Biomedical subjects

Adam R Clarke

Publications and source records attributed to Adam R Clarke.

At least 37 records · Page 2Linked to original sources

PubMed2005

The effect of methylphenidate on response inhibition and the event-related potential of children with attention deficit/hyperactivity disorder.

Children with Attention Deficit/Hyperactivity Disorder (AD/HD) appear to be deficient in inhibitory processes, as reflected in behavioural and electrophysiological measures. This study examined the effect of methylphenidate (MPH) on response inhibition in children with AD/HD. Event-related potentials (ERPs) and skin conductance level (SCL) were recorded from 18 boys with AD/HD and 18 controls while they performed a cued Go/Nogo task with 70% Go probability. All participants performed the task twice, with an hour interval between test sessions. At the beginning of this interval children with AD/HD took their normal morning dose of MPH. The AD/HD group showed lower SCL than controls pre-medication, a difference not found subsequent to the administration of MPH. While the AD/HD group made more overall errors (omission+commission) pre-medication, and continued to make more omission errors than controls post-medication, the groups became comparable on the number of commission errors, suggesting MPH ameliorates deficits in response inhibition. Children with AD/HD displayed enhanced N1 and P2 amplitudes, and reduced N2 amplitudes relative to controls. These differences were not significant post-medication, at least partly attributable to the action of MPH. This study is unusual in the concurrent examination of electrodermal and electrophysiological measures of medication effects in children with AD/HD, with the retesting of both the AD/HD and control groups allowing a more valid estimate of the effects of medication, rather than assuming that retesting does not have a substantial impact.

Adolescent↗

PubMed2005

Event-related potentials in attention-deficit/hyperactivity disorder of the predominantly inattentive type: an investigation of EEG-defined subtypes.

Research using resting EEG measures suggests that there is a large variability in EEG profiles within the common behaviourally defined DSM-IV subtypes of AD/HD. Within the predominantly inattentive type of AD/HD (AD/HDin), two distinct groups have been identified, based on underlying EEG activity that represents either cortical hypoarousal, or a maturational lag (with EEGs resembling those of younger children). This study investigated whether ERPs from an inter-modal oddball task could differentiate AD/HDin children from controls, and whether the ERPs of the two previously identified subgroups of AD/HDin children with different EEG profiles differ on this task. AD/HDin children (N=54), 27 identified as cortically hypoaroused and 27 as maturationally lagged, and 27 age-matched controls, were presented with an inter-modal oddball task in which the non-target stimulus was a counter-phasing checkerboard and the target was a 2000 Hz auditory tone. Stimuli (20% targets) were presented at a fixed ISI of 1.03 s and participants were required to count all targets. The task successfully differentiated the AD/HDin children from controls, with AD/HDin children having smaller N1, P2 and P3 amplitudes to both the auditory targets and the visual non-targets. These results were interpreted as indicating a generalised stimulus registration, facilitation and processing deficit in AD/HDin. The only difference between the EEG-defined subtypes was a relative increase in left-frontal N1 amplitude in the cortically hypoaroused group. This was interpreted as indicating reduced frontal lateralisation of N1 in the maturational lag group, supporting its labelling, and anomalous lateralisation in the cortical hypoarousal group.

Arousal↗

PubMed2005

EOG correction: a comparison of four methods.

EOG correction is a class of techniques that account for ocular artifact in the electroencephalogram (EEG) by subtracting electrooculographic data from the EEG. The purpose of this study was to evaluate four of these correction techniques (Verleger, Gasser, & Mocks, 1982 [VGM]; Gratton, Coles, & Donchin, 1983 [GCD]; Semlitsch, Presslich, Schuster, & Anderer, 1986 [SPSA]; Croft & Barry, 2000 [CB]). Blinks, vertical eye movements (VEM), and horizontal eye movements (HEM) from 26 subjects were corrected using these techniques, and eye movement event-related potentials computed to aid validation. HEMs were corrected better by CB, VGM/GCD then SPSA, VEMs by CB, VGM/GCD then SPSA, and blinks by CB, SPSA, GCD and then VGM, with the advantage of CB substantial for blinks (eta2>.72), VEMs (eta2>.60), and HEMs (eta2>.27). It is argued that the CB procedure adequately accounts for ocular artifact in the EEG. Reasons for the limitations of the other procedures are discussed.

