The sensing of rotational and translational optic flow by the primate optokinetic system.

In primates, 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 semi-circular canals and otolith organs, respectively. Two independent visual tracking systems deal with any residual disturbances of gaze (global optic flow) and are manifest in the two components of the optokinetic response: the indirect or delayed component (OKNd) and the direct or early component (OKNe). I 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 residual 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 seems to have evolved much more recently in frontal-eyed animals and, I suggest, acts as a backup to the TVOR--also recently evolved?--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. This sensitivity to absolute depth is also shared by OKNe, consistent with the idea that OKNe 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 and disparity, which I suggest help the system to segregate the object of regard from other elements in the scene. I also suggest that highly complex optic flow patterns (such as those experienced by the moving observer who looks a little off to one side of his direction of heading) are dealt with by a third visual tracking mechanism--the smooth pursuit system--that spatially filters visual motion inputs so as to exclude all but the motion of the object of interest (local optic flow).