HUMAN OCULAR RESPONSES TO TRANSLATION OF THE OBSERVER AND OF THE SCENE - DEPENDENCE ON VIEWING DISTANCE

Recent experiments on monkeys have indicated that the eye movements in duced by brief translation of either the observer or the visual scene are a linear function of the inverse of the viewing distance. For the movements of the observer, the room was dark and responses were attrib uted to a translational vestibule-ocular reflex (TVOR) that senses the motion through the otolith organs; for the movements of the scene, wh ich elicit ocular following, the scene was projected and adjusted in s ize and speed so that the retinal stimulation was the same at all dist ances. The shared dependence on viewing distance was consistent with t he hypothesis that the TVOR and ocular following are synergistic and s hare central pathways. The present experiments looked for such depende ncies on viewing distance in human subjects. When briefly accelerated along the interaural axis in the dark, human subjects generated compen satory eye movements that were also a linear function of the inverse o f the viewing distance to a previously fixated target. These responses , which were attributed to the TVOR, were somewhat weaker than those p reviously recorded from monkeys using similar methods. When human subj ects faced a tangent screen onto which patterned images were projected , brief motion of those images evoked ocular following responses that showed statistically significant dependence on viewing distance only w ith low-speed stimuli (10 degrees/s). This dependence was at best weak and in the reverse direction of that seen with the TVOR, i.e., respon ses increased as viewing distance increased. We suggest that in genera ting an internal estimate of viewing distance subjects may have used a confounding cue in the ocular-following paradigm the size of the proj ected scene - which was varied directly with the viewing distance in t hese experiments (in order to preserve the size of the retinal image). When movements of the subject were randomly interleaved with the move ments of the scene - to encourage the expectation of ego-motion - the dependence of ocular following on viewing distance altered significant ly: with higher speed stimuli (40 degrees/s) many responses (63%) now increased significantly as viewing distance decreased, though less vig orously than the TVOR. We suggest that the expectation of motion resul ts in the subject placing greater weight on cues such as vergence and accommodation that provide veridical distance information in our exper imental situation: cue selection is context specific.