Synchronization, anticipation, and consistency in motor timing of children with dimensionally defined attention deficit hyperactivity behaviour

MEG event-related desynchronization and synchronization deficits during basic somatosensory processing in individuals with ADHD

Background

Attention-Deficit/Hyperactivity Disorder (ADHD) is a prevalent, complex disorder which is characterized by symptoms of inattention, hyperactivity, and impulsivity. Convergent evidence from neurobiological studies of ADHD identifies dysfunction in fronto-striatal-cerebellar circuitry as the source of behavioural deficits. Recent studies have shown that regions governing basic sensory processing, such as the somatosensory cortex, show abnormalities in those with ADHD suggesting that these processes may also be compromised.

Methods

We used event-related magnetoencephalography (MEG) to examine patterns of cortical rhythms in the primary (SI) and secondary (SII) somatosensory cortices in response to median nerve stimulation, in 9 adults with ADHD and 10 healthy controls. Stimuli were brief (0.2 ms) non-painful electrical pulses presented to the median nerve in two counterbalanced conditions: unpredictable and predictable stimulus presentation. We measured changes in strength, synchronicity, and frequency of cortical rhythms.

Results

Healthy comparison group showed strong event-related desynchrony and synchrony in SI and SII. By contrast, those with ADHD showed significantly weaker event-related desynchrony and event-related synchrony in the alpha (8–12 Hz) and beta (15–30 Hz) bands, respectively. This was most striking during random presentation of median nerve stimulation. Adults with ADHD showed significantly shorter duration of beta rebound in both SI and SII except for when the onset of the stimulus event could be predicted. In this case, the rhythmicity of SI (but not SII) in the ADHD group did not differ from that of controls.

Conclusion

Our findings suggest that somatosensory processing is altered in individuals with ADHD. MEG constitutes a promising approach to profiling patterns of neural activity during the processing of sensory input (e.g., detection of a tactile stimulus, stimulus predictability) and facilitating our understanding of how basic sensory processing may underlie and/or be influenced by more complex neural networks involved in higher order processing.

Temporal lobe dysfunction in medication-naïve boys with attention-deficit/hyperactivity disorder during attention allocation and its relation to response variability

BACKGROUND

Patients with attention-deficit/hyperactivity disorder (ADHD) typically show fronto-striatal abnormalities during functions of cognitive control. In this study we investigate whether medication-naïve children with ADHD are impaired in temporo-parietal neural networks that mediate purely perceptual attention allocation to a behaviorally neutral oddball task. Furthermore, we explore the relationship between the neural substrates of attention allocation and response variability, typically increased in patients.

METHOD

Event-related functional magnetic resonance imaging was used to compare brain activation of 17 medication-naïve boys with ADHD with that of 18 handedness- and IQ-matched healthy comparison boys during a visual oddball task that required the same response to oddball and standard trials. Furthermore, to explore the relationship between behavioral dispersion and attention networks, regression analyses were conducted between response variability and brain activation networks.

RESULTS

Patients showed significantly reduced brain activation in left and right superior temporal lobes, basal ganglia, and posterior cingulate during the oddball versus standard contrast. The activation differences in superior temporal lobes correlated inversely with response variability in control subjects but not in patients with ADHD.

CONCLUSIONS

Brain abnormalities in patients with ADHD are not confined to fronto-striatal networks mediating executive functions but are also observed in temporo-striatal and cingulate regions during perceptive visual attention processes. Furthermore, temporal lobe dysfunction in the context of perceptual attention might be related to their behavioral problems with response variability.

Time perception deficit in children with ADHD

Time perception deficit has been demonstrated in children with attention deficit hyperactivity disorder (ADHD) by using time production and time reproduction tasks. The impact of motor demand, however, has not yet been fully examined. The current study, which is reported herein, aimed to investigate the pure time perception of Chinese children with ADHD by using a duration discrimination task. A battery of tests that were specifically designed to measure time perception and other related abilities, such as inhibition, attention, and working memory, was administered to 40 children with ADHD and to 40 demographically matched healthy children. A repeated measure MANOVA indicated that children with ADHD showed significantly higher discrimination thresholds than did healthy controls, and there was an interaction effect between group and duration. Pairwise comparison indicated that children with ADHD were less accurate in discriminating duration at either target duration. Working memory (Corsi blocks task) was related to the discrimination threshold at a duration of 800 ms after controlling for full-scale IQ (FIQ) in the ADHD group, but this did not survive the Bonferroni correction. The results indicated that children with ADHD may have perceptual deficits in time discrimination. They needed a greater difference between the comparison and target intervals to discriminate the short, median, and long durations reliably. This study provides further support for the existence of a generic time perception deficit, which is probably due to the involvement of a dysfunctional fronto-striato-cerebellar network in this capacity, especially the presence of deficits in basic internal timing mechanisms.

