THE NEURAL PROCESSING OF 3-D VISUAL INFORMATION - EVIDENCE FROM EYE-MOVEMENTS

Primates have several reflexes that generate eye movements to compensa te for bodily movements that would otherwise disturb their gaze and un dermine their ability to process visual information. Two vestibule-ocu lar reflexes compensate selectively for rotational and translational d isturbances of the head, and each has visual backups that operate as n egative feedback tracking mechanisms to deal with any residual disturb ances of gaze. Of particular interest here are three recently discover ed visual tracking mechanisms that specifically address translational disturbances and operate in machine-like fashion with ultra-short late ncies (<60 ms in monkeys, <85 ms in humans). These visual reflexes dea l with motions in all three dimensions and operate as automatic servos , using preattentive parallel processing to provide signals that initi ate eye movements before the observer is even aware that there has bee n a disturbance. This processing is accomplished by visual filters eac h tuned to a different feature of the binocular images located in the immediate vicinity of the plane of fixation. Two of the reflexes use b inocular stereo cues and the third is tuned to particular patterns of optic flow associated with the observer's forward motion. Some stereoa nomalous subjects show tracking deficits that can be attributed to a l ack of just one subtype of cortical cell encoding motion in one partic ular direction in a narrow depth plane centred on fixation. Despite th eir rapid, reflex nature, all three mechanisms rely on cortical proces sing and evidence from monkeys supports the hypothesis that all are me diated by the medial superior temporal (MST) area of cortex. Remarkabl y, MSI seems to represent the first stage in cortical motion processin g at which the visual error signals driving each of the three reflexes are fully elaborated at the level of individual cells.