Postural invariance in three-dimensional reaching and grasping movements

The question of whether the final arm posture to be reached is determined i n advance during prehension movements remains widely debated. To address th is issue, we designed a psychophysical experiment in which human subjects w ere instructed to reach and grasp, with their right arm, a small sphere pre sented at various locations. In some trials the sphere remained stationary, while in others (the perturbed trials) it suddenly jumped, at movement ons et, to a new unpredictable position. Our data indicate that the final confi guration of the upper limb is highly predictable for a given location of th e sphere. For movements directed at stationary objects, the variability of the final arm posture was very small in relation to the variability allowed by joint redundancy. For movements directed at "jumping" objects, the init ial motor response was quickly amended, allowing an accurate SI-asp. The fi nal ann posture reached at the end of the perturbed trials was neither diff erent from nor more variable than the final arm posture reached at the end of the corresponding stationary trials (i.e. the trials sharing the same fi nal object location). This latter result is not trivial, considering both j oint redundancy and the motor reorganization imposed by the change in spher e location. In contrast to earlier observations, our data cannot be account ed for by biomechanical or functional factors. Indeed, the spherical object used in the present study did not constrain the final arm configuration or the hand trajectory. When considered together, our data support the idea t hat the final posture to be reached is planned in advance and used as a con trol variable by the central nervous system.