthy controls and a blunted response in dAMPH users. Collectively, our findings of a blunted hemodynamic response in monoaminergic regions, in combination with indications for lower striatal DAT binding and poorer behavioral measures are likely to represent DAergic dysfunction in dAMPH users, although NAergic dysfunction may also play a role.

On the temporal characteristics of performance variability in attention deficit hyperactivity disorder (ADHD)

Increased intra-subject variability of reaction times (ISV-RT) is one of the most consistent findings in attention-deficit/hyperactivity disorder (ADHD). Although the nature of this phenomenon is still unclear, it has been hypothesised to reflect interference from the Default Mode Network (DMN). So far, ISV-RT has been operationally defined either as a frequency spectrum of the underlying RT time series, or as a measure of dispersion of the RT scores distribution. Here, we use a novel RT analysis framework to link these hitherto unconnected facets of ISV-RT by determining the sensitivity of different measures of RT dispersion to the frequency content of the underlying RT time series. N=27 patients with ADHD and N=26 healthy controls performed several visual N-back tasks. Different measures of RT dispersion were repeatedly modelled after individual frequency bands of the underlying RT time series had been either extracted or suppressed using frequency-domain filtering. We found that the intra-subject standard deviation of RT preserves the "1/f noise" characteristic typical of human RT data. Furthermore and most importantly, we found that the ex-Gaussian parameter τ is rather exclusively sensitive to frequencies below 0.025 Hz in the underlying RT time series and that the particularly slow RTs, which nourish τ, occur regularly as part of an quasi-periodic, ultra-slow RT fluctuation. Overall, our results are compatible with the idea that ISV-RT is modulated by an endogenous, slowly fluctuating process that may reflect DMN interference.

Change blindness in children with ADHD: a selective impairment in visual search?

OBJECTIVE

This study evaluated change blindness and visual search efficiency in children with ADHD in searching for central and marginal changes.

METHOD

A total of 36 drug-naïve children (18 ADHD/18 controls) performed a flicker task that included changes in objects of central or marginal interest. The task required observers to search for a change until they detected it.

RESULTS

Children with ADHD performed more slowly and less accurately than did typically developing children, specifically in detecting marginal-interest changes.

CONCLUSION

In contrast to more standard visual search tasks, flicker tasks seem to be more sensitive to highlight focused attention deficits in children diagnosed with ADHD. Concretely, ADHD attentional deficits were more apparent when the task involved serial top-down strategies.

RETRACTED: A comparison of tantrum behavior profiles in children with ASD, ADHD and comorbid ASD and ADHD

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editor in Chief, after review by an independent panel of experts, due to evidence of a compromised peer review process and the failure of one author to disclose significant conflicts of interest. An independent peer-review process is a cornerstone of scientific integrity that allows for research to be scrutinized before publication to ensure that conclusions are anchored in sound methodology and objective interpretation of the results. Equally important is that the readership of research is fully informed about any potential competing interests that may have influenced the research process. This article is being retracted because it did not include a declaration of a conflict of interest of one author in relation to diagnostic tools which the paper endorses. The same author was also the Editor in Chief of the journal at the time of publication and evidence indicates that the paper was accepted without any independent peer review by external reviewers. The article is therefore retracted in accordance with the ethical guidelines of Elsevier: https://www.elsevier.com/about/policies/publishing-ethics and the Committee on Publication Ethics (COPE) https://publicationethics.org/files/retraction-guidelines.pdf.

Neurocognitive effects of methylphenidate on ADHD children with different DAT genotypes: a longitudinal open label trial

The variable number of tandem repeat polymorphism in the 3'-untranslated region of the dopamine transporter gene (DAT) may influence the variability of the therapeutic response to methylphenidate (MPH) in Attention Deficit/Hyperactivity Disorder (ADHD). For this reason we evaluated the neuropsychological functioning after a prolonged period of MPH treatment and after a specific time from MPH suspension. Relationship between DAT VNTR genotypes and neurocognitive response to MPH was analyzed in a sample of 108 drug-naive ADHD patients. The performance of children with ADHD on measures of working memory, inhibition and planning was assessed at 4, 8 and 24 weeks and at 8 weeks after MPH withdrawal. Patients with 9/9 genotype evidenced an improvement in response inhibition and working memory only at 4 weeks of treatment, in planning at 24 weeks of therapy and after 8 weeks of MPH suspension. Patients with 9/10 showed an improvement in response inhibition at 4, 8 and 24 weeks of treatment, in planning at 24 weeks and after 8 weeks of MPH suspension. Patients with 10/10 evidenced an improvement in response inhibition and working memory at 4, 8 and 24 weeks of treatment and in planning at 4, 8 and 24 weeks of treatment and after 8 weeks of suspension. These results indicate that the 9/9 ADHD genotype has a different response at 24 weeks treatment with MPH. 10/10 DAT allele seems to be associated with an increased expression level of the dopamine transporter and seems to mediate the MPH treatment response in ADHD patients.

