Adam R. Clarke

A review of electrophysiology in attention-deficit/hyperactivity disorder: I. Qualitative and quantitative electroencephalography

OBJECTIVE This article reviews the electroencephalography (EEG) literature in relation to attention-deficit/hyperactivity disorder (AD/HD). METHODS The review briefly outlines the history of the disorder, focusing on the changing diagnostic systems which both reflect and constrain research into AD/HD. Both qualitative and quantitative EEG studies are examined, and their results are discussed in relation to various models of AD/HD. Implications of these data for future research and development in AD/HD are considered. RESULTS In terms of resting EEG, elevated relative theta power, and reduced relative alpha and beta, together with elevated theta/alpha and theta/beta ratios, are most reliably associated with AD/HD. Theta/alpha and theta/beta ratios also discriminate diagnostic subgroups of AD/HD. Recent studies of EEG heterogeneity in this disorder indicate the existence of different profiles of cortical anomalies which may cut across diagnostic types. CONCLUSIONS The research to date has identified a substantial number of EEG correlates of AD/HD which hold promise for improving our understanding of the brain dysfunction(s) underlying the disorder. Further work in this field may benefit from a broader conceptual approach, integrating EEG and other measures of brain function.

A review of electrophysiology in attention-deficit/hyperactivity disorder: II. Event-related potentials

OBJECTIVE This article reviews the event-related potential (ERP) literature in relation to attention-deficit/hyperactivity disorder (AD/HD). METHODS ERP studies exploring various aspects of brain functioning in AD/HD are reviewed, ranging from early preparatory processes to a focus on the auditory and visual attention systems, and the frontal inhibition system. Implications of these data for future research and development in AD/HD are considered. RESULTS A complex range of ERP deficits has been associated with the disorder. Differences have been reported in preparatory responses, such as the contingent negative variation. In the auditory modality, AD/HD-related differences are apparent in all components from the auditory brain-stem response to the late slow wave. The most robust of these is the reduced posterior P3 in the auditory oddball task. There are fewer studies of the visual attention system, but similar differences are reported in a range of components. Results suggesting an inhibitory processing deficit have been reported, with recent studies of the frontal inhibitory system indicating problems of inhibitory regulation. CONCLUSIONS The research to date has identified a substantial number of ERP correlates of AD/HD. Together with the robust AD/HD differences apparent in the EEG literature, these data offer potential to improve our understanding of the specific brain dysfunction(s) which result in the disorder. Increased focus on the temporal locus of the information processing deficit(s) underlying the observed range of ERP differences is recommended. Further work in this field may benefit from a broader conceptual approach, integrating EEG and ERP measures of brain function.

Age and sex effects in the EEG: development of the normal child

OBJECTIVES This study investigated age-related changes and sex differences in the EEGs of normal children. METHODS Forty boys and 40 girls, between the ages of 8 and 12 years, participated in this study. The EEG was recorded during an eyes-closed resting condition and Fourier transformed to provide estimates for total power, absolute and relative power in the delta, theta, alpha and beta bands, and for theta/alpha and theta/beta ratios. RESULTS Absolute delta activity decreased with age. Relative delta and theta decreased and alpha and beta increased with increasing age. The theta/alpha and theta/beta ratios decreased with increasing age. All of these indicated a developmental reduction in slow wave activity. Maturational differences were found in the rates of change between the midline and the two hemispheres. In the absolute delta and the theta/beta ratio, the midline and the two hemispheres became more equipotential with age. In the beta band, power increased at a greater rate than in the two hemispheres. Sex differences were found, with males having less theta and more alpha than females. CONCLUSIONS These results indicated that maturation occurs earlier at the midline than in the two hemispheres. Females were also found to have a developmental lag in the EEG compared with males.