Conduct Problems in Adolescence: Three Domains of Inhibition and Effect of Gender

Behavioral and neuropsychological evidence has often shown that children with conduct disorder drawn from clinical populations show problems with impulsivity and are impaired in motor tasks of inhibitory control. We explored the relation between conduct problems and inhibitory deficits in a community sample. Three domains of inhibition were explored in 54 adolescents with conduct problems compared to 53 age, IQ, sex, school, and ethnicity matched controls in 2 geographical areas. Motor response inhibition was assessed using the Stop task, verbal inhibition using the Hayling Sentence Completion test, and cognitive interference inhibition using the Stroop task. Adolescents with conduct problems showed deficits in executive and inhibitory processes of the motor response inhibition task, but were unimpaired in cognitive or verbal inhibitory control. No sex differences were observed and impairments were independent of IQ or geographical location. Findings suggest that motor deficits, rather than higher cognitive forms of inhibitory control, are a specific deficit in children with conduct problems, that girls are as affected as boys, and that motor impulsiveness is a dimensional trait associated with conduct problems, as it is also observed in the community.

Response variability in Attention-Deficit/Hyperactivity Disorder: a neuronal and glial energetics hypothesis

BACKGROUND

Current concepts of Attention-Deficit/Hyperactivity Disorder (ADHD) emphasize the role of higher-order cognitive functions and reinforcement processes attributed to structural and biochemical anomalies in cortical and limbic neural networks innervated by the monoamines, dopamine, noradrenaline and serotonin. However, these explanations do not account for the ubiquitous findings in ADHD of intra-individual performance variability, particularly on tasks that require continual responses to rapid, externally-paced stimuli. Nor do they consider attention as a temporal process dependent upon a continuous energy supply for efficient and consistent function. A consideration of this feature of intra-individual response variability, which is not unique to ADHD but is also found in other disorders, leads to a new perspective on the causes and potential remedies of specific aspects of ADHD.

THE HYPOTHESIS

We propose that in ADHD, astrocyte function is insufficient, particularly in terms of its formation and supply of lactate. This insufficiency has implications both for performance and development: H1) In rapidly firing neurons there is deficient ATP production, slow restoration of ionic gradients across neuronal membranes and delayed neuronal firing; H2) In oligodendrocytes insufficient lactate supply impairs fatty acid synthesis and myelination of axons during development. These effects occur over vastly different time scales: those due to deficient ATP (H1) occur over milliseconds, whereas those due to deficient myelination (H2) occur over months and years. Collectively the neural outcomes of impaired astrocytic release of lactate manifest behaviourally as inefficient and inconsistent performance (variable response times across the lifespan, especially during activities that require sustained speeded responses and complex information processing).

TESTING THE HYPOTHESIS

Multi-level and multi-method approaches are required. These include: 1) Use of dynamic strategies to evaluate cognitive performance under conditions that vary in duration, complexity, speed, and reinforcement; 2) Use of sensitive neuroimaging techniques such as diffusion tensor imaging, magnetic resonance spectroscopy, electroencephalography or magnetoencephalopathy to quantify developmental changes in myelination in ADHD as a potential basis for the delayed maturation of brain function and coordination, and 3) Investigation of the prevalence of genetic markers for factors that regulate energy metabolism (lactate, glutamate, glucose transporters, glycogen synthase, glycogen phosphorylase, glycolytic enzymes), release of glutamate from synaptic terminals and glutamate-stimulated lactate production (SNAP25, glutamate receptors, adenosine receptors, neurexins, intracellular Ca2+), as well as astrocyte function (alpha1, alpha2 and beta-adrenoceptors, dopamine D1 receptors) and myelin synthesis (lactate transporter, Lingo-1, Quaking homolog, leukemia inhibitory factor, and Transferrin).

IMPLICATIONS OF THE HYPOTHESIS

The hypothesis extends existing theories of ADHD by proposing a physiological basis for specific aspects of the ADHD phenotype - namely frequent, transient and impairing fluctuations in functioning, particularly during performance of speeded, effortful tasks. The immediate effects of deficient ATP production and slow restoration of ionic gradients across membranes of rapidly firing neurons have implications for daily functioning: For individuals with ADHD, performance efficacy would be enhanced if repetitive and lengthy effortful tasks were segmented to reduce concurrent demands for speed and accuracy of response (introduction of breaks into lengthy/effortful activities such as examinations, motorway driving, assembly-line production). Also, variations in task or modality and the use of self- rather than system-paced schedules would be helpful. This would enable energetic demands to be distributed to alternate neural resources, and energy reserves to be re-established. Longer-term effects may manifest as reduction in regional brain volumes since brain areas with the highest energy demand will be most affected by a restricted energy supply and may be reduced in size. Novel forms of therapeutic agent and delivery system could be based on factors that regulate energy production and myelin synthesis. Since the phenomena and our proposed basis for it are not unique to ADHD but also manifests in other disorders, the implications of our hypotheses may be relevant to understanding and remediating these other conditions as well.