Blinking↗

PubMed2004

Age and gender effects in EEG coherence: II. Boys with attention deficit/hyperactivity disorder.

OBJECTIVE: This study investigated intra-hemispheric and inter-hemispheric EEG coherences as a function of age in boys with different subtypes of Attention Deficit/Hyperactivity Disorder (AD/HD), in comparison with a control group of normal boys. METHODS: Three groups of 40 children (AD/HD combined type, AD/HD inattentive type, and normal controls) participated. Each group contained 8 males in each 1 year band from 8 to 12 years. EEG was recorded from 21 sites during an eyes-closed resting condition. Wave-shape coherence was calculated for 8 intra-hemispheric electrode pairs (4 in each hemisphere), and 8 inter-hemispheric electrode pairs, within each of the delta, theta, alpha and beta bands. RESULTS: Developmental effects in intra-hemispheric coherences at shorter and longer inter-electrode distances generally supported Thatcher's two-compartment model. Control boys showed evidence of development in longer-range inter-hemispheric coherences which was not apparent in AD/HD boys. Boys with AD/HD of the combined type showed qualitatively different anomalies than boys with AD/HD of the inattentive type. CONCLUSIONS: EEG coherences in normal boys of this age range develop systematically with age in a non-linear fashion. Boys with AD/HD do not show this development. They display coherence anomalies which differ in nature between DSM-IV subtypes, suggesting differences which are not relatable to simple symptom severity. SIGNIFICANCE: The data reported here indicate differences in the development of EEG coherences in boys with AD/HD, and point to differences between the AD/HD subtypes which may help understanding of the disorder.

Age Factors↗

PubMed2004

Suppression of P50 evoked potential component, schizotypal beliefs and smoking.

Suppression of the P50 component of the evoked potential is an electrophysiological index of sensory gating that is blunted in schizophrenia spectrum disorders. Although P50 suppression is thought to be related to symptomatology, this is yet to be shown. The failure to demonstrate this relation has led some to argue that P50 suppression and symptomatology are not related. However, a possible confound has recently been corroborated [i.e., chronic smoking is related to superior P50 suppression [Crawford et al., Neuroscience Letters 317 (2002) 151]], and a relation has been found in questionnaire-defined individuals with indications of schizotypy [i.e., psychometric schizotypy is related to poor P50 suppression [Croft et al., Biological Psychiatry 50 (2001) 441]]. The present study attempted to replicate and extend both studies by examining P50 suppression, smoking histories, psychometric schizotypy and NEO Five-Factor Inventory (NEO-V) scores in 37 healthy participants. Replicating Crawford et al., P50 suppression was better in the heavier smokers. Providing a partial replication of Croft et al., P50 suppression was inversely related to schizotypy scores in participants who smoked little or not at all; however, P50 suppression was positively related to schizotypy in heavier smokers. Covarying for age and NEO-V scale scores had little effect on these relations. The findings provide evidence of important confounds that would limit our ability to detect P50 suppression/symptom relations in schizophrenia.

Adolescent↗

PubMed2004

Age and gender effects in EEG coherence: I. Developmental trends in normal children.

OBJECTIVE: This study investigated intra-hemispheric and inter-hemispheric EEG coherences as a function of age and gender in normal children. METHODS: Two groups of 40 boys and 40 girls, each containing 8 normal children in each 1-year band from 8 to 12 years, participated. EEG was recorded from 21 sites during an eyes-closed resting condition. Wave-shape coherence was calculated for 8 intra-hemispheric electrode pairs (4 in each hemisphere), and 8 inter-hemispheric electrode pairs, within each of the delta, theta, alpha and beta bands. RESULTS: There was consistent evidence of coherences developing across this age range, particularly in long-range intra- and inter-hemispheric electrode pairs. Coherences were larger in the left hemisphere than the right. Coherences also appeared to develop further in males than females, across several regions and frequency bands, suggesting that females lag males in this aspect of brain development. CONCLUSIONS: EEG coherences in normal children of this age range develop systematically with age. These developmental effects vary substantially with gender, brain region and frequency bands. SIGNIFICANCE: The data reported here provide basic norms for coherence development in different brain regions in normal children.

