Abstract
Short interval intracortical inhibition (SICI) of motor cortex, measured by transcranial magnetic stimulation (TMS) in a passive (resting) condition, has been suggested as a neurophysiological marker of hyperactivity in attention-deficit/hyperactivity disorder (ADHD). The aim of this study was to determine motor excitability in a go/nogo task at stages of response preparation, activation and suppression in children with ADHD, depending on the level of hyperactivity and impulsivity. Motor evoked potentials were recorded in 29 typically developing children and 43 children with ADHD (subdivided in two groups with higher and lower levels of hyperactivity/impulsivity; H/I-high and H/I-low). In the H/I-high group, SICI was markedly reduced in the resting condition and during response preparation. Though these children were able to increase SICI when inhibiting a response, SICI was still reduced compared to typically developing children. Interestingly, SICI at rest and during response activation were comparable, which may be associated with their hypermotoric behaviour. In the H/I-low group, response activation was accompanied by a pronounced decrease of SICI, indicating reduced motor control in the context of a fast motor response. In summary, different excitability patterns were obtained for the three groups allowing a better understanding of dysfunctional response activation and inhibition processes within the motor cortex in children with ADHD.
Highlights
Attention-deficit/hyperactivity disorder (ADHD) is one of the most frequently diagnosed child psychiatric disorders with a prevalence rate of about 5% in all school-aged children [1]
Time course analysis in a response inhibition task identified different patterns of motor excitability for children with ADHD and a higher or lower level of hyperactivity and impulsivity compared to typically developing children
In the H/I-low group, response activation was accompanied by a marked decrease of Short interval intracortical inhibition (SICI), which might reflect a reduced motor control due to the subsequent fast motor response
Summary
Attention-deficit/hyperactivity disorder (ADHD) is one of the most frequently diagnosed child psychiatric disorders with a prevalence rate of about 5% in all school-aged children [1]. Response inhibition refers to the interruption and suppression of inappropriate responses in order to adjust flexibly to changing claims and settings [3,4] and was thought to be the core dysfunction in ADHD and the key to a better understanding and interpretation of this disorder. Moderate effect sizes along with studies that found no differences compared to healthy controls [7] suggest that weaknesses in executive functions are neither necessary nor sufficient to explain all cases of ADHD [8] These inconsistent results are not surprising considering other factors, such as the heterogeneity of the disorder and differences in task difficulties [5,6,9]. There is not a one-to-one correspondence between genes and certain aspects of ADHD; specific executive dysfunctions are not necessarily involved
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