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 defective (LD) patients to adapt to oscillopsia. METHODS Visual function and eye movements were assessed in seven normal subjects and four LD patients with oscillopsia due to absent vestibulo-ocular reflex. Temporal vision was assessed by measurement of threshold sensitivity for detection of a target which moved across a flickering, spatially uniform background field. Spatial vision was investigated by measurements of threshold sensitivity for the detection of a target moving across a spatially modulated background in the form of square wave gratings. Velocity discrimination was assessed with drifting gratings. All measurements were made under static conditions and during oscillatory movement of either the visual stimulus or the subject (1 Hz, peak velocity 50 degrees/s). RESULTS Temporal responses--Normal subjects and LD patients exhibited similar responses while static and under body oscillation. Spatial responses--The two groups achieved similar results under static conditions but body oscillation reduced threshold sensitivities and shifted the spatial response function towards lower spatial frequencies in the LD patients only. Similar changes in the spatial responses were seen during oscillation of the visual stimulus but these occurred in both normal subjects and LD patients. Velocity discrimination--Two LD patients achieved normal velocity discrimination but the other two showed abnormal responses to visual stimulus movement; one displayed a loss of velocity discrimination during whole body oscillation, and the other mismatched the velocity of two moving grating stimuli. CONCLUSIONS The changes in the spatial responses are attributed to the presence 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 processing. The abnormal velocity discrimination may relate to the progressive improvement in oscillopsia reported by LD patients.