PERCEPTION OF HORIZONTAL HEAD AND TRUNK ROTATION - MODIFICATION OF NECK INPUT FOLLOWING LOSS OF VESTIBULAR FUNCTION

Chronic loss of vestibular function modifies the role of neck afferent s in human perception of self-motion. We characterized this change by comparing the self-motion perception of patients with chronic vestibul ar loss (Ps) to that of normal subjects (Ns). Stimuli consisted of sin usoidal horizontal rotations (0.025-0.4 Hz) of the trunk relative to t he head (neck stimulation) and/or of the head in space (vestibular sti mulation). Perception of head rotation relative to the trunk, of trunk rotation in space, or of head rotation in space was assessed in terms of gain and phase (veridical perception, G = 1 and psi = 0-degrees) a s well as detection threshold using a pointing procedure. (1) Percepti on of head rotation relative to the trunk (neck proprioception). Ps' d etection threshold of head-to-trunk rotation was normal (i.e. similar to that of Ns) across all frequencies tested. Also, with peak angular velocities above 5-degrees/s, the gain of their perception was approxi mately normal. When peak velocity was decreased below this value, howe ver, either by lowering stimulus frequency with peak displacement kept constant (+/-8-degrees) or by decreasing peak displacement at constan t frequency (0.05 Hz), the gain increased above unity, unlike in Ns. I n contrast, the phase remained normal (approximately 0-degrees). (2) P erception of trunk rotation in space. Ps perceived their trunks as sta tionary during neck stimulation and all vestibular-neck combinations a t medium to low frequencies. At 0.4 Hz, however, Ps consistently perce ived the trunk rotation, conceivably due to somatosensory self-motion cues arising from high body acceleration. In contrast, Ns perceive a t runk-in-space rotation with the neck stimulation and most of the stimu lus combinations across the whole frequency range tested. Ns perceived their trunks as stationary only during head rotation on the stationar y trunk (presumed to reflect a mutual cancellation of neck and vestibu lar signals). (3) Perception of head rotation in space. In Ps, unlike Ns, this perception always resembled that of head rotation relative to the trunk. (4) When Ps were presented with a visual or somatosensory space reference (not motion cues), their perception of trunk and head rotation in space became approximately normal. (5) We suggest that the re are basically two changes in the neck-induced self-motion perceptio n associated with chronic vestibular loss. First, neck proprioception shows a non-linear gain that overemphasizes low stimulus velocities, f or unknown reasons. Second, the neck signal which normally is used for the perception of trunk rotation in space is suppressed (Ps in the da rk, deprived of any space reference, resort to the notion that their t runks are stationary). The change in Ps' perception of head rotation i n space is attributed to the former two changes (assuming that they su perimposed their notion of head-on-trunk rotation on that of a station ary trunk).