Monkeys have two vestibulo-ocular reflexes that deal independently with rotational and translational disturbances of the observer. On the basis of our previous recordings, we have hypothesized that each has its own independent visual backup. One of the critical findings in the evolution of this new concept was that the visual and vestibular ocular responses to translational disturbances of the scene and of the observer, respectively, are both linear functions of the inverse of the viewing distance, presumably reflecting their shared structure and function. We now report that a similar dependence on proximity is also true of human vestibular ocular responses to translation of the observer but not of the visual ocular responses to translation of the scene (called early ocular following or optokinetic responses). Mental set and stimulus context are known to be very important for the perception of linear motion due to translation of the scene (linearvection), which is much more vivid when the subject knows that he/she is in a situation in which linear bodily motion can occur. However, the apparent insensitivity of human ocular following to proximity, as reported here, was not simply due to the improbability of linear motion of the observer: It was still the case when the translational disturbances of the scene were randomly alternated with or even superimposed on translational disturbances of the observer. This and other evidence lead us to suggest that human optokinetic responses lack many of the characteristics of the monkey and are mediated mainly by another tracking mechanism the smooth pursuit system, which in the monkey shows little dependence on viewing distance. During the course of these studies, it was apparent that the early ocular following responses elicited by sudden translational disturbances of the visual scene were much more vigorous in humans when superimposed on an ongoing tracking response. This is consistent with the idea that a fixation mechanism must be disabled for ocular following to proceed with maximum vigor.
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