A test of motor (not executive) planning in developmental coordination disorder and autism

Grip selection tasks have been used to test ‘planning’ in both autism and developmental coordination disorder (DCD). We differentiate between motor and executive planning and present a modified version of a motor planning task. Participants reached-and-grasped a cylinder in one of two orientations before turning it clockwise or anticlockwise. On half the trials, the turning action only resulted in a comfortable final posture at the cost of making a harder initial reach-to-grasp action; ending comfortably has been taken as the evidence of ‘planning’. We hypothesised that initial grip selection (easier or harder) would be dominated by motoric developmental status. Adults always selected an initial grip that resulted in a comfortable end-state when reaching with their dominant hand, but occasionally ended uncomfortably with their non-dominant hand. Most 9-14 year old children with and without autism also showed this ‘end state comfort’ bias, compared with only half of children aged 5-8 years. In contrast, children with developmental coordination disorder were biased towards selecting the simplest (minimal rotation) initial movement, even at the cost of end state comfort. Our results are best understood in terms of motor planning, with selection of an easier initial grip resulting from poor reach-to-grasp control rather than an executive planning deficit. The absence of differences between children with autism and controls may reflect the low demand this task actually places on executive planning abilities.