Human optokinetic nystagmus in response to moving binocularly disparate stimuli.

Physiological and behavioral evidence shows that the directionally preponderant subcortical control of optokinetic nystagmus (OKN) in lower mammals is supplemented in higher mammals by bidirectional cortical control. It is hypothesized that this cortical control allows higher mammals to cope with the parallactic movement of the scene produced by linear motion of the body. In particular, it is hypothesized that a coupling between OKN and stereopsis allows higher mammals to stabilize the images of objects within the plane of fixation while ignoring motion signals from objects at other distances. According to this hypothesis the gain of the slow phase of OKN should be highest for binocularly fused moving stimuli and attenuated for binocularly disparate displays. The results of Experiment 1 confirmed this prediction although the effects of accommodation were not ruled out completely. In Experiment 2 a display moving in one direction was presented across the central retina at the same time as one moving in the opposite direction was presented in the upper and lower periphery. It was found that subjects do not show OKN in the direction of the peripheral display unless it is binocularly fused and the central display is disparate. In Experiment 3 a stationary display of dots was superimposed on a moving display. It was found that OKN is not inhibited by the stationary display when it has a horizontal disparity and the moving display is fused. Experiment 4 found that horizontal OKN is disrupted by the sudden introduction of a vertical disparity in the stimulus. Since accommodative state was kept constant in the last three experiments, the data show that binocular disparities can help a person to stabilize selectively the image of one moving display while ignoring conflicting motion signals from another display.