OBJECT MOTION PERCEPTION DURING EGO-MOTION - PATIENTS WITH A COMPLETELOSS OF VESTIBULAR FUNCTION VS NORMALS

Object motion perception was assessed in avestibular patients and norm al controls. Two experiments were conducted, in which subjects were re quired to assess the motion of a visual stimulus with respect to earth . In the first experiment, we measured the velocity at which a briefly presented (200 ms) grating was perceived as earth fixed, while the su bject maintained fixation on a visual target fixed relative to the bod y, during whole-body yaw rotation (VOR suppression). In this experimen tal setup, the influence of the semicircular canal signals on object m otion perception was evaluated. The avestibular patients judged the gr ating to be stationary with respect to earth, when it was moving at th e same velocity as their body, whereas for normal controls, the gratin g was perceived as stationary when it moved at a velocity slower than their body motion, but greater than zero. The difference between the t wo subject groups was significant, and showed the strong contribution of the vestibular system to object motion perception. Similarly, a mea surement of the velocity at which a grating was perceived as stationar y was obtained during smooth pursuit eye movements. In this experiment the contribution of the efference copy of the oculomotor signal and p roprioceptive signals to object motion perception were assessed. As wi th the first experiment, the normal controls displayed a more veridica l sense of object motion perception than the patients, although the di fference was only just significant. We suggest that the difference cou ld be an adaptive change in the patients perception of motion, which a llows them to reduce the effects of oscillopsia.