Adaptation to oscillopsia: a psychophysical and questionnaire investigation.

In this study we explore the reasons why patients with bilateral vestibular failure report disparate degrees of oscillopsia. Twelve bilateral labyrinthine-defective (LD) subjects and twelve normal healthy controls were tested using a self- versus visual-motion psychophysical experiment. The LD subjects also completed a questionnaire designed to quantify the severity of handicap caused by oscillopsia. Additional standardized questionnaires were completed to identify the role of personality, personal beliefs and affective factors in adaptation to oscillopsia. During the psychophysical experiment subjects sat on a motorized Barany chair whilst viewing a large-field projected video image displayed on a screen in front of them. The chair and video image oscillated sinusoidally at 1 Hz in counter-phase at variable amplitudes which were controlled by the subject but constrained, so that the net relative motion of the chair and video image always resulted in a sinusoid with a peak velocity of 50 degrees /s. The subject's task was to find the ratio of chair versus video image motion that subjectively produced the 'most comfortable visual image'. Eye movements were recorded during the experiment in order that the net retinal image slip at the point of maximum visual comfort could be measured. The main findings in the LD subjects were that, as a group, they selected lower chair motion amplitude settings to obtain visual comfort than did the normal control subjects. Responses to the questionnaires highlighted considerable variation in reported handicap due to oscillopsia. Greater oscillopsia handicap scores were significantly correlated with a greater external locus of control (i.e. the perception of having little control over one's health). Retinal slip speed was negatively correlated with oscillopsia handicap score so that patients who suffered the greatest retinal slip were those least handicapped by oscillopsia. The results suggest that adaptation to oscillopsia is partly related to the patient's personal attitude to the recovery process and partly associated with the development of tolerance to the movement of images on the retina during self-motion. The latter is likely to be related to previously described changes in visual motion sensitivity in these patients.