EEG analysis in Attention-Deficit/Hyperactivity Disorder: a comparative study of two subtypes

This study investigated differences in the EEG between children with Attention-Deficit/Hyperactivity Disorder of the Combined Type, Attention-Deficit/Hyperactivity Disorder of the Predominantly Inattentive Type and control subjects. All subjects were between the ages of 8 and 12 years, and groups were matched on age and gender. The EEG was recorded during an eyes-closed resting condition from 21 monopolar derivations and these were clustered into nine regions prior to analysis. One minute of trace was analysed using Fourier transformation to obtain both absolute and relative power estimates in the delta, theta, alpha and beta frequency bands. The patient groups were found to have greater levels of theta and deficiencies of alpha and beta in comparison to the control group. Children with Attention-Deficit/Hyperactivity Disorder of the Predominantly Inattentive type were found to be significantly different from those of the Combined type in the same measures, appearing to be closer to the normal profiles. The general results support a maturational lag model of the central nervous system in Attention Deficit/Hyperactivity Disorder. The differences between the subtypes suggest a difference in the severity of the disorder rather than a different neurological dysfunction.

EEG-defined subtypes of children with attention-deficit/hyperactivity disorder

OBJECTIVES This study investigated the presence of EEG clusters within a sample of children with the combined type of attention-deficit/hyperactivity disorder (ADHD). METHODS Subjects consisted of 184 boys with ADHD and 40 age-matched controls. EEG was recorded from 21 sites during an eyes-closed resting condition and Fourier transformed to provide estimates for total power, and relative power in the delta, theta, alpha and beta bands, and for the theta/beta ratio. Factor analysis was used to group sites into 3 regions, covering frontal, central and posterior regions. These data were subjected to cluster analysis. RESULTS Three distinct EEG clusters of children with ADHD were found. These were characterized by (a) increased slow wave activity and deficiencies of fast wave, (b) increased high amplitude theta with deficiencies of beta activity, and (c) an excess beta group. CONCLUSIONS These results indicate that children with ADHD do not constitute a homogenous group in EEG profile terms. This has important implications for studies of the utility of EEG in the diagnosis of ADHD. Efforts aimed at using EEG as a tool to discriminate ADHD children from normals must recognize the variability within the ADHD population if such a tool is to be valid and reliable in clinical practice.

EEG power and coherence in autistic spectrum disorder

OBJECTIVE Autistic spectrum disorder (ASD) has been defined as a neurodevelopmental disorder with associated deficits in executive function, language, emotional, and social function. ASD has been associated with pathophysiology in cerebral organization. The current study investigated quantitative EEG findings in twenty children diagnosed with autistic disorders as compared to 20 controls matched for gender, age and IQ. METHODS The EEG was recorded during an eyes-closed resting condition and topographical differences in cerebral functioning were examined using estimates of absolute, relative, and total power, as well as intrahemispheric and interhemispheric coherences. RESULTS There were group differences in power, intrahemispheric and interhemispheric coherences. Findings included excessive theta, primarily in right posterior regions, in autistics. There was also a pattern of deficient delta over the frontal cortex and excessive midline beta. More significantly, there was a pattern of underconnectivity in autistics compared to controls. This included decreased intrahemispheric delta and theta coherences across short to medium and long inter-electrode distances. Interhemispherically, delta and theta coherences were low across the frontal region. Delta, theta and alpha hypocoherence was also evident over the temporal regions. Lastly, there were low delta, theta and beta coherence measurements across posterior regions. CONCLUSIONS These results suggest dysfunctional integration of frontal and posterior brain regions in autistics along with a pattern of neural underconnectivity. This is consistent with other EEG, MRI and fMRI research suggesting that neural connectivity anomalies are a major deficit leading to autistic symptomatology. SIGNIFICANCE This paper reports the largest integrated study of EEG power and coherence during a resting state in children suffering autism spectrum disorder.

