Reaction-time-dependent differences in the initial movement direction of fast goal-directed arm movements

Abstract The relation between the initial movement direction of fast target-directed arm movements and the reaction times of these movements was studied in responses to single- and double-step stimuli. To enlarge the range of reaction times an auditory stimulus could be presented in combination with the visual stimulus. In responses to double-step stimuli, the initial direction of the movement changed gradually as a function of reaction time. Surprisingly, in a number of cases the responses to single-step stimuli also showed an initial movement direction that depended on reaction time. Movements with very short reaction times were not initiated in the direction of the target. The initial movement direction shifted gradually towards the direction of the target location as a function of reaction time. We suggest that some of the subjects move their hand towards a ‘default’ target location when information about the visual target is not yet available.