The dopamine transporter haplotype and reward-related striatal responses in adult ADHD

Attention deficit/hyperactivity disorder (ADHD) is a highly heritable disorder and several genes increasing disease risk have been identified. The dopamine transporter gene, SLC6A3/DAT1, has been studied most extensively in ADHD research. Interestingly, a different haplotype of this gene (formed by genetic variants in the 3' untranslated region and intron 8) is associated with childhood ADHD (haplotype 10-6) and adult ADHD (haplotype 9-6). The expression of DAT1 is highest in striatal regions in the brain. This part of the brain is of interest to ADHD because of its role in reward processing is altered in ADHD patients; ADHD patients display decreased striatal activation during reward processing. To better understand how the DAT1 gene exerts effects on ADHD, we studied the effect of this gene on reward-related brain functioning in the area of its highest expression in the brain, the striatum, using functional magnetic resonance imaging. In doing so, we tried to resolve inconsistencies observed in previous studies of healthy individuals and ADHD-affected children. In a sample of 87 adult ADHD patients and 77 healthy comparison subjects, we confirmed the association of the 9-6 haplotype with adult ADHD. Striatal hypoactivation during the reward anticipation phase of a monetary incentive delay task in ADHD patients was again shown, but no significant effects of DAT1 on striatal activity were found. Although the importance of the DAT1 haplotype as a risk factor for adult ADHD was again demonstrated in this study, the mechanism by which this gene increases disease risk remains largely unknown.

Musicians change their tune: how hearing loss alters the neural code

Individuals with sensorineural hearing loss have difficulty understanding speech, especially in background noise. This deficit remains even when audibility is restored through amplification, suggesting that mechanisms beyond a reduction in peripheral sensitivity contribute to the perceptual difficulties associated with hearing loss. Given that normal-hearing musicians have enhanced auditory perceptual skills, including speech-in-noise perception, coupled with heightened subcortical responses to speech, we aimed to determine whether similar advantages could be observed in middle-aged adults with hearing loss. Results indicate that musicians with hearing loss, despite self-perceptions of average performance for understanding speech in noise, have a greater ability to hear in noise relative to nonmusicians. This is accompanied by more robust subcortical encoding of sound (e.g., stimulus-to-response correlations and response consistency) as well as more resilient neural responses to speech in the presence of background noise (e.g., neural timing). Musicians with hearing loss also demonstrate unique neural signatures of spectral encoding relative to nonmusicians: enhanced neural encoding of the speech-sound's fundamental frequency but not of its upper harmonics. This stands in contrast to previous outcomes in normal-hearing musicians, who have enhanced encoding of the harmonics but not the fundamental frequency. Taken together, our data suggest that although hearing loss modifies a musician's spectral encoding of speech, the musician advantage for perceiving speech in noise persists in a hearing-impaired population by adaptively strengthening underlying neural mechanisms for speech-in-noise perception.

Sustained attention and frailty in the older adult population

OBJECTIVES

We investigated whether sustained attention performance and variability were associated with prefrailty and frailty in the older adult population.

METHOD

A total of 4,317 participants aged 50 years and over from the Irish Longitudinal Study on Ageing (TILDA) completed a comprehensive health assessment. Frailty was defined by low gait speed, low grip strength, unintentional weight loss, self-reported exhaustion, and low physical activity. Scores of greater than or equal to 3, 1-2, and 0 indicated that participants were frail, prefrail, and nonfrail, respectively. Multinomial logistic regression computed associations between frailty state and measures of performance and variability on the Sustained Attention to Response Task (SART). Cognitive processing speed and executive function were also measured.

RESULTS

Mean reaction time (RT; odds ratio [OR] = 1.13, p < .05) and RT variability reflective of the top-down aspect of sustained attention (OR = 1.11, p < .05) were associated with prefrailty in the 50-64 age group. Mean RT (OR = 1.72, p < .05) was associated with frailty and RT variability (OR = 1.22, p < .01) with prefrailty in the 65+ age group. Results remained significant following adjustments for cognitive processing speed, executive function, chronic conditions, medications, age, and gender.

DISCUSSION

Sustained attention performance and variability were associated with prefrailty and frailty in the older adult population and may represent a novel, objective, and modifiable cognitive marker of frailty progression.

