OCULOMOTOR RESPONSE TO LINEAR ACCELERATION AS INDUCED BY COUNTER-ROTATION IN SUPINE SUBJECTS

Background: Horizontal nystagmus occurs in response to sinusoidal line ar accelerations directed along an upright subject's Y (interaural) ax is, and is proposed to be mediated by an utricular otolith mechanism. Hypothesis: The otolith organs, composed of the utricles and saccules, provide a unique set of signals for any linear acceleration in 3-dime nsional space. A supine subject under alternate changing directions of linear acceleration as induced by counter-rotation will receive alter nate stimulation along the Y as well as the Z (dorsoventral) axis. We hypothesized that alternate horizontal and vertical nystagmus would be elicited as a result oi the changing direction of linear acceleration . Methods: A group oi eight subjects in the supine position were expos ed to counterrotation at 0.16, 0.25, and 0.33 Hz. Vertical and horizon tal eye movements were recorded simultaneously using the El-Mar eye an d head tracking system. Results: Horizontal nystagmus was observed in ail supine subjects. The direction of the slow phase of nystagmus chan ged with directional changes in linear acceleration. Reversals in the direction of eye movements lagged behind the reversals in the directio n of the acceleration. However, only two subjects exhibited alternatin g horizontal and vertical nystagmus as a result of changing axis or li near acceleration, from ''along the Y axis'' to ''along the Z axis.'' Conclusion: We propose that the nystagmus induced in the supine subjec t was provoked by linear acceleration and largely an otolith-mediated reflex. The lack of vertical response could be due to the relative pau city of vestibular afferents information along the dorsoventral axis.