Ocular compensation for self-motion. Visual mechanisms.

In monkeys, there are several reflexes that generate eye movements to compensate for the observer's own movements. Two vestibuloocular reflexes compensate selectively for rotational (RVOR) and translational (TVOR) disturbances of the head, receiving their inputs from the semicircular canals and otolith organs, respectively. Two independent visual tracking systems that deal with residual disturbances of gaze are manifest in the two components of the optokinetic response: the indirect or delayed component (OKNd) and the direct or early component (OKNe). We hypothesize that OKNd--like the RVOR--is phylogenetically old, being found in all animals with mobile eyes, and that it evolved as a backup to the RVOR to compensate for rotational disturbances of gaze. Indeed, optically induced changes in the gain of the RVOR result in parallel changes in the gain of OKNd, consistent with the idea of shared pathways as well as shared functions. In contrast, OKNe--like the TVOR--seems to have evolved much more recently in frontal-eyed animals and, we suggest, acts as a backup to the TVOR to deal primarily with translational disturbances of gaze. Frontal-eyed animals with good binocular vision must be able to keep both eyes directed at the object of regard irrespective of proximity, and in order to achieve this during translational disturbances, the output of the TVOR is modulated inversely with the viewing distance. OKNe shares this sensitivity to absolute depth, consistent with the idea that it is synergistic with the TVOR and shares some of its central pathways. There is evidence that OKNe is also sensitive to relative depth cues such as motion parallax, which we suggest helps the system to segregate the object of regard from other elements in the scene. However, there are occasions when the global optic flow cannot be resolved into a single vector useful to the oculomotor system (e.g., when the moving observer looks towards the direction of heading). We suggest that on such occasions a third independent tracking mechanism, the smooth pursuit system, is deployed to stabilize gaze on the local feature of interest. In this scheme, the pursuit system has an attentional focusing mechanism that spatially filters the visual motion inputs driving the oculomotor system. The major distinguishing features of the 3 visual tracking mechanisms are summarized in Table 1.