Age and sex effects in the EEG: differences in two subtypes of attention-deficit/hyperactivity disorder

OBJECTIVES This study investigated age-related changes and sex differences in the EEGs of two groups of children with attention-deficit/hyperactivity disorder (ADHD) combined type and ADHD predominantly inattentive type, in comparison with a control group of normal children. METHODS Forty boys and forty girls were included in each group. The EEG was recorded during an eyes-closed resting condition and Fourier transformed to provide estimates for total power, absolute and relative power in the delta, theta, alpha and beta bands, and for theta/alpha and theta/beta ratios. RESULTS Total power, relative alpha, and the theta/alpha and theta/beta ratios were differentiated between all 3 groups. Sex differences between the ADHD subjects and the control group were greater in males than females and matured faster in males. With increasing age, the EEG of the ADHD inattentive group was found to change at a similar rate to the changes found in the normal group, with the differences in power levels remaining constant. In the ADHD combined group, the power was found to change at a greater rate than in the ADHD inattentive group, with power levels of the two ADHD groups becoming similar with age. CONCLUSIONS These results are supportive of a two-component model of ADHD, with the hyperactive/impulsive component maturing with age and the inattentive component remaining more stable.

Electroencephalogram differences in two subtypes of Attention-Deficit/Hyperactivity Disorder

This study investigated EEG differences between children with two subtypes of Attention-Deficit/Hyperactivity Disorder (ADHD) and normal control subjects. EEG was recorded during an eyes-closed resting condition and Fourier transformed to provide absolute and relative power estimates for the delta, theta, alpha, and beta bands, and the mean frequency for each band was calculated. Ratio coefficients were also calculated between frequency bands. Mean group differences were found in the theta, alpha, and beta bands between all three groups. Similarly, differences were found between all three groups for the theta/alpha and theta/beta ratios and for the mean frequency of the total EEG. These results support a model of ADHD resulting from a developmental deviation rather than a maturational lag in the central nervous system. Differences between the clinical groups in frontal activity suggest that different neuroanatomical systems are involved in the different subtypes of ADHD.

Acute mobile phone operation affects neural function in humans

OBJECTIVES Mobile phones (MP) are used extensively and yet little is known about the effects they may have on human physiology. There have been conflicting reports regarding the relation between MP use and the electroencephalogram (EEG). The present study suggests that this conflict may be due to methodological differences such as exposure durations, and tests whether exposure to an active MP affects EEG as a function of time. METHODS Twenty-four subjects participated in a single-blind fully counterbalanced cross-over design, where both resting EEG and phase-locked neural responses to auditory stimuli were measured while a MP was either operating or turned off. RESULTS MP exposure altered resting EEG, decreasing 1-4 Hz activity (right hemisphere sites), and increasing 8-12 Hz activity as a function of exposure duration (midline posterior sites). MP exposure also altered early phase-locked neural responses, attenuating the normal response decrement over time in the 4-8 Hz band, decreasing the response in the 1230 Hz band globally and as a function of time, and increasing midline frontal and lateral posterior responses in the 30-45 Hz band. CONCLUSIONS Active MPs affect neural function in humans and do so as a function of exposure duration. The temporal nature of this effect may contribute to the lack of consistent results reported in the literature.

Inhibitory motor control in children with attention-deficit/hyperactivity disorder: event-related potentials in the stop-signal paradigm

BACKGROUND The aim of the study was to investigate the inhibitory control of an ongoing motor response and to identify underlying neural deficiencies, manifested in event-related potentials, that cause poorer inhibitory performance in children with attention-deficit/hyperactivity disorder. METHODS A stop-signal paradigm with a primary visual task and auditory stop signal was used to compare performance in 13 children with attention-deficit/hyperactivity disorder and 13 control children, while event-related potentials were recorded simultaneously. RESULTS Children with attention-deficit/hyperactivity disorder showed poorer inhibitory performance through a slower inhibitory process. Inhibitory processing of auditory stop signals was marked by a frontal N2 component that was reduced in the attention-deficit/hyperactivity disorder group relative to controls. A central positive component (P3) was associated with the success of inhibiting a response, but there were no group differences in its amplitude or latency. CONCLUSIONS Findings support the hypothesis of deficient inhibitory control in children with attention-deficit/hyperactivity disorder. Slower inhibitory processing appears to be due to a specific neural deficiency that manifests in the processing of the stop signal as attenuated negativity in the N2 latency range.