FALSE PERCEPTION OF MOTION IN A PATIENT WHO CANNOT COMPENSATE FOR EYE-MOVEMENTS

We are usually unaware of the motion of an image across our retina tha t results from our own movement. For instance, during slow-tracking ey e movements we do not mistake the shift of the image projected onto th e retina for motion of the world around us, but instead perceive a sta ble world. Following early suggestions by von Helmoltz(1), it is commo nly believed that this spatial stability is achieved by subtracting th e retinal motion signal from an internal reference signal, such as a c opy of the movement command (efference copy)(2-4). Object motion is pe rceived only if the two differ. Although this concept is widely accept ed, its anatomical underpinning remains unknown. Here we describe the case of a patient with bilateral extrastriate cortex lesions, sufferin g from false perception of motion due to an inability to take eye move ments into account when faced with self-induced retinal image slip. Th is is indicated by the fact that during smooth-pursuit eye movements, he perceives motion of the stationary world at a velocity that corresp onds to the velocity of his eye movement; that is, he perceives the ra w retinal image slip uncorrected for his own eye movements. We suspect that this deficiency reflects damage of a distinct parieto-occipital region that disentangles self-induced and externally induced visual mo tion by comparing retinal signals with a reference signal encoding eye movements and possibly ego-motion in general.