EFFECT OF OTOLITH DYSFUNCTION - IMPAIRMENT OF VISUAL-ACUITY DURING LINEAR HEAD MOTION IN LABYRINTHINE DEFECTIVE SUBJECTS

Visual symptoms emerging after the loss of vestibular function are usu ally attributed to the dysfunction of semicircular canal vestibulo-ocu lar reflexes, as they have been shown to stabilize vision during angul ar head movements. However natural head displacements involve both ang ular and linear motion, and therefore visual instability may occur bec ause of defective otolith-ocular reflexes (OORs) which are the eye mov ements evoked by linear head acceleration. In this paper the relations hip between OORs and visual acuity during linear head motion was studi ed in normal subjects and 14 patients with bilateral loss of caloric r esponses. OORs were elicited in darkness by step acceleration (0.24 g) of the whole body along the interaural axis. Latency, slow phase velo city and asymmetry of the OOR were measured from the desaccaded and av eraged electrooculographic trace. Visual acuity was assessed during si nusoidal lateral oscillation of the subject viewing an earth-fixed tar get, and vice versa with the subject stationary and the target moving at 0.5, 1.0 and 1.5 Hz. The task was to recognize numbers flashing up on a three digit light-emitting diode visual display. Normal subjects had symmetrical OORs with short latencies (<130 ms). In patients, OORs were either absent (n = 2) or abnormal with asymmetries (n = 8), dimi nished velocities (n = 4) or prolonged latencies (n = 6). At high freq uency oscillation (1.5 Hz), normal subjects invariably recognized more numbers during self-motion compared with target motion, whereas most patients did not. In patients, abnormal dynamic visual acuity was corr elated with absent or delayed OOR responses. This is the first demonst ration of a functional role of the OORs in that they contribute to vis ual stabilization during high frequency linear head motion. Bilateral vestibular failure commonly affects the OORs and thereby compromises d ynamic visual acuity.