Origin of eye movements induced by high frequency rotation of the head

Perception of stability of the visual world and control of ocular fixation and tracking are altered in subjects submitted to high frequency vibration. Studies of the eye movements induced in man by passively rotating the head sinusoidally around a vertical axis show that beyond 8 Hz, the amplitude of the eye movements increases and reaches 2.5 times the amplitude of head movement at 30 hz. The high amplitude eye oscillation may, at least in part, explain the perception of visual world instability and the decrease of visuo-oculomotor system performance in man submitted to high frequency vibration. Two interpretations of this phenomenon have been proposed (9). High amplitude eye movements induced at high frequency may be due to either a non-linearity of the vestibulo-ocular reflex (VOR) or mechanical resonance oscillations of the orbital apparatus. To test these hypotheses, baboons were trained to fixate visual targets. Each animal's head was rigidly attached to a rotating frame through a block of dental cement bolted to the skull. Head rotation was produced by a servo-controlled vibrator. Rotations in the frequency range 1 to 20 Hz were successively applied with the animal in darkness or fixating a stationary target. The results showed that gain curves obtained with baboons are similar to those obtained with man. Paralysis of the muscles of one eye by injection of lidocaine disclosed a behavioral asymmetry of the two eyes at low frequency. The paralysed eye showed no movement below 8 Hz, while the normal eye behaved as in the normal situation. Beyond 8 Hz, the gain of the treated eye increased gradually so that beyond 12 Hz, the two eyes responded.(ABSTRACT TRUNCATED AT 250 WORDS)