There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
In a series of experiments, we studied the differences between natural target-directed
grasping movements and 'pantomimed' movements directed towards remembered objects.
Although subjects continued to scale their hand opening for object size when pantomiming,
grip formation and other kinematic variables differed significantly from those seen
in normal target-directed actions. This was true whether the subjects had just seen
the target object 2 sec before (Experiments 1 and 2) or whether the target object
was still present and they were simply required to pantomime the grasping movement
beside it (Experiment 3). We argued that these pantomimed reaches were being driven
by stored perceptual information about the object, and were not utilizing the normal
visuomotor control systems that direct actions in real time. This interpretation received
strong support from observations of a patient with visual form agnosia who was also
tested. In an earlier report, we had shown that 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 remember
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 or understanding was supported by the fact that
she could convincingly pantomime actions to imagined, familiar objects, the sizes
of which were known to her (Experiment 4). All these results suggest that the mechanisms
underlying the formation of perceptual representations of objects are quite independent
of those mediating on-line visuomotor control.