Ocular Stabilization During Oscillatory Head Movements

EVEN during steady visual fixation of some object of interest, the eye makes continual small movements—called "physiological nystagmus"—which seem to aid in the maintenance of visual contrast.1The fluctuations in image position which are produced by these eye movements are on the order of seconds or minutes of arc; however, visual acuity declines markedly whenever extensive image movements occur,2and vertebrates show a variety of mechanisms which control postural and ocular stability in a manner that tends to minimize such movements during intervals of visual fixation.3,4 The vestibular apparatus plays an important role in this ocular stabilization.3,5The semicircular canals—which are primarily affected by rotational movements of the head—have commonly been viewed as acceleration sensors,6-8and understeady-stateconditions the position of the cupula does depend directly upon angular acceleration.9However, head rotations are generally brief, and therefore analysis in terms of