Oscillopsia: visual function during motion in the absence of vestibulo-ocular reflex

Objectives-To investigate (1) the effects of loss of vestibular function on spatiotemporal vision and (2) the mechanisms which enable labyrinthine def ective (LD) patients to adapt to oscillopsia. Methods-Visual function and eye movements were assessed in seven normal sub jects and four LD patients with oscillopsia due to absent vestibule-ocular reflex. Temporal vision was assessed by measurement of threshold sensitivit y for detection of a target which moved across a flickering, spatially unif orm background field. Spatial vision was investigated by measurements of th reshold sensitivity for the detection of a target moving across a spatially modulated background in the form of square wave gratings. Velocity discrim ination was assessed with drifting gratings. All measurements were made und er static conditions and during oscillatory movement of either the visual s timulus or the subject (1 Hz, peak velocity 50 degrees/s). Results Temporal responses-Normal subjects and LD patients exhibited simila r responses while static and under body oscillation. Spatial responses-The two groups achieved similar results under static cond itions but body oscillation reduced threshold sensitivities and shifted the spatial response function towards lower spatial frequencies in the LD pati ents only. Similar changes in the spatial responses were seen during oscill ation of the visual stimulus but these occurred in both normal subjects and LD patients. Velocity discrimination-Two LD patients achieved normal velocity discrimina tion but the other two showed abnormal responses to visual stimulus movemen t; one displayed a loss of velocity discrimination during whole body oscill ation, and the other mismatched the velocity of two moving grating stimuli. Conclusions-The changes in the spatial responses are attributed to the pres ence of retinal slip during visual stimulus motion in all subjects or body oscillation in the LD patients. It is concluded that any visual adaptation to oscillopsia achieved by the LD patients does not influence the measured spatial response functions, which arise at an early stage of visual process ing. The abnormal velocity discrimination may relate to the progressive imp rovement in oscillopsia reported by LD patients.