Date of Award


Degree Type


Degree Name

Doctor of Philosophy


This thesis was designed to test the validity of several recent claims concerning the nature of the visuomotor mechanisms involved in the control of human prehension. The first goal of this research was to test the theory that the reaching and grasping components of a prehensile act are generated through two independent visuomotor channels: one which extracts object location information to control the reaching limb, and another which computes intrinsic object characteristics to determine appropriate hand shaping and guide object recognition (Jeannerod, 1984; Jeannerod & Decety, 1990). This proposal received little support. Thus, it was found that the control mechanisms underlying transport of the limb and grip formation were affected by similar task constraints (including the size and location of the target object, the availability of visual feedback, and changes in the distal requirements of the task), and that bilateral parietal lesions interfered with these control mechanisms without compromising object recognition ability. These findings suggested that a single visuomotor channel was involved in the control of prehension.;It was also found that onset of hand closure did not coincide with the appearance of any particular kinematic marker in the reaching component. This argued against Jeannerod's (1984) proposal that coordination between the reach and the grasp is achieved by a center which generates synchronized bursts of activity in two separate visuomotor channels. Moreover, both maximum grip aperture and the timing of the hand's closure and descent onto the target object were found to be affected by the availability of visual feedback, a finding which suggests that grip formation is not controlled by a feedback-independent, central motor program (cf. Jeannerod, 1984). Finally, evidence was presented to suggest that visuomotor programs undergo rapid decay, but that some information about objects and their locations can nonetheless be stored briefly, although this might involve the activation of two distinct working memory systems.;Taken together, the results are consistent with the notion of a hierarchically-organized motor control center, located in the posterior parietal lobe, which is responsible for optimizing performance under a variety of conditions through the coordination of different effector systems and the anticipation of operating constraints.



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