Inattention, hyperactivity, and emergent literacy: different facets of inattention relate uniquely to preschoolers' reading-related skills

Although extant studies indicate that there is a strong association between attention deficit/hyperactivity disorder and reading ability in elementary school children, knowledge regarding the relation between inattentive and hyperactive/impulsive behaviors and emergent literacy in preschool children is less established. This study examined the unique and overlapping relations between measures that assess inattention and hyperactivity/impulsivity and emergent literacy skills in preschool children. Participants included 204 preschool children (M age = 56 months, 50.9% female, 79.8% European American). Behavioral rating scales were completed by teachers, and the Continuous Performance Test (CPT) and the Test of Preschool Early Literacy were completed by the preschoolers. Across measures, inattention was a unique correlate of emergent literacy skills, whereas hyperactivity/impulsivity was not. Both rating scales and the CPT indices of inattention were uniquely associated with emergent literacy skills. These results suggest that these measures are assessing different manifestations of inattention that are both unique correlates of early reading skills.

Sustaining attention to simple tasks: a meta-analytic review of the neural mechanisms of vigilant attention

Maintaining attention for more than a few seconds is essential for mastering everyday life. Yet, our ability to stay focused on a particular task is limited, resulting in well-known performance decrements with increasing time on task. Intriguingly, such decrements are even more likely if the task is cognitively simple and repetitive. The attentional function that enables our prolonged engagement in intellectually unchallenging, uninteresting activities has been termed vigilant attention. Here we synthesized what we have learned from functional neuroimaging about the mechanisms of this essential mental faculty. To this end, a quantitative meta-analysis of pertinent neuroimaging studies was performed, including supplementary analyses of moderating factors. Furthermore, we reviewed the available evidence on neural time-on-task effects, additionally considering information obtained from patients with focal brain damage. Integrating the results of both meta-analysis and review, we identified a set of mainly right-lateralized brain regions that may form the core network subserving vigilant attention in humans, including dorsomedial, mid- and ventrolateral prefrontal cortex, anterior insula, parietal areas (intraparietal sulcus, temporoparietal junction), and subcortical structures (cerebellar vermis, thalamus, putamen, midbrain). We discuss the potential functional roles of different nodes of this network as well as implications of our findings for a theoretical account of vigilant attention. It is conjectured that sustaining attention is a multicomponent, nonunitary mental faculty, involving a mixture of (a) sustained/recurrent processes subserving task-set/arousal maintenance and (b) transient processes subserving the target-driven reorienting of attention. Finally, limitations of previous studies are considered and suggestions for future research are provided.

Dopamine transporter genotype predicts attentional asymmetry in healthy adults

A number of recent studies suggest that DNA variation in the dopamine transporter gene (DAT1) influences spatial attention asymmetry in clinical populations such as ADHD, but confirmation in non-clinical samples is required. Since non-spatial factors such as attentional load have been shown to influence spatial biases in clinical conditions, here we sought to determine whether any association between DAT1 genotype and spatial bias might be moderated by non-spatial attentional load. Healthy adults were asked to react to sudden onset peripheral targets while demand on non-spatial attention was manipulated via a central task. Participants were genotyped for a DAT1 variable number of tandem repeat (VNTR) polymorphism. The 10-repeat allele of this variant is a replicated susceptibility allele for ADHD and has been shown to associate with spatial bias. As expected, an overall leftward asymmetry/pseudoneglect was observed when the data were averaged across the entire sample. When data were stratified by DAT1 genotype, individuals lacking homozygosity for the 10-repeat DAT1 allele (non-10/10) showed a pronounced leftward bias that was significantly different from zero. In line with past reports from children with ADHD, this leftward bias was attenuated in individuals who were homozygous for the DAT1 10-repeat allele (10/10), suggestive of relatively weaker right hemisphere dominance for spatial attention. This effect of DAT1 genotype on spatial bias was not modulated by non-spatial attention load. These data confirm in healthy adult participants both the existence and the direction of the relationship previously reported between DAT1 genotype and spatial bias in children with ADHD. These data add to a growing body of evidence showing that spatial attentional asymmetry is a stable quantitative trait, with individual differences in this trait significantly predicted by common DNA variation in the DAT1 gene.

