In children with attention-deficit/hyperactivity disorder, less task-related up-modulation of motor cortex during response inhibition

Objective

The aim of this study was to identify a quantitative, brain-based measure reflecting impaired response inhibition in children with attention-deficit/hyperactivity disorder (ADHD).

Methods

In this cross-sectional study, we used transcranial magnetic stimulation (TMS) to evoke potentials in hand muscle during both a simple reaction time and a response inhibition task in 8-to-12-year-old children, 41 with ADHD (42% girls, 76% white, mean age 10.3 years) and 38 typically developing controls (53% girls, 74% white, mean age 9.8 years). We used mixed-model linear regressions of evoked potential amplitudes to compare motor cortex excitability at (1) task-onset (“START”: 550 ms prior to action); (2) preparing-to-go (“GO”: 150 ms prior to action); and (3) selecting-to-stop (“STOP”: 150 ms after stop cue). We hypothesized that task-related up-modulation of motor cortex excitability (motor evoked potential amplitudes) would depend both on task (STOP > GO > START) and on diagnosis (controls > patients).

Results

Motor cortex up-modulation was significantly greater for STOP trials than during GO or START. Children with ADHD had both worse response inhibition performance (longer stop-signal reaction times) and significantly less task effect on motor cortex up-modulation. The largest diagnostic difference in motor cortex activation occurred during STOP trials. Reduced up-modulation during stopping was also associated with higher parent-rated symptom severity.

Interpretation

Our findings suggest that motor cortex up-modulation of excitability, assessed indirectly by TMS motor evoked potentials, reflects the cognitive load during response inhibition tasks and may be a quantitative, brain-based indicator of impaired response inhibition in children with ADHD.