Oculocentric frames of reference for limb movement

We have reviewed evidence that suggests that the target for limb motion is encoded in a retinocentric frame of reference. Errors in pointing that are elicited by an illusion that distorts the perceived motion of a target are strongly correlated with errors in gaze position. The modulations in the direction and speed of ocular smooth pursuit and of the hand show remarkable similarities, even though the inertia of the arm is much larger than that of the eye. We have suggested that ocular motion is constrained so that gaze provides an appropriate target signal for the hand. Finally, ocular and manual tracking deficits in patients with cerebellar ataxia are very similar. These deficits are also consistent with the idea that a gaze signal provides the target for hand motion; in some cases limb ataxia would be a consequence of optic ataxia rather than reflecting a deficit in the control of limb motion per se. These results, as well as neurophysiological data summarized here, have led us to revise a hypothesis we have previously put forth to account for the initial stages of sensorimotor transformations underlying targeted limb motions. In the original hypothesis, target location and initial arm posture were ultimately encoded in a common frame of reference tied to somatosensation, i.e. a body-centered frame of reference, and a desired change in posture was derived from the difference between the two. In our new scheme, a movement vector is derived from the difference between variables encoded in a retinocentric frame of reference. Accordingly, gaze, with its exquisite ability to stabilize a target image even under dynamic conditions, would be used as a reference signal. Consequently, this scheme would facilitate the processing of information under conditions in which the body and the target are moving relative to each other.