Dopaminergic polymorphisms associated with time-on-task declines and fatigue in the Psychomotor Vigilance Test

Prolonged demands on the attention system can cause a decay in performance over time known as the time-on-task effect. The inter-subject differences in the rate of this decline are large, and recent efforts have been made to understand the biological bases of these individual differences. In this study, we investigate the genetic correlates of the time-on-task effect, as well as its accompanying changes in subjective fatigue and mood. N = 332 subjects performed a 20-minute test of sustained attention (the Psychomotor Vigilance Test) and rated their subjective states before and after the test. We observed substantial time-on-task effects on average, and large inter-individual differences in the rate of these declines. The 10-repeat allele of the variable number of tandem repeats marker (VNTR) in the dopamine transporter gene and the Met allele of the catechol-o-methyl transferase (COMT) Val158Met polymorphism were associated with greater vulnerability to time-on-task. Separately, the exon III DRD4 48 bp VNTR of the dopamine receptor gene DRD4 was associated with subjective decreases in energy. No polymorphisms were associated with task-induced changes in mood. We posit that the dopamine transporter and COMT genes exert their effects by increasing dopaminergic tone, which may induce long-term changes in the prefrontal cortex, an important mediator of sustained attention. Thus, these alleles may affect performance particularly when sustained dopamine release is necessary.

Becoming consistent: developmental reductions in intraindividual variability in reaction time are related to white matter integrity

Cognitive development is known to involve improvements in accuracy, capacity, and processing speed. Less is known about the role of performance consistency, and there has been virtually no empirical examination of the neural underpinnings of within-person variability in development. In a sample of 92 healthy children and adolescents aged 8-19 years, we aimed to characterize age-related changes in trial-to-trial intraindividual variability (IIV) of reaction time (RT) and to test whether IIV is related to white matter (WM) integrity as indexed by diffusion tensor imaging. IIV was quantified as the SD of correct RTs in a speeded arrow flanker task, and Tract-Based Spatial Statistics was used to test relationships with diffusion characteristics. Large age-related reductions in IIV in both simple congruent trials and more complex incongruent trials were found. Independently of sex, age, and median RT (mRT), lower IIV was associated with higher fractional anisotropy and lower overall diffusivity. Effects were seen for IIV in one or both trial types in the corticospinal tract, the left superior longitudinal fasciculus, the uncinate fasciculus, the forceps minor, and in the genu and splenium of the corpus callosum. There were no significant associations between mRT and any of the diffusion indices. The findings support the proposition that developmental reductions in IIV reflect maturation of WM connectivity and highlight the importance of considering within-person variability in theories of cognitive development and its neurobiological foundation.

Intra-individual variability among children with ADHD on a working memory task: an ex-Gaussian approach

Intra-individual variability on a computer-based working memory task was examined among 25 children/adolescents with ADHD and 24 typically developing peers. Participants completed the Visual Serial Addition Task (VSAT) and reaction time data were fit to an ex-Gaussian distribution. ADHD participants demonstrated significantly more variable performance than controls, and effects of working memory load were observed. Event rate, however, had no influence on group differences in performance. Follow-up correlations revealed associations between VSAT performance and ADHD symptomatology. This study supports intra-individual variability as a hallmark feature of ADHD beyond the domain of response inhibition and reinforces the need to consider variability in ADHD more broadly.

Falls and falls efficacy: the role of sustained attention in older adults

BACKGROUND

Previous evidence indicates that older people allocate more of their attentional resources toward their gait and that the attention-related changes that occur during aging increase the risk of falls. The aim of this study was to investigate whether performance and variability in sustained attention is associated with falls and falls efficacy in older adults.

METHODS

458 community-dwelling adults aged ≥ 60 years underwent a comprehensive geriatric assessment. Mean and variability of reaction time (RT), commission errors and omission errors were recorded during a fixed version of the Sustained Attention to Response Task (SART). RT variability was decomposed using the Fast Fourier Transform (FFT) procedure, to help characterise variability associated with the arousal and vigilance aspects of sustained attention.The number of self-reported falls in the previous twelve months, and falls efficacy (Modified Falls Efficacy Scale) were also recorded.

RESULTS

Significant increases in the mean and variability of reaction time on the SART were significantly associated with both falls (p < 0.01) and reduced falls efficacy (p < 0.05) in older adults. An increase in omission errors was also associated with falls (p < 0.01) and reduced falls efficacy (p < 0.05). Upon controlling for age and gender affects, logistic regression modelling revealed that increasing variability associated with the vigilance (top-down) aspect of sustained attention was a retrospective predictor of falling (p < 0.01, OR = 1.14, 95% CI: 1.03-1.26) in the previous year and was weakly correlated with reduced falls efficacy in non-fallers (p = 0.07).

CONCLUSIONS

Greater variability in sustained attention is strongly correlated with retrospective falls and to a lesser degree with reduced falls efficacy. This cognitive measure may provide a novel and valuable biomarker for falls in older adults, potentially allowing for early detection and the implementation of preventative intervention strategies.