Event rate and event-related potentials in ADHD

BACKGROUND

It has been repeatedly found that performance of children with attention deficit hyperactivity disorder (ADHD) is more impaired when a long inter-stimulus interval (ISI) is used than when a short ISI is used. According to the cognitive-energetic model, this may reflect difficulty in remaining in an optimal motor activation state because of insufficient effort allocation.

METHOD

Event-related potentials (ERPs) were evaluated during a Go/No-Go task that incorporates a condition with a fast and a slow presentation rate.

RESULTS

ADHD, whether or not comorbid with oppositional defiant/conduct disorder (ODD/CD), was associated with a steeper increase in reaction time (RT) from the fast to the slow condition accompanied by a missing increment of the parietal P3. Speed of responding was found to be correlated with P3 amplitude. In the fast condition, children with ADHD made more errors of commission, accompanied by a smaller No-Go N2, a component thought to be related to inhibition; however, after controlling for ODD/CD these differences disappeared.

CONCLUSIONS

The association between the steeper increase in RT and reduced parietal P3s may indicate that the children with ADHD did not allocate enough extra effort to adjust to a potentially under-activated state. However, the event rate effects could not account for all of the differences between groups and also early automatic information processing stages seem disturbed in this disorder as indexed by larger P2 amplitudes. Alternative explanations are discussed.

ADHD and DCD: A relationship in need of research

Although the connection between attention deficit hyperactivity disorder (ADHD) and developmental coordination disorder (DCD) has been recognized for several decades, little research has examined the relationship between these two disorders. This paper draws attention to the contribution the cognitive-energetic model (CEM) can make in determining the specific nature of these two disorders. An information processing approach such as the CEM is a useful model to identify specific and overlapping mechanisms that are disrupted in these two disorders. This paper describes the CEM and reviews the research so-far in relation to the model. The CEM has several advantages over other models: First, it predicts which task variables will be independent or may interact, thus enabling the investigator to determine the success/failure of the task manipulation(s) and identify spurious findings. Second, the CEM links energetic factors to task variables and predicts both additive and interactive effects. Third, the CEM enables executive processes such as selective attention, working memory and inhibition to be related to both energetic and lower level processes. Fourth, by employing the CEM in clinical research, taxonomy of deficits can be derived enabling further insight into the nature of the disorders and their specific neuropsychological dysfunction. Suggestions are made for future research into the neuropsychological deficits associated with ADHD and DCD.

Temporal information processing in ADHD: Findings to date and new methods

The ability to perceive and represent time is a fundamental but complex cognitive skill that allows us to perceive and organize sequences of events and actions, and to anticipate or predict when future events will occur. It is a multidimensional construct, and a variety of methods have been used to understand timing performance in ADHD samples, which makes it difficult to integrate findings across studies. While further replication is needed, growing evidence links ADHD to problems in several aspects of temporal information processing, including duration discrimination, duration reproduction, and finger tapping. Neuroimaging studies of ADHD have also implicated cerebellar, basal ganglia, and prefrontal regions of the brain, which are believed to subserve temporal information processing. This line of research implicates more basic cognitive mechanisms than previously linked with ADHD and challenges researchers to develop and utilize innovative, multidisciplinary, scientific methods to dissect the various components of temporal information processing. Recent advances in neuroimaging, such as magnetoencephalography in collaboration with structural magnetic resonance imaging, can discriminate temporal processing at the level of a millisecond. This approach can lay the groundwork to provide a more precise understanding of neural network activity during different aspects and stages of temporal information processing in ADHD.

Lack of prefrontal repetitive transcranial magnetic stimulation effects in time production processing

The aim of the present study was to determine the effects of high frequency repetitive transcranial magnetic stimulation (rTMS) over different neuroanatomical areas [left and right doroslateral prefrontal cortex (DLPFC) and right cerebellar hemisphere] on time production task. The study was performed in 16 healthy right-handed men with a cross-over, within subject repeated measures design. There were four rTMS conditions: baseline without stimulation, high frequency rTMS over right, left DLPFC and over right cerebellum. The volunteers were asked to produce a 3-min interval by internal counting. The rTMS was applied during the task. No significantly differences were observed in absolute error scores in time estimation task with any rTMS condition. This preliminary study does not support the role of the prefrontal lobe in time production processes.

