Visual-vestibular interaction in humans during earth-horizontal axis rotation.

Visual-vestibular interaction (VVI) using 60 degrees/s constant velocity earth horizontal axis (EHA) yaw rotation was measured in 7 human subjects. This so-called 'barbecue spit' rotation stimulated both the horizontal semicircular canals and the otolith organs. Subjects were tested with their eyes open in the dark (EOD), while fixating upon a target rotating with them (FIX), and while observing stationary optokinetic stripes (VVR). The resulting nystagmus slow component velocity (SCV) was analyzed. During EOD, subjects showed an exponentially decaying SCV response with a time constant of between 10 and 15 s that decayed to a non-zero baseline value (bias). Superimposed was a cyclic activity, modulation, whose period equalled the time for a complete revolution of the subject. During FIX, the average value of SCV was nearly zero indicating almost complete abolition of the exponential decay and bias components. The modulation component was reduced by half. During VVR, an exponential decay was observed in most subjects and the average value of the bias component nearly equalled that of the velocity of rotation. Modulation during VVR varied on a cycle-by-cycle basis. On average, the modulation component was nearly twice that for the EOD condition. We conclude that visual-vestibular interactions during EHA differ significantly from those during rotation about the vertical; specifically, there is a non-linear interaction between linear acceleration and optokinetic nystagmus.