Coordinated responses following mechanical perturbation of the arm during prehension.

We have investigated how the control of hand transport and of hand aperture are coordinated in prehensile movements by delivering mechanical perturbations to the hand transport component and looking for coordinated adjustments in hand aperture. An electric actuator attached to the subject's right arm randomly pulled the subject backwards, away from the target, or pushed them towards it, during a quarter of the experimental trials. A compensatory adjustment of hand aperture followed the immediate, mechanical effects of the perturbation of hand transport. The adjustment appeared to return the subject towards a stereotyped spatial relation between hand aperture and hand transport. These spatial patterns suggest how the two components may be coordinated during prehension. A simple model of this coordination, based on coupled position feedback systems, is presented.