Atypical motor and sensory cortex activation in attention-deficit/hyperactivity disorder: a functional magnetic resonance imaging study of simple sequential finger tapping

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) has been shown to be associated with anomalous motor development, including excessive overflow movements. The neurological basis of these deficits has not been established. Functional magnetic resonance imaging (fMRI) was used to determine whether differences in brain activation during sequential finger tapping are present in children with ADHD compared with typically developing control subjects.

METHODS

Twenty-two right-handed children between 8 and 12 years old, 11 with ADHD and 11 typically developing control subjects closely matched for age and gender, performed self-paced sequential finger tapping during fMRI acquisition.

RESULTS

There were no significant between-group differences in speed of sequential finger tapping. The between-group whole-brain comparison showed greater magnitude of activation for control subjects than children with ADHD in the right superior parietal lobe during both right-handed and left-handed finger tapping. The region-of-interest analysis within Brodmann Area 4 revealed that children with ADHD showed a significantly smaller extent of fMRI activation in the primary motor cortex contralateral to the finger-sequencing hand.

CONCLUSIONS

Despite similar speed of sequential finger tapping, children with ADHD showed decreased contralateral motor cortex and right parietal cortex activation during both right-handed finger sequencing (RHFS) and left-handed finger sequencing (LHFS). The fMRI findings suggest that children with ADHD have anomalous development of cortical systems necessary for execution of patterned movements.

Age and medication effects on rhythmic responses in ADHD: Possible oscillatory mechanisms?

Voluntary motor responses in children with attention-deficit hyperactivity disorder (ADHD) may reflect underlying neuronal oscillatory mechanisms. The aims of this study were: (1) to corroborate the presence of rhythmic motor abnormalities on tapping test in children with attention-deficit hyperactivity disorder, shown in previous studies; (2) to delineate the characteristics of ADHD children demonstrating these rhythmic abnormalities; (3) to assess the impact of methylphenidate (Ritalin) on this abnormal motor phenomenon. The study was designed in a double blind manner. Sixty-four ADHD children aged 6-12 years and 60 matched controls underwent a finger tapping test (1-5 Hz). We measured the abnormal rhythmic tapping with and without methylphenidate treatment in a double blind design. Conventional statistical analysis was used to assess the correlation of the presence of motor abnormality with various clinical characteristics (such as degree of hyperactivity/impulsivity). Fifty-six percent of children with ADHD compared to 8% of controls demonstrated abnormal rhythmic responses at a mean frequency of 3.1+/-0.9 Hz. These erroneous voluntary movements were more common in children with a greater degree of hyperactivity, in younger children but not significantly altered by methylphenidate treatment. Surprisingly, abnormal rhythmic responses showed less variability than found for responses for which the tapping was adequately paced, suggesting different motor control processes for normal and abnormal paced tapping. We speculate that rhythmic tapping responses reflect abnormal oscillatory mechanisms involved in ADHD, exacerbated in younger children with ADHD and in those in whom the hyperactivity/impulsivity is more pronounced.

Tapping and anticipation performance in attention deficit hyperactivity disorder

The objective of this study was to examine the precise timing of a motor response in a sample of adolescents with ADHD and comparison participants. 46 participants with ADHD (M age=15.6, SD=1.4; 40 boys) and 44 control participants (M age=15.3, SD=1.4; 40 boys) were recruited through a metropolitan hospital. Participants were administered a tapping task and an anticipation task. Adolescents with ADHD displayed significantly more intra-individual variability on the visual 1000-msec. frequency interval of the tapping task and displayed lower accuracy on the cued and uncued trials of the anticipation task than comparison participants. Intra-individual variability on the tapping task was correlated with intra-individual variability on the anticipation task within both the ADHD and control groups. These findings suggest that adolescents with ADHD have impairments in both the cognitive representation and motor production of the precise timing of a motor response.

Neurologic examination abnormalities in children with bipolar disorder or attention-deficit/hyperactivity disorder

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) and bipolar disorder (BPD) are frequently comorbid and overlapping diagnoses. To move beyond diagnosis toward unique pathophysiology, we evaluated both ADHD and BPD children for neurologic examination abnormalities (NEAs) in comparison with normal control (NC) children.

METHODS

We performed the Revised Physical and Neurological Examination for Soft Signs in three groups (ADHD, BPD, NC). Then, a rater blind to diagnosis evaluated their motor performance. Results were analyzed with a multiple analysis of covariance.

RESULTS

Subjects with ADHD were impaired on repetitive task reaction time. In contrast, pediatric BPD subjects, both with and without comorbid ADHD, were impaired on sequential task reaction time.

