The vertical optokinetic nystagmus (OKN) of 10 normal subjects and the optokinetic afternystagmus (OKAN) of 3 subjects were measured with the magnetic search coil technique. In order to assess the relative contributions of various retinal areas to the up-down asymmetry in OKN the central and peripheral visual fields were selectively stimulated in four OKN conditions. In the full-field OKN condition the stimulus was a 61 degrees x 64 degrees display of moving random-dots. Overall, full-field OKN gains elicited by upward motion were significantly higher than those elicited by downward motion at stimulus velocities between 30 and 70 degrees/s. In the periphery-only OKN condition a 3 degrees or 6 degrees-wide vertical band occluded the center of the full-field display. Nine of the 10 subjects displayed OKN in this condition. For 6 subjects, the addition of the 6 degrees band to the full field resulted in an increase in the up-down asymmetry at stimulus velocities above 30 degrees/s. For the other three subjects there was a decline in the gains of both upward and downward OKN when the 3 degrees or 6 degrees band was present; the result was directionally symmetric OKN gains. In the central-strip OKN condition only a 6 degrees-wide central vertical strip of moving dots was visible. The gains of central-strip OKN were not significantly different from the full-field responses. A servo controlled centrally-located 10 degrees x 6 degrees moving display was used in the center-only OKN condition. In this condition both upward and downward gains were attenuated and there was no up-down asymmetry. OKAN was measured following a 50-s exposure to either the full-field or center-only OKN display. The stimulus velocity was 30 degrees/s. After viewing the full-field display the 3 subjects displayed OKAN with slow phases upward following upward OKN but there was no downward OKAN following downward OKN. In contrast, there was no consistent directional asymmetry following exposure to the center-only display. The disappearance of the upward preponderance in OKN and OKAN with occlusion of the peripheral retina suggests that the directional asymmetry in vertical OKN exists in the slow OKN system.

No abstract

A 30-deg-high horizontally rotating random-dot display was presented to the central field, and with its more central edge at vertical eccentricities of 0, 2.5, 5, and 10 deg above or below the horizon. Stimulus velocities of 25–100 deg/s and two directions of motion were presented. The mean gain of the slow phases of optokinetic nystagmus (OKN) for five subjects was significantly higher when the stimulus was presented to the lower visual field than when the stimulus was presented to the upper field. This difference was most pronounced when the display was displaced 5 deg from the fovea and moving below 100 deg/s. Our results are consistent with existing psychophysical and physiological evidence for the superiority of the upper retina. In addition, four of the five observors showed significant directional asymmetries.

Optokinetic nystagmus (OKN) is suppressed if attention is directed to a centrally placed afterimage superimposed on a moving display. Imagining a stationary object has little or no effect. An afterimage does not provide the retinal slip and misfoveation error signals provided by a stationary object and we have shown that an effective error signal does not arise from occlusion or masking of the display by the afterimage. Although a lack of relative motion between afterimage and moving display could indicate when OKN gain is one, there is no unique relative motion signal associated with a gain of zero. Subjects could partially inhibit the vestibulo-ocular reflex (VOR) in the dark when they imagined a head-fixed object. They could suppress the response more effectively by attending to an afterimage, but the suppression was still only partial. When OKN and VOR were evoked simultaneously, pursuit movements of the eyes could not be suppressed until the vestibular inputs had subsided. We conclude that signals associated with OKN, are fully available to the mechanism that assesses the headcentric motion of objects but that signals associated with VOR are only partially available to that mechanism.

No abstract