Directional asymmetry of nystagmus elicitation in humans during step and sinusoidal modes of lateral linear acceleration.

We investigated nystagmus elicitation in 50 normal subjects who were exposed interaurally to linear acceleration with step (rectangular) and sinusoidal modes of oscillation using a linear accelerator. Relatively strong G-loads of 0.3-0.5 G at a 10 m stroke were applied to subjects who looked at a memorized target in darkness, with the head and trunk tightly restrained in the upright sitting position. Horizontal and vertical eye movements were recorded by electrooculography (EOG). Various levels of G-directional preponderance (DP), including completely one-sided, were observed similarly in either stimulus mode, strongly suggesting that directional asymmetry in nystagmus elicitation may be a functional characteristic in the otolith-ocular response, in contrast to the canal-ocular response. The effects of G-load increase were less congruent between the two stimulus modes. In the step-mode oscillation, the desaccaded slow eye position which corresponds to the vestibuloocular reflex (VOR) was saw-toothed in shape as was the stimulus velocity curve, but the baseline often drifted slowly and DP-dependently in the direction opposite to the fast phase of nystagmus. When the slow phase velocity (SPV), a slope of the saw-tooth, was adjusted mathematically for such slow drift, it revealed that the adjusted SPVs were almost symmetrical between rightward and leftward G-directions. These results suggest that DP generation is separate from VOR generation which is primarily symmetrical.