title: Speed, variability, and timing of motor output in ADHD: which measures are useful for endophenotypic research?
abstract: Attention-Deficit/Hyperactivity Disorder (ADHD) shares a genetic basis with motor coordination problems and probably motor timing problems. In line with this, comparable problems in motor timing should be observed in first degree relatives and might, therefore, form a suitable endophenotypic candidate. This hypothesis was investigated in 238 ADHD-families (545 children) and 147 control-families (271 children). A motor timing task was administered, in which children had to produce a 1,000 ms interval. In addition to this task, two basic motor tasks were administered to examine speed and variability of motor output, when no timing component was required. Results indicated that variability in motor timing is a useful endophenotypic candidate: It was clearly associated with ADHD, it was also present in non-affected siblings, and it correlated within families. Accuracy (under- versus over-production) in motor timing appeared less useful: Even though accuracy was associated with ADHD (probands and affected siblings had a tendency to under-produce the 1,000 ms interval compared to controls), non-affected siblings did not differ from controls and sibling correlations were only marginally significant. Slow and variable motor output without timing component also appears present in ADHD, but not in non-affected siblings, suggesting these deficits not to be related to a familial vulnerability for ADHD. Deficits in motor timing could not be explained by deficits already present in basic motor output without a timing component. This suggests abnormalities in motor timing were predominantly related to deficient motor timing processes and not to general deficient motor functioning. The finding that deficits in motor timing run in ADHD-families suggests this to be a fruitful domain for further exploration in relation to the genetic underpinnings of ADHD.
authors: Department of Clinical Neuropsychology, VU University Amsterdam, Van der Boechorststraat 1, Amsterdam 1081 BT, The Netherlands. nnj.rommelse@psy.vu.nl
authors: Nanda N J
authors: NN
authors: Rommelse
authors: Marieke E
authors: ME
authors: Altink
authors: Jaap
authors: J
authors: Oosterlaan
authors: Leo
authors: L
authors: Beem
authors: Cathelijne J M
authors: CJ
authors: Buschgens
authors: Jan
authors: J
authors: Buitelaar
authors: Joseph A
authors: JA
authors: Sergeant
mesh_terms: D000328:Adult; D001289:Attention Deficit Disorder with Hyperactivity / Q000235:genetics / Q000503:physiopathology*; D002648:Child; D005190:Family; D005191:Family Characteristics; D005260:Female; D007839:Functional Laterality; D006801:Humans; D007360:Intelligence; D007398:Interpersonal Relations; D008297:Male; D009043:Motor Activity; D009426:Netherlands; D010290:Parents; D010641:Phenotype; D012016:Reference Values; D015203:Reproducibility of Results; D035781:Siblings; D013997:Time Factors