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 usually attributed to the dysfunction of semicircular canal vestibulo-ocular reflexes, as they have been shown to stabilize vision during angular head movements. However, natural head displacements involve both angular and linear motion, and therefore visual instability may occur because of defective otolith-ocular reflexes (OORs) which are the eye movements evoked by linear head acceleration. In this paper, the relationship between OORs and visual acuity during linear head motion was studied in normal subjects and 14 patients with bilateral loss of caloric responses. OORs were elicited in darkness by step acceleration (0.24 g) of the whole body along the interaural axis. Latency, slow phase velocity and asymmetry of the OOR were measured from the desaccaded and averaged electrooculographic trace. Visual acuity was assessed during sinusoidal lateral oscillation of the subject viewing an earth-fixed target, 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), diminished velocities (n = 4) or prolonged latencies (n = 6). At high frequency 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 correlated with absent or delayed OOR responses. This is the first demonstration of a functional role of the OORs in that they contribute to visual stabilization during high frequency linear head motion. Bilateral vestibular failure commonly affects the OORs and thereby compromises dynamic visual acuity.