Coordination of hand aperture with the spatial path of hand transport

We have investigated the coordination of hand aperture with the spatial path of hand transport in prehensile movement by comparing straight prehensile movements with curved movements, in which subjects had to pass over a "via point" marked on the worksurface before picking up an object in the target location. Spatial plots of hand aperture against hand transport showed that the preshaping of the hand to prepare an appropriate grasp was delayed in the curved movements relative to the straight movements, with most of the preshaping of the hand occurring after passing the via point, even when the via point occurred late in the course of the movement. The postponement of hand preshaping was apparently not due to subjects' segmenting the movement into two completely separate portions preceding and following the via point, since some degree of hand opening often occurred before the via point. We suggest that the delay in hand opening in curved movements involves a scheduling process, which uses information about hand transport to set an appropriate hand aperture.