Visual-vestibular interactions were assessed for eleven human subjects
during earth-horizontal axis rotation. The apparatus consisted of a r
otating chair and an independently controlled rotating optokinetic sur
round. Subjects underwent ten different test runs where vestibular and
optokinetic stimuli were given independently and in combination. The
resultant nystagmus slow component velocity was analyzed. When vestibu
lar stimuli were given, the typical slow component velocity response c
onsisted of an exponential decay to a non-zero baseline value (bias co
mponent). Superimposed on this was a cyclic modulation of the slow com
ponent velocity whose period was equal to the time required for one co
mplete revolution. Our data indicate that the addition of visual input
to otolith input does not affect the slow component velocity modulati
on component during earth horizontal axis rotation. The average bias c
omponent during otolith stimulation alone was much lower than the stim
ulus velocity. The bias component during optokinetic stimulation produ
ce velocity dependent saturation. Thus, neither input alone was adequa
te to produce a bias component that matched the higher stimulus veloci
ties. In contrast, the average bias component during otolith-visual in
teraction runs produced responses that were nearly equal to the relati
ve target velocity. This occurred despite large individual variability
of the otolith alone and optokinetic response alone. Thus, the brain
compensates to match the target velocity when otolith and visual stimu
li are presented together.