CONCLUSIONS

This differential pattern of NEAs by diagnosis suggests pathophysiologic differences between ADHD and BPD in children. Repetitive motor performance requires inhibition of nonrelevant movements; ADHD subjects' impairment in this domain supports the hypothesis that ADHD involves a core deficit of fronto-striato-basal ganglia neurocircuitry. In contrast, BPD subjects' impaired sequential motor performance is consistent with behavioral data showing impaired attentional set-shifting and reversal learning in BPD subjects. Further study, going beyond symptom description to determine pathophysiologic differences, is required to refine neuronal models of these often comorbid diagnoses.

Time Perception: Modality and Duration Effects in Attention-Deficit/Hyperactivity Disorder (ADHD)

Time perception performance was systematically investigated in adolescents with and without attention-deficit/hyperactivity disorder (ADHD). Specifically, the effects of manipulating modality (auditory and visual) and length of duration (200 and 1000 ms) were examined. Forty-six adolescents with ADHD and 44 controls were administered four duration discrimination tasks and two control tasks, and a set of standardized measures. Participants with ADHD had higher thresholds than controls on all of the duration discrimination tasks, with the largest effect size obtained on the visual 1000 ms duration discrimination task. No group differences were observed on the control tasks. Visual-spatial memory was found to be a significant predictor of visual and auditory duration discrimination at longer intervals (1000 ms) in the ADHD sample, whereas auditory verbal working memory predicted auditory discrimination at longer intervals (1000 ms) in the control sample. These group differences suggest impairments in basic timing mechanisms in ADHD.

Neuropsychological correlates of ADHD symptoms in preschoolers

The authors examined the neuropsychological status of 22 preschoolers at risk for attention-deficit/hyperactivity disorder (ADHD) and 50 matched control children, using measures of nonverbal working memory, perceptual and motor inhibition, and memory for relative time. All tasks included paired control conditions, which allowed for the isolation of discrete executive function constructs. Group differences were evident on several measures of neuropsychological functioning; however, after accounting for nonexecutive abilities, no deficits could be attributed to specific functions targeted by the tasks. Performance on executive measures was not related to objective indices of activity level or ratings of ADHD symptoms. Yet, the fact that at-risk preschoolers were highly symptomatic casts doubt on whether executive function deficits and/or frontostriatal networks contribute etiologically to early behavioral manifestations of ADHD.

Modeling attention-deficit/hyperactivity disorder: a critical appraisal of the cognitive-energetic model

A number of theoretical models of attention-deficit/hyperactivity disorder (ADHD) have emerged in recent years that may be used as systematic guides for clinical research. The cognitive-energetic model (CEM) proposes that the overall efficiency of information processing is determined by the interplay of three levels: computational mechanisms of attention, state factors, and management/executive function (EF). The CEM encompasses both top-down and bottom-up processes and draws attention to the fact that ADHD causes defects at all three levels. These include cognitive mechanisms, such as response output; energetic mechanisms, such as activation and effort; and management/EF deficits. Increasing evidence suggests that inhibition deficits associated with ADHD may, at least in part, be explained in terms of an energetic dysfunction. The activation and effort energetic pools appear most relevant to ADHD, being directly related to response organization; however, further testing of CEM is critically dependent on the development of direct measures of these energetic pools. The CEM is a comprehensive model of ADHD but is not without limitations. In particular, further research is required to define more specifically the relationship between process dysfunction and state dysregulation in ADHD.

Executive functions and methylphenidate response in subtypes of attention-deficit/hyperactivity disorder

BACKGROUND

Oculomotor tasks are a well-established means of studying executive functions and frontal-striatal functioning in both nonhuman primates and humans. Attention-deficit/hyperactivity disorder (ADHD) is thought to implicate frontal-striatal circuitry. We used oculomotor tests to investigate executive functions and methylphenidate response in two subtypes of ADHD.

METHODS

Subjects were boys, aged 11.5-14 years, with ADHD-combined (n = 10), ADHD-inattentive (n = 12), and control subjects (n = 10). Executive functions assessed were motor planning (tapped with predictive saccades), response inhibition (antisaccades), and task switching (saccades-antisaccades mixed).

RESULTS

The ADHD-combined boys were impaired relative to control subjects in motor planning (p < .003) and response inhibition (p < .007) but not in task switching (p > .92). They were also significantly impaired relative to ADHD-inattentive boys, making fewer predictive saccades (p < .03) and having more subjects with antisaccade performance in the impaired range (p < .04). Methylphenidate significantly improved motor planning and response inhibition in both subtypes.

