Brain, Vol 120, Issue 6 1005-1013, Copyright © 1997 by Oxford University Press
T Lempert, CC Gianna, MA Gresty and AM Bronstein
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.
ARTICLES
Effect of otolith dysfunction. Impairment of visual acuity during linear head motion in labyrinthine defective subjects
Medical Research Council Human Movement and Balance Unit, National Hospital for Neurology and Neurosurgery, London, UK.
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