DIFFERENCES IN THE VISUAL CONTROL OF PANTOMIMED AND NATURAL GRASPING MOVEMENTS

In a series of experiments, we studied the differences between natural target-directed grasping movements and 'pantomimed' movements directe d towards remembered objects. Although subjects continued to scale the ir hand opening for object size when pantomiming, grip formation and o ther kinematic variables differed significantly from those seen in nor mal target-directed actions. This was true whether the subjects had ju st seen the target object 2 sec before (Experiments 1 and 2) or whethe r the target object was still present and they were simply required to pantomime the grasping movement beside it (Experiment 3). We argued t hat these pantomimed reaches were being driven by stored perceptual in formation about the object, and were not utilizing the normal visuomot or control systems that direct actions in real time. This interpretati on received strong support from observations of a patient with visual form agnosia who was also tested. In an earlier report, we had shown t hat this patient showed anticipatory scaling of her grasp despite her inability to discriminate between objects perceptually on the basis of size. The present study showed, however, that the requirement to reme mber an object even briefly, or to pantomime an action beside it, was enough to completely disrupt her visuomotor scaling (Experiments 2 and 3). That this reflected a Failure of perception rather than imagery o r understanding was supported by the fact that she could convincingly pantomime actions to imagined, familar objects, the sizes of which wer e known to her (Experiment 4). All these results suggest that the mech anisms underlying the formation of perceptual representations of objec ts are quite independent of those mediating on-line visuomotor control .