Short-latency eye movements: evidence for rapid, parallel processing of optic flow

As we go about our daily activities we view the world from a constantly shifting platform and some visual functions are compromised if the images on the retina are not reasonably stable. For example, visual acuity begins to deteriorate when retinal image speeds exceed a few degrees per second (Westheimer & McKee, 1975). There are a number of visual reflexes that help to stabilize our gaze on particular objects of interest by generating eye movements to offset our head movements. However, it is important to remember that these visual mechanisms normally operate in close synergy with vestibuloocular reflexes that rely on two types of end-organ embedded in the base of the skull: the semicircular canals, which are selectively sensitive to angular accelerations of the head, and the otolith organs, which are selectively sensitive to linear accelerations (Goldberg & Fernandez, 1975). Thus, the vestibular end-organs decompose head movements into their angular and linear components and support two quite independent reflexes, the RVOR and TVOR, that compensate selectively for rotational and translational disturbances of the head respectively with latencies <10 msec. These vestibular reflexes operate open-loop—because their output, eye movement, does not influence their input, head movement—and neither is perfect, hence motion of the observer must often be associated with some residual retinal image motion and this is where the visual stabilization mechanisms become involved. However, the visual end-organs — the two retinas — see all visual disturbances, regardless of whether they result from rotation and/or translation of gaze so that if any visual decomposition is to be done it must be by signal processing in the central nervous system (CNS). It is our contention that the visual system does attempt to perform such decomposition, using visual filters to sense the pattern of optic flow and thereby infer the observer’s motion and the eye movements that best compensate for that motion.