CONCLUSIONS

ADHD-combined but not ADHD-inattentive boys showed impairments on motor planning and response inhibition. These deficits might be mediated by brain structures implicated specifically in the hyperactive/impulsive symptoms. Methylphenidate improved oculomotor performance in both subtypes; thus, it was effective even when initial performance was not impaired.

Motivational effects on motor timing in attention-deficit/hyperactivity disorder

OBJECTIVE

This study was designed to clarify whether poor performance of children with attention-deficit/hyperactivity disorder (ADHD) on motor timing tasks reflects a true deficit in the temporal organization of motor output or is due to a lack of intrinsic motivation.

METHOD

Eighteen children with ADHD (age 8-12) were compared with 18 age- and gender-matched normal controls with respect to timing precision, timing variability, and the frequency of extreme under- and overestimations during a 1-second interval production task. Monetary reward, response cost, and no reward were implemented to manipulate motivation.

RESULTS

Children with ADHD produced significantly more inaccurate and more variable time intervals and exhibited a larger number of extreme over- and underestimations than control children. Although all children performed significantly better when monetary incentives were applied, group differences were not eliminated.

CONCLUSIONS

In this study, no evidence was found for a motivational deficit as an explanation for impaired performance on a time production task in ADHD. Rather, results provide clear support for a generic motor timing deficit, probably due to a dysfunctional frontostriatocerebellar network involved in temporal aspects of motor preparation.

Towards an understanding of unique and shared pathways in the psychopathophysiology of ADHD

Most attention deficit hyperactivity disorder (ADHD) research has compared cases with unaffected controls. This has led to many associations, but uncertainties about their specificity to ADHD in contrast with other disorders. We present a selective review of research, comparing ADHD with other disorders in neuropsychological, neurobiological and genetic correlates. So far, a specific pathophysiological pathway has not been identified. ADHD is probably not specifically associated with executive function deficits. It is possible, but not yet established, that ADHD symptoms may be more specifically associated with motivational abnormalities, motor organization and time perception. Recent findings indicating common genetic liabilities of ADHD and other conditions raise questions about diagnostic boundaries. In future research, the delineation of the pathophysiological mechanisms of ADHD needs to match cognitive, imaging and genetic techniques to the challenge of defining more homogenous clinical groups; multi-site collaborative projects are needed.

Variability in time reproduction: difference in ADHD combined and inattentive subtypes

OBJECTIVE

To examine the relationship between time reproduction, performance variability, and sustained attention deficits in children with attention-deficit/hyperactivity disorder (ADHD) combined (ADHD-C) and inattentive (ADHD-I) subtypes, relative to matched controls.

METHOD

Participants (age range 7.1-14.1 years) performed a time reproduction task. A subset of the ADHD group was also tested on the Sustained Attention to Response Test. Absolute discrepancy, accuracy coefficient, and intraindividual variability scores on the time reproduction task were compared across the three groups (ADHD-C: N = 20; ADHD-I: N = 19; controls: N = 44) and correlated with the Sustained Attention to Response Test.

RESULTS

First, significantly better performance was observed in matched controls than in children with ADHD on the time reproduction task. Second, there was a significant difference between the two ADHD subtypes in the variability of the size of errors made at high time intervals (36-60 seconds). Third, intraindividual performance variability in the direction (over- versus underestimations) of time reproductions correlated with sustained attention performance.

CONCLUSIONS

Children with ADHD varied more in the size and direction of their time reproduction errors than control children. Those with ADHD-C demonstrated more intraindividual variability than did those with ADHD-I in the size of their errors. The data provide support for a relationship between sustained attention and time reproduction. This relationship has previously been inferred from common right-lateralized neural circuitry that is thought to subserve these processes.

EUNETHYDIS -- searching for valid aetiological candidates of Attention-Deficit Hyperactivity Disorder or Hyperkinetic Disorder

BACKGROUND

To step up research in ADHD, exchange of ideas, working together on key theoretical models and cooperative studies are necessary.

OBJECTIVE

To report about a European approach with strong links to the rest of the world.

METHOD

European Network on Hyperkinetic Disorders (Eunethydis) studies of Attention -- Deficit Hyperactivity Disorder (ADHD) or Hyperkinetic Disorder (HKD) is briefly reviewed in the context of the international effort to discover the aetiology of the disorder.

RESULTS

There are promising neurobiological, neurophysiological and neuropsychological candidates to explain the nature of ADHD/HKD.

CONCLUSION

Eunethydis has shown to be a fruitful platform for ADHD research and has good resources for its further development.

Time reproduction in finger tapping tasks by children with attention-deficit hyperactivity disorder and/or dyslexia

AIM

Deficits in timing and sequencing behaviour in children with dyslexia and with attention-deficit hyperactivity disorder have already been identified. However many studies have not controlled for comorbidity between dyslexia and ADHD. This study investigated timing performance of children with either dyslexia or ADHD, or ADHD + dyslexia or unaffected children using a finger tapping paradigm.

