To our knowledge, this is the first study, which examines the influence of response speed and medication on movement-related potentials in ADHD, separating pre- and post-movement processing. Contralateral focal activation of the premotor and primary motor cortex during motor response programming (iMP’) exhibited a significantly reduced lateralization in children with ADHD compared to healthy controls, especially when reaction times were short. For the motor memory trace (mPINV’) this effect was not found, mPINV’ amplitude was even non-significantly higher (i.e. more negative) in trials with slow responses. Unlike control children, unmedicated children with ADHD showed significantly larger mPINV’ than iMP’ amplitudes. While in healthy controls shorter reaction times were associated with more negative iMP’ amplitudes, this pattern was reversed or at least absent in ADHD subjects. This finding appears plausible as several structural 
and functional findings 
point towards maturation delays in ADHD. In younger children, iMP amplitudes still show a positive polarity, while polarity changes in adolescents 
. Thus the inverse association between reaction time and iMP’ amplitude in children with ADHD compared to healthy control children may point towards the interpretation that reduced iMP’ amplitudes in children with ADHD in our sample may result from a delayed motor cortex maturation. The fact that children with ADHD showed a more negative mPINV’ than iMP’ amplitude fits to this idea, as this pattern is usually found in younger children 
. A delayed maturation would also explain why the results contradicted our initial hypothesis: If in ADHD fast responses are associated with more positive potentials and in healthy control children fast responses are associated with more negative potentials, differences will be largest in fast response trials even though lapses of attention may occur more often in subjects with ADHD. A delayed maturation could also explain why MPH did not normalize movement-related potentials completely.
Short reaction times may be taken as indication of better concentration on the task. Movement kinetics themselves (speed of the movement, muscle force) have been found to be largely independent of lateralized MRP amplitudes 
. Thus, differences in the motor cortex recruitment by higher cortical areas (indicated by differences in response speed 
) may have crucially influenced iMP’ amplitude but could not account for all the diagnosis-related deficits because ADHD children did not present negative iMP’ amplitudes even for fast responses.
Previous studies of lateralised ERP in ADHD support our findings and have shown a reduced contingent negative variation 
and diminished lateralised ERP within a time range compatible to the iMP 
. However, this was not a unanimous finding, as the lateralised readiness potential was not reduced in all studies 
. fMRI research has reported reduced activity of the contralateral motor cortex in ADHD in a finger-tapping task 
. Increased PINV amplitudes in children with ADHD and tic disorder have been described under conditions of loss or lack of control 
. Our study further refines these findings and contributes to separating motor 
and cognitive PINV components 
Methylphenidate tended to normalize prolonged response latencies in ADHD children like in previous studies 
. MPH also tended to normalize lateralized movement-related potentials in agreement with previous literature 
as it led to a significant reduction of the difference between mPINV’ and iMP’ amplitude in children with ADHD. The interaction between COMPONENT and MEDICATION only reached significance level when we controlled for reaction time. The inclusion of reaction time as a covariate did not make a big difference (the p-value of the interaction was p
0.14 without and p
0.03 with reaction time as a covariate). However, even so, the fact that the statistical significance of the COMPONENT x MEDICAT’ION effect depended on the inclusion of reaction time as a covariate could indicate that the interaction effect was caused by either the reaction time or by the medication effect. This confounding effect could not be resolved statistically. A reduction of iMP’ and mPINV’ differences under MPH would be in line with findings in a different sample showing that dopamine affects pre- and post-movement potentials in a different manner, with distraction leading to lower pre- and higher post-movement processing 
. First generation antipsychotics (dopamine antagonists) and Parkinsons’s disease also affect pre- and post-movement potentials in a different way 
. However, even under MPH children with ADHD did not present negative iMP’ amplitudes. Additionally, a reduction of mPINV’ latency under MPH becomes evident in and may point towards medication related changes in response movement kinetics rather than a true mPINV’ amplitude reduction.
4.1 Maturational Changes in the Motor Cortex in ADHD
Taken together, these findings indicate a qualitative difference in focal motor cortex activation in ADHD, which cannot be compensated for by medication or top-down control when only trials with short reaction times are taken into account. Previous studies have repeatedly shown a polarity reveral during childhood 
and explanations for the change in pre-movement MRP polarity during childhood have referred to a substitution of axodendritic by axosomatic synapses in the primary motor cortex 
. The polarity of a surface EEG potential is determined by the depth of the postsynaptic potentials in the cortex: Excitatory activity in superficial (apical dendrites) and inhibitory activity in deeper layers (cell somata) produce surface negativity. Qualitative changes in primary motor cortex seem more likely than increased inhibition of unwanted movements in younger children 
. Younger children show an event-related desynchronization in the alpha band during response preparation like adolescents do 
. Moreover, transcranial magnetic stimulation studies point towards a disinhibition instead of increased inhibition during response preparation also in 6–10 year-old children 
. These facts show that a polarity reversal of movement-related potentials due to maturational changes in the cortex appear more likely than other explanations. There was a largely symmetric time-course at C3 and C4 except for the iMP’ and mPINV time intervals in fast responses of healthy control children (). The potential at C3 during the iMP was more negative compared to C4 in (fast responding) healthy children while children with ADHD show a rather symmetric positivity at C3 and C4. Taken into account that this deviation of C3 from C4 specifically affected the iMP interval right before movement onset (), the most likely explanation is a true negative potential in C3. However, due to the overlap with the P300 complex, we cannot completely exclude contributions from positive potentials at C4.
The identification of lateralized movement-related potentials with their characteristic time course 
, their specific modulation by stimulant medication and the absence of a baseline contamination provide strong support for the assumption that the previously reported diminished P300 
did not contribute to our results.
We would like to emphasize, that ADHD is not a simple maturation delay, as findings about differences which persist into adulthood demonstrate. Some aspects of maturation seem to be delayed in ADHD and may contribute to (though not fully explain) ADHD pathology 
A limitation of our study is the small sample size and that data from a continuous performance test were re-analyzed. The fact that subjects had to be prepared to inhibit their responses in some trials may have influenced our results. Future studies should include a standardized characterization of clinical motor problems in the examined sample and employ a wider range of motor paradigms ranging from freely selected spontaneous movements to pre-programmed movement sequences.
Response speed crucially modulates lateralized MRP amplitudes. Surprisingly, the most pronounced differences between ADHD and healthy control children were found in trials with fast reaction times, i.e. good concentration. The inverse association of response speed and iMP’ amplitude in the control and the ADHD group pointed towards a maturation delay in the motor system of ADHD children in our sample. Stimulant medication tended to normalize response speed, but did not normalize iMP’ amplitudes, giving further support to the hypothesis that the substitution of axodendritic by axosomatic synapses may be delayed in the motor cortex in ADHD children. This hypothesis warrants further investigation.