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 expose d interaurally to linear acceleration with step (rectangular) and sinusoida l modes of oscillation using a linear accelerator. Relatively strong G-load s of 0.3-0.5 G at a 10 m stroke were applied to subjects who looked at a me morized target in darkness, with the head and trunk tightly restrained in t he upright sitting position. Horizontal and vertical eye movements were rec orded by electrooculography (EOG). Various levels of G-directional preponde rance (DP), including completely one-sided, were observed similarly in eith er stimulus mode, strongly suggesting that directional asymmetry in nystagm us elicitation may be a functional characteristic in the otolith-ocular res ponse, in contrast to the canal-ocular response. The effects of G-load incr ease were less congruent between the two stimulus modes. In the step-mode o scillation, the desaccaded slow eye position which corresponds to the vesti buloocular reflex (VOR) was saw-toothed in shape as was the stimulus veloci ty curve, but the baseline often drifted slowly and DP-dependently in the d irection opposite to the fast phase of nystagmus. When the slow phase veloc ity (SPV), a slope of the saw-tooth, was adjusted mathematically for such s low drift, it revealed that the adjusted SPVs were almost symmetrical betwe en rightward and leftward G-directions. These results suggest that DP gener ation is separate from VOR generation which is primarily symmetrical. (C) 2 001 Elsevier Science Ireland Ltd and the Japan Neuroscience Society. All ri ghts reserved.