METHOD

Four groups of children (ADHD x Dyslexia) with a total of 68 children were compared using a four factorial design with two between-subject factors (ADHD (yes/no), dyslexia (yes/no)) and two within-subject factors, inter-stimulus interval (263, 500, 625, 750, 875 and 1000 ms) and tapping condition (free tapping, synchronous tapping, and unpaced tapping). In addition the complexity of rhythm reproduction pattern (unpaced tapping) was varied (simple/complex).

RESULTS

No significant differences were found either in the ability of the ADHD or the dyslexia groups to sustain a self-chosen free tapping rate or to generate a stable inter-response interval either by synchronising to a signal or in reproducing a given interval without the previous pacing signal. Response averages showed the expected asynchrony and variability. In rhythm pattern reproduction the groups did not differ significantly in their ability to reproduce rhythms. However, a significant two way interaction effect between dyslexia and complexity was apparent indicating that the difference in levels of performance for simple versus complex rhythms was more pronounced for dyslexia than for the two other groups.

CONCLUSION

The results indicate that motor timing ability in the millisecond range below 1000 ms in children with ADHD and/or dyslexia is intact. The performance of the comorbid group was revealed to be similar to the performance of the single disorder groups, but both the dyslexic groups were relatively worse than either the ADHD-only or the unimpaired group at reproducing complex versus simple rhythms.

Time perception deficits in attention-deficit/ hyperactivity disorder and comorbid reading difficulties in child and adolescent samples

BACKGROUND

Our objective was to investigate time perception in Attention-Deficit/Hyperactivity Disorder (ADHD) with and without comorbid reading difficulties (RD) in child and adolescent participants.

METHOD

In study 1, 50 children with ADHD (31 ADHD, 19 ADHD+RD) and age-matched healthy controls (n = 50) completed three psychophysical tasks: duration discrimination (target duration of 400 ms versus a foil duration), frequency discrimination (a control condition to evaluate general perceptual ability), and a duration estimation task using the method of reproduction for intervals of 400 ms, 2000 ms, and 6000 ms. Study 2 used the same tasks with an adolescent sample (35 ADHD, 24 ADHD+RD, 39 controls).

RESULTS

In both studies, children and adolescents with ADHD and ADHD+RD displayed some impairments in duration discrimination and the precision with which they reproduced the intervals on the estimation task, particularly the shorter 400 ms interval. The most severe impairments tended to occur in the comorbid ADHD+RD group. No impairments were found on the frequency discrimination task. ADHD participants also displayed significant intra-individual variability in their performance on the estimation task. Finally, short-term and working memory, estimated full-scale IQ, and teacher report of hyperactivity/impulsivity were found to differentially predict performance on the time perception measures in the adolescent clinical sample.

CONCLUSIONS

Deficits in duration discrimination, duration estimation, and intra-individual performance variability may have cascaded effects on the temporal organisation of behaviour in children and adolescents with ADHD and ADHD+RD.

Motor timing deficits in community and clinical boys with hyperactive behavior: the effect of methylphenidate on motor timing

In a previous paper we showed that community children with hyperactive behavior were more inconsistent than controls in the temporal organization of their motor output. In this study we investigated: (1) various aspects of motor timing processes in 13 clinically diagnosed boys with attention deficit hyperactivity disorder (ADHD) who were compared to 11 community boys with hyperactive behavior and to a control group and (2) the effect of methylphenidate on the motor timing processes in the clinical group with ADHD in a double blind, cross-over, medication-placebo design, including 4 weeks of medication. The clinical group with ADHD, like the community group with hyperactivity, showed greater variability in sensorimotor synchronization and in sensorimotor anticipation relative to controls. The clinical group was also impaired in time perception, which was spared in the community group with hyperactivity. The persistent, but not the acute dose, of methylphenidate reduced the variability of sensorimotor synchronization and anticipation, but had no effect on time perception. This study shows that motor timing functions are impaired in both clinical and community children with hyperactivity. It is the first study to show the effectiveness of persistent administration of methylphenidate on deficits in motor timing in ADHD children and extends the use of methylphenidate from the domain of attentional and inhibitory functions to the domain of executive motor timing.

