HORIZONTAL OTOLITH-OCULAR RESPONSES IN HUMANS AFTER UNILATERAL VESTIBULAR DEAFFERENTATION

We studied horizontal eye movements evoked by lateral whole body trans lation in nine patients who underwent vestibular nerve section. Preope ratively, all had preