Vestibular Perception following Acute Unilateral Vestibular Lesions.

Little is known about the vestibulo-perceptual (VP) system,
particularly after a unilateral vestibular lesion. We investigated
vestibulo-ocular (VO) and VP function in 25 patients with
vestibular neuritis (VN) acutely (2 days after onset) and after
compensation (recovery phase, 10 weeks). Since the effect of VN on
reflex and perceptual function may differ at threshold and
supra-threshold acceleration levels, we used two stimulus
intensities, acceleration steps of 0.5°/s(2) and velocity steps of
90°/s (acceleration 180°/s(2)). We hypothesised that the vestibular
lesion or the compensatory processes could dissociate VO and VP
function, particularly if the acute vertiginous sensation
interferes with the perceptual tasks. Both in acute and recovery
phases, VO and VP thresholds increased, particularly during
ipsilesional rotations. In signal detection theory this indicates
that signals from the healthy and affected side are still fused,
but result in asymmetric thresholds due to a lesion-induced bias.
The normal pattern whereby VP thresholds are higher than VO
thresholds was preserved, indicating that any 'perceptual noise'
added by the vertigo does not disrupt the cognitive decision-making
processes inherent to the perceptual task. Overall, the parallel
findings in VO and VP thresholds imply little or no additional
cortical processing and suggest that vestibular thresholds
essentially reflect the sensitivity of the fused peripheral
receptors. In contrast, a significant VO-VP dissociation for
supra-threshold stimuli was found. Acutely, time constants and
duration of the VO and VP responses were reduced - asymmetrically
for VO, as expected, but surprisingly symmetrical for perception.
At recovery, VP responses normalised but VO responses remained
shortened and asymmetric. Thus, unlike threshold data,
supra-threshold responses show considerable VO-VP dissociation
indicative of additional, higher-order processing of vestibular
signals. We provide evidence of perceptual processes (ultimately
cortical) participating in vestibular compensation, suppressing
asymmetry acutely in unilateral vestibular lesions.