Aging↗

PubMed2004

Event-related potentials in the auditory oddball as a function of EEG alpha phase at stimulus onset.

OBJECTIVE: We aimed to examine the relation between the phase of electroencephalogram (EEG) alpha activity at stimulus onset and event-related potentials (ERPs) in a fixed-inter-stimulus interval auditory 'oddball' task, using a novel conceptualisation of orthogonal phase effects (cortical negativity versus positivity, negative driving versus positive driving, waxing versus waning). METHODS: EEG responses to button-press targets, from 14 subjects presented with 4 blocks of 150 stimuli (50% target probability), were examined. Pre-stimulus alpha activity (8-13 Hz) at Pz was assessed for each trial by digital filtering of the EEG. The alpha cycle at Pz, starting from a negative-going zero crossing, was used to sort trials into 4 phases, for which ERPs were derived from both the filtered and unfiltered EEG activity at Fz, Cz, and Pz. RESULTS: Preferred brain states in this paradigm were indicated by an 8% greater occurrence of negative driving than positive driving, and a 33% greater occurrence of waxing than waning phases. Negative driving phases were associated with increased N1 latencies and decreased N2 amplitudes. Latencies of N1 and P2 were reduced in waxing phases. These reflected systematic changes in alpha frequency and amplitude at stimulus onset. CONCLUSIONS: In a fixed-inter-stimulus interval paradigm, component frequencies of the EEG are dynamically adjusted in order to provide brain states at the moment of stimulus presentation which differentially affect the EEG correlates of stimulus processing. SIGNIFICANCE: The results add to our understanding of the genesis of the ERP, indicating the importance of the dynamic interplay between instantaneous EEG activity and stimulus processing reflected in the ERP.

Acoustic Stimulation↗

PubMed2004

EEG differences in children as a function of resting-state arousal level.

OBJECTIVE: This study aimed to explore the basis of a theoretical position which has major impact in the current literature on attention-deficit/hyperactivity disorder (AD/HD) - that the ratio of theta to beta power in the EEG (the theta/beta ratio) represents central nervous system (CNS) arousal. METHODS: Resting state topographic EEG activity was investigated as a function of CNS arousal in normal right-handed boys. Arousal was defined in terms of electrodermal activity, which has a long history in Psychology as a measure of CNS arousal. RESULTS: Relative delta, theta, and beta power, and the theta/beta ratio, failed to differ significantly between age-matched groups which differed markedly in skin conductance level and non-specific electrodermal fluctuations. The high-arousal group showed significantly lower levels of relative alpha power, particularly in posterior and hemispheric regions, with higher alpha frequencies in these areas. CONCLUSIONS: These data fail to support the theoretical linkage between the theta/beta ratio and CNS arousal. Further work is needed to disentangle the different correlates of arousal and task-related activation, particularly in cognitive and attentional-processing terms. SIGNIFICANCE: The outcomes of this study, in addition to clarifying the nature of EEG markers of CNS arousal, have important implications for our understanding of AD/HD, as they require re-evaluation of current models of the disorder.

Analysis of Variance↗

PubMed2004

Factors associated with treatment compliance in young people following an emergency department presentation for deliberate self-harm.

This study investigated factors associated with patient non-compliance with follow-up treatment after a presentation to an Emergency Department (ED) for deliberate self-harm (DSH). 56 patients under 24 years and 20 parents participated in this study. Subjects were interviewed by telephone after they had attended or missed a follow-up appointment following a presentation to an ED for DSH. Convenience of the appointment time and the patients' beliefs about whether counseling would help them were found to differentiate attending and non-attending patients. The attitudes of parents also had a major influence on decisions to attend or not attend an appointment. These results support the use of a therapeutic intervention in the ED, targeting both patients and their parents' attitudes toward counseling.

