The ocular following responses elicited by brief unexpected movements of the visual scene were studied in 10 rhesus monkeys. Test patterns were either random dots or sine-wave gratings [spatial frequency (Fs) 0.046-1.06 cycles per degree (c/degree)]. Test stimuli were velocity steps [speed (V) 5-400 degrees/s] of 100-ms duration, applied 50 ms after spontaneous saccades to avoid saccadic intrusions. Eye velocity response profiles were nonmonotonic and idiosyncratic, but consistent and closely time-locked to stimulus onset. Two measures of response amplitude were used: initial peak in eye velocity (ei), and average final eye velocity over the period of 110-140 ms measured from stimulus onset (ef). Using random dot patterns, response latencies were short, e.g., when the criterion for onset was an eye acceleration of 100 degrees/s2, mean latency (+/- SE) for eight monkeys with a 40 degrees/s test ramp was 51.5 +/- 0.6 ms. Using gratings of low spatial frequency (Fs less than 0.5 c/degree), latency was inversely related to, and solely a function of, contrast and temporal frequency, Ft (where Ft = V X Fs). We conclude from the latter that ocular following is triggered by local changes in luminance, and propose a model of the detection mechanism that reproduces all the essential features of these data. Moderate low-pass spatial filtering ("blurring") of the random dot pattern, by interposing a sheet of ground glass between the animal and the scene, progressively increased the response latency and decreased ef, but ei was either little affected or increased. When used with gratings, the ground glass simply reduced the contrast (range: 0.5-0.003), with very similar consequences for ocular following: latency increased and ef decreased, but ei changed little over the first decade of contrast reduction, increased over the second, and began to show attenuation (often pronounced) only at the lowest contrast. We suggest that these anomalous increases in ei with reductions in contrast are secondary to the delay in response onset and might be explained if the motion detectors responsible for triggering ocular following act as a gate for integrated retinal slip inputs to the tracking system proper: the delay in detection causes a buildup in the error signal driving the tracking response. En masse movement of the visual field was not the optimal stimulus for ocular following.(ABSTRACT TRUNCATED AT 400 WORDS)

The vestibulo-ocular reflex functions to prevent head movements from disturbing retinal images by generating compensatory eye movements to offset the head movements. In the monkey--the species mainly under consideration here--this reflex is machine-like and very effective. In the short-term, the VOR operates as an open-loop control system without the benefit of feedback and its performance is fixed and immutable: No matter what pattern of eye-head coordination the animal uses to view external objects, there is a continuing need for the VOR and it continues to operate; however, should the VOR consistently fail to stabilize the retinal images during head turns, it will gradually undergo long-term adaptive gain changes that restore, that stability. This adaptive capability is ultimately dependent upon vision, and a variety of optical devices that disturb the visual input normally associated with lead turns have been used to induce large changes in the reflex. Insofar as the monkey is concerned, all of the available evidence suggests to us that the modifiable elements underlying these long-term adjustments are located in the brainstem vestibular pathways and not, as previously suggested by others, in the floccular lobes of the cerebellum. However, the flocculus does appear to have an important, inductive role in the adaptive process providing at least part of the error signal guiding the long-term adjustments in the brainstem. In our view, the VOR is a particularly well-defined example of a plastic system and promises to be a most useful model for studying the cellular mechanisms underlying memory and learning the central nervous system.

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The ocular-following responses elicited by brief unexpected movements of the visual scene were studied in human subjects. Response latencies varied with the type of stimulus and decreased systematically with increasing stimulus speed but, unlike those of monkeys, were not solely determined by the temporal frequency generated by sine-wave stimuli. Minimum latencies (70-75 ms) were considerably shorter than those reported for other visually driven eye movements. The magnitude of the responses to sine-wave stimuli changed markedly with stimulus speed and only slightly with spatial frequency over the ranges used. When normalized with respect to spatial frequency, all responses shared the same dependence on temporal frequency (band-pass characteristics with a peak at 16 Hz), indicating that temporal frequency, rather than speed per se, was the limiting factor over the entire range examined. This suggests that the underlying motion detectors respond to the local changes in luminance associated with the motion of the scene. Movements of the scene in the immediate wake of a saccadic eye movement were on average twice as effective as movements 600 ms later: post-saccadic enhancement. Less enhancement was seen in the wake of saccade-like shifts of the scene, which themselves elicited weak ocular following, something not seen in the wake of real saccades. We suggest that there are central mechanisms that, on the one hand, prevent the ocular-following system from tracking the visual disturbances created by saccades but, on the other, promote tracking of any subsequent disturbance and thereby help to suppress post-saccadic drift. Partitioning the visual scene into central and peripheral regions revealed that motion in the periphery can exert a weak modulatory influence on ocularfollowing responses resulting from motion at the center. We suggest that this may help the moving observer to stabilize his/her eyes on nearby stationary objects.