Vestibularly evoked potentials (VESTEPs) of the horizontal semicircular canals under different body positions in...

We recorded vestibularly evoked potentials (VESTEPs) using natural vestibular stimuli simulating the form of a normal head movement ("raised cosine"). Its smooth course was designed to minimize all possible sources of artifacts of mechanical, electrical, and physiological origin. This motion profile was applied to subjects (Ss) sitting upright, thus stimulating the horizontal semicircular canals alone, as well as when they were tilted in different positions in steps of 30 degrees about the interaural y-axis, which had the effect of modulating the otolithic contribution to the horizontal VESTEP. The transient bell-shaped VESTEP recorded in the upright sitting S was more and more superimposed or replaced by a sustained negativity the more the S was tilted toward a horizontal position. Thus, the VESTEP gradually changes from a velocity response in the upright S (canals only) to a position response in the lying S (canals plus otoliths). Differences in the VESTEPs obtained for clockwise (cw) and counterclockwise (ccw) turns in the tilted S were found. The reason for this is not clear at present and requires further experimentation. However, the VESTEPs obtained under conditions involving backward tilts and cw turns corresponded to those obtained with forward tilts and ccw turns. A similar relationship was found for the VESTEPs obtained with backward tilts and ccw turns and those obtained with forward tilts and cw turns. The physiological basis of this is discussed.