Journal Article↗

PubMed2003

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.

Acoustic Stimulation↗

PubMed2003

Effects of stimulant medications on the EEG of children with Attention-Deficit/Hyperactivity Disorder Predominantly Inattentive type.

Stimulant medications are the most commonly-used treatments for Attention-Deficit/Hyperactivity Disorder (ADHD) in North America and Australia, although it is still not entirely known how these medications work. This study investigated the effects of stimulant medications on the EEG of children with the Inattentive type of ADHD. An initial 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. Theta/alpha and theta/beta ratios were also calculated. Subjects were placed on a 6-month trial of a stimulant and a second EEG was recorded at the end of the trial. Subjects were included in this study only if they showed a good clinical response during the trial. The unmedicated ADHD group had significantly greater absolute and relative theta, less relative alpha, and higher theta/alpha and theta/beta ratios than the control group. The stimulant medications resulted in a normalisation of the EEG, with changes in the theta, alpha and beta bands being most evident. These results suggest that stimulants act to increase cortical arousal in children with ADHD, normalising their EEG.

Adolescent↗

PubMed2003

Preferred EEG brain states at stimulus onset in a fixed interstimulus interval auditory oddball task, and their effects on ERP components.

Previous work has indicated the importance of ongoing EEG activity in the elicitation of the event-related potential (ERP), supporting the conceptualisation of the ERP in terms of amplification and attenuation of component frequencies in the EEG. We investigated the importance of the phase of narrow-band EEG activity in generating N1 and P2 components in the auditory ERP. An auditory oddball paradigm requiring a button-press response to targets, with fixed interstimulus interval (ISI) and 15% target probability, was utilised. The continuous EEG at Cz was recorded from 16 subjects as the raw data set. Offline digital filtering was used to separate the EEG into 13 narrow bands from 1 to 13 Hz. For each band, the phase at the onset of each non-target stimulus was determined. These were used to sub-average the unfiltered data stream at each of four phases for each of 13 frequencies for each subject. Phase effects were examined in terms of two orthogonal dimensions of electrical brain activity: Cortical negativity and negative driving. Stimulus onset varied as a function of these dimensions in a non-random fashion across frequency, indicating the preferential occurrence of particular phases, interpretable as preferred brain states. Large differential effects were also apparent in N1 and P2 amplitudes. These data indicate important aspects of brain dynamics, suggesting that in a fixed-ISI paradigm the component frequencies of the EEG are dynamically adjusted in order to provide particular brain states at stimulus occurrence to facilitate the brain's processing of the stimulus.

Acoustic Stimulation↗

PubMed2003

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.

Attention Deficit Disorder with Hyperactivity↗

PubMed2003

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.

Attention Deficit Disorder with Hyperactivity↗

PubMed2003

EEG activity in girls with attention-deficit/hyperactivity disorder.

OBJECTIVE: This study investigated the EEG of girls with Attention-deficit/hyperactivity disorder (ADHD). METHODS: Subjects consisted of 100 girls with ADHD between the ages of 8 and 12 years and 40 age- and gender-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. Factor analysis was used to group sites into 3 regions, covering frontal, central and posterior regions. The total ADHD group was compared to the control group as well as the data being subjected to cluster analysis. RESULTS: The ADHD subjects had greater total power, more relative theta, and less relative delta, alpha and beta than controls. Cluster analysis indicated the presence of two distinct EEG clusters of girls with ADHD. These were (a) a large subgroup characterized by increased total power, more relative theta, and less relative delta and beta than control subjects; and (b) a small subgroup with a substantially-increased amount of high amplitude theta activity, with deficiencies in all other bands. CONCLUSIONS: These results indicate that girls with ADHD exhibit abnormalities in their EEGs, but there is far less variance in their EEG profiles than is found in boys with the disorder. The results also suggest that there may be distinct groups of girls with ADHD who are not being referred for clinical treatment. Recommendations are made for further research in this population. This study is significant in that it is the first major study to separately investigate the EEG of girls with ADHD.

