Abstract
Acute unilateral loss of vestibular function (UVL) may cause ocular cyclotorsion, skew deviation, head tilt and leaning of the trunk toward the lesioned side. This response (in part) may result from asymmetry of the vestibulo-ocular and vestibulo-spinal reflexes but could be thought of as a realignment of the body with a tilted perception of the subjective postural vertical (SPV) due to vestibular asymmetry. Subjective perception of static uprightness is normal in UVL, probably because proprioception can readily be used for tilt estimation. Accordingly, we examined SPV when proprioceptive cues were changing rapidly. Subjects were challenged to fly a simulator earth level against discrete imposed roll tilts of 10°-28°. They flew in calm conditions and when perturbed by roll oscillations at 1 Hz, 5° peak simulating turbulence. Normal subjects achieved a mean level flight attitude of 0.5° right SD 0.7° when calm and 0.6° right SD 1.3° when turbulent. Patients with unilateral vestibular nerve section flew tilted slightly to the side of their lesion, mean 2.6° SD 2.1° when calm, p < 0.05, and during turbulence by a mean 7.4° SD 2.6°, p < 0.01. We conclude that vestibular signals are used more to determine SPV when proprioception changes rapidly and that the lateralized tilt observed in UVL patients suggests that a unilateral vestibular lesion, probably of the otolith, causes a bias of the SPV.
