To investigate the effect of asymmetrical vestibular input on the perceived straight-ahead direction, we compared 7 subjects (age
yrs, mean ± SD) who had chronic (
mos) unilateral vestibular deafferentation with 10 age matched controls (age
younger controls (age
yrs). Despite the age difference, the two control groups performed similarly and were therefore pooled. Eye and head movements were recorded using search coils as subjects underwent 30 s trials of sinusoidal, whole body oscillation (0.4–2 Hz, peak velocities
/s) in darkness while attempting to maintain gaze on a remembered target 5 m distant. As a control, most stimulus oscillations were randomly superimposed on an imperceptible, constant velocity of
/s that produced a whole-body offest of
by the end of the trial. Following oscillation, subjects remained motionless in darkness and were asked to orient both gaze and a manipulandum to the remembered target location. In control subjects, mean final gaze and manipulandum positions were within
of the target for all testing conditions. There was no dependence of final gaze and manipulandum positions on the frequency or velocity of the preceding whole-body oscillations (
). In four of seven unilaterally deafferented subjects there was an ipsilesional bias of final eye position of
. These subjects moved both eye and manipulandum to the ipsilesional side, with the error increasing at higher stimulus velocities. For the
/s peak head velocity, mean ipsilesional gaze bias ranged from
and mean manipulandum bias ranged from
. Although the errors depended on velocity
), errors were independent of frequency (
). In the remaining three subjects with vestibular deafferentation, final gaze and manipulandum positions were not statistically different from controls. Early gain (eye velocity / head velocity) of the VOR averaged
for the first 10 s of all trials and was similar in all groups (
). Gain during the final 10 s gain averaged
for control subjects, but was significantly lower at
for unilaterally deafferented subjects, whose eye positions reached the limit of the ocular motor range. We conclude that many humans with chronic unilateral vestibular deafferentation have a large ipsilesional dynamic bias of eye position and the perceived straight ahead direction reflecting persistent asymmetry of vestibular processing.