Interval length and time-use by children with AD/HD: a comparison of four models

Predictions made by 4 competing models of time use by children with AD/HD were tested using a computerized version of the Matching Familiar Figures Test in 2 studies. In Study 1 teacher-identified AD/HD children (N = 10) and non-AD/HD controls (N = 10) completed the task under 3 different trial duration conditions (5, 10, and 15 s). In Study 2 the same task was completed by a group of children with a diagnosis of Hyperkinetic Disorder (N = 12) and controls (N = 12). In both studies AD/HD children performed poorly on trials of both 5- and 15-s duration but as well as controls on the 10-s trials. This quadratic function provided support for the State Regulation Deficit model of time use in AD/HD. The value of tailoring the temporal features of learning contexts to the conceptual style of AD/HD children is discussed.

Evidence for a pure time perception deficit in children with ADHD

BACKGROUND

Deficits have been found previously in children with ADHD on tasks of time reproduction, time production and motor timing, implicating a deficit in temporal processing abilities, which has been interpreted as either secondary or primary to core executive dysfunctions. The aim of this study was to explore further the abilities of hyperactive children in skills of time estimation, using a range of time perception tasks in different temporal domains.

METHOD

Time estimation was tested in a verbal estimation task of 10 seconds. Time reproduction was also acquired for two time intervals of 5 and 12 seconds. A temporal discrimination task aimed to determine the idiosyncratic threshold of minimum time interval (in milliseconds) necessary to distinguish two intervals differing by approximately 300 milliseconds. Twenty-two children diagnosed with ADHD were compared to 22 healthy children, matched for age, handedness and working memory skills.

RESULTS

Children with ADHD were significantly impaired in their time discrimination threshold: on average, time intervals had to be 50 ms longer for the hyperactive children in order to be discriminated when compared with controls. Children with ADHD also responded earlier on a 12-second reproduction task, which however only approached significance after controlling for IQ and short-term memory. No group differences were found for the 5-second time reproduction or verbal time estimation tasks.

CONCLUSIONS

The findings suggest that children with ADHD perform poorly on time reproduction tasks which load heavily on impulsiveness and attentional processes and they also suggest that these children may have a perceptual deficit of time discrimination, which may only be detectable in brief durations which differ by several hundred milliseconds. A temporal perception deficit in the range of milliseconds in ADHD may impact upon other functions such as perceptual language skills and motor timing.

Neuropsychological analyses of impulsiveness in childhood hyperactivity

BACKGROUND

Neuropsychological analyses of impulsiveness are needed to refine assessment of attention-deficit hyperactivity disorder (ADHD).

AIMS

To investigate specific impairments in hyperactive children in a neuropsychological task battery of impulsiveness, the Maudsley Attention and Response Suppression (MARS) task battery, and to identify the neural substrates.

METHOD

Impulsiveness was assessed using different tasks of inhibitory control and time management (MARS) in 55 children with ADHD, other diagnoses and controls. Functional magnetic resonance images were obtained from adolescents with and without ADHD during three of the tasks.

RESULTS

Children with ADHD, but not psychiatric controls, were impaired on tests of response inhibition, but not of motor timing. Reduced right prefrontal activation was observed in hyperactive adolescents during higher level inhibition and delay management, but not during simple sensorimotor coordination.

CONCLUSIONS

Attention-deficit hyperactivity disorder is characterised by specific deficits in tasks of motor response inhibition, but not motor timing, and by dysfunction of frontostriatal brain regions.

Drugs used in the treatment of attention-deficit/hyperactivity disorder affect postsynaptic firing rate and oscillation without preferential dopamine autoreceptor action

BACKGROUND

Current theories propose that low doses of catecholaminergic stimulants reduce symptoms in patients with attention-deficit/hyperactivity disorder by acting on autoreceptors to reduce catecholaminergic transmission; few data are available that directly address this hypothesis.

METHODS

We investigated the autoreceptor and postsynaptic receptor actions of systemically administered stimulants on dopaminergic systems in rats with single-unit recording in the substantia nigra pars compacta and globus pallidus, respectively.

RESULTS

Dose-response curves for rate indicated that the potencies of the indirect-acting agonists methylphenidate and D-amphetamine at dopaminergic autoreceptors were not greater than at postsynaptic receptors; in fact, D-amphetamine was more potent postsynaptically. In addition to effects on firing rate, spectral/wavelet analyses indicated that these drugs had prominent effects on postsynaptic multisecond oscillations. These oscillations were shifted by stimulants from baseline periods of approximately 30 sec to periods of 5-10 sec. Effects on pattern were found at doses as low as 1.0 mg/kg (methylphenidate) and 0.2 mg/kg (D-amphetamine). At this latter dose, D-amphetamine had little effect presynaptically.

CONCLUSIONS

These and prior results demonstrate that there is no autoreceptor-preferring dose range of catecholaminergic stimulants; these drugs at low doses are unlikely to reduce motor activity by this mechanism. Nonetheless, they might affect attentive and cognitive processes by modulating multisecond temporal patterns of central activity.