Arousal↗

PubMed2003

Effects of stimulant medications on children with attention-deficit/hyperactivity disorder and excessive beta activity in their EEG.

OBJECTIVE: This study investigated the effects of stimulant medications on the Electroencephalography (EEG) of children with the combined type of attention-deficit/hyperactivity disorder (ADHD) together with excessive beta activity in their EEG. METHODS: Twenty ADHD and 20 control subjects participated in this study. EEG was recorded from 21 sites during an eyes-closed resting condition and Fourier transformed to provide estimates for total power and absolute and relative power in the delta, theta, alpha and beta bands, and for the theta/alpha and theta/beta ratios. Subjects were placed on a 6-month trial of a stimulant medication and a second EEG was recorded at the end of the trial. RESULTS: The unmedicated ADHD group had greater absolute and relative beta, less absolute and relative alpha, a higher theta/alpha and a lower theta/beta ratio than the control group. In the frontal regions the ADHD group had an increase in total power, absolute theta, absolute and relative beta and the theta/beta ratio, with greater relative delta, relative theta, and absolute and relative alpha in posterior regions. With medication use, absolute beta activity and frontal total power decreased, although these changes represented a reduction in power, not a normalisation. CONCLUSIONS: These results indicate that ADHD children with excessive beta activity in their EEG are probably not hypoaroused as previously suggested, and that beta activity in these children is probably only associated with the impulsivity and/or hyperactivity aspects of the disorder, but may not necessarily be associated with inattention. SIGNIFICANCE: This study is the first to investigate effects of stimulant medications on the EEG of ADHD children with excessive beta activity in their EEG.

Attention Deficit Disorder with Hyperactivity↗

PubMed2002

Effects of stimulant medications on the EEG of children with attention-deficit/hyperactivity disorder.

RATIONALE: Stimulant medications are the most commonly used treatments for attention deficit/hyperactivity disorder (ADHD) in North America and Australia, although it is still not entirely known how these medications work. OBJECTIVES: This study aimed to investigate the effects of stimulant medications on the EEG of children with the Combined subtype of ADHD. METHOD: An initial 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. Theta/alpha and theta/beta ratios were also calculated. Subjects were placed on a 6-month trial of a stimulant and a second EEG was recorded at the end of the trial. RESULTS: The ADHD group had significantly greater absolute delta and theta, less posterior absolute beta, more relative theta, and less relative alpha than the control group, which is typical of EEG studies of children with ADHD. The use of stimulant medications resulted in normalisation of the EEG, primarily evident in changes in the theta and beta bands. CONCLUSIONS: These results suggest that stimulants act to increase cortical arousal in children with ADHD, normalising their brain activity.

Adolescent↗

PubMed2002

Children with attention-deficit/hyperactivity disorder and comorbid oppositional defiant disorder: an EEG analysis.

This study investigated EEG differences between two groups of children with attention-deficit/hyperactivity disorder combined type (ADHD), with or without comorbid oppositional defiant disorder (ODD), and normal control subjects. Each group consisted of 20 males. All subjects were between the ages of 8 and 12 years, and groups were matched on age. EEG was recorded during an eyes-closed resting condition from 21 monopolar derivations, which were clustered into nine regions for analysis. The EEGs were Fourier transformed to provide absolute and relative power estimates for the delta, theta, alpha and beta bands. Values were also calculated for the theta/alpha and theta/beta ratios. The ADHD groups had more absolute and relative theta than the control group. Regionally, the ADHD groups had less relative alpha and more relative delta in posterior regions, and less relative beta in the frontal regions, than the control group. These differences were also apparent in both ratio measures. Only two significant topographic differences were found between the ADHD groups, with both of these being less deviant from normality in the ADHD+ODD group than the ADHD group. These results indicate that EEG correlates of ADHD are not clouded by the presence of comorbid ODD, which suggests possible applications in clinical practice.

Alpha Rhythm↗