Abstract
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°/s2 and velocity steps of 90°/s (acceleration 180°/s2). 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.
Highlights
The vestibulo-ocular reflex (VOR) has been extensively investigated in health and disease [1]
We investigated threshold and supra-threshold vestibulo-ocular (VO) and vestibulo-perceptual (VP) function in 25 patients in the acute and recovery stages of vestibular neuritis (VN)
We report that VO and VP thresholds show similar patterns of response in the acute stage after VN
Summary
The vestibulo-ocular reflex (VOR) has been extensively investigated in health and disease [1]. Little is known about the functional properties of the vestibulo-perceptual (VP) system, following an acute peripheral unilateral vestibular lesion. Their precise distribution remains obscure [2], cortical vestibular networks presumably mediate perception of whole-body motion. As with all psychophysical systems, decisions (‘‘in which direction am I moving?’’) are based on the detection of signals (stimuli), such as semicircular canal afferent information, against a background of activity, or noise. As with other psychophysical systems, the VP system has a defined threshold at which a signal becomes recognised relative to the noise level [4,5]. These higher perceptual thresholds in humans probably reflect the cortical processing of vestibular signals during decision making in direction recognition tasks [7]
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