Abstract
In seven normal subjects, smooth pursuit eye movements (SPEM), elicited by sinusoidal spot target oscillation, were measured with various backgrounds : the spot target was presented, (1) in darkness (dark), (2) with a stationary textured background (textured), (3) with a moving textured background which was phase-locked to the spot target oscillation at a background/spot velocity ratio of 0.5, 0.75, 1.0 (coupled), 1.25 and 1.5. Spot target oscillation frequencies ranging from 0.2 to 1.6 Hz and peak velocities ranging from 19 to 150 deg/s were employed. Eye movements were measured by a video-based image analyzer (ISCAN system). From the results, compared with the “dark” condition, SPEM gain increased on the “coupled” condition and decreased on the “textured” condition. The influence of background on SPEM gain was significant at higher frequencies and higher peak velocity oscillations, in which the subject failed to continuously keep the target on the fovea because of the low SPEM gain. However, SPEM phase delay were similar for “dark”, “textured” and “coupled” conditions. Since the responses during “coupled” and “textured” conditions are most likely elicited by rapid-rise optokinetic reflex (OKR) interaction, we conclude that the SPEM system is similar to the rapid-rise OKR system. When a phase-locked textured background was presented, “the velocity ratio of 1.0 (coupled)” condition was most prominent in the SPEM gain, i.e. “the velocity ratio of 1.25 and 1.5” conditions never exceeded the “coupled” condition despite the faster background movement. This indicates that optokinetic stimulus (OKS) in the peripheral visual field may be neglected as a driving source of eye movement response when the OKS velocity differed from spot target velocity and only OKS with a velocity equal to the spot target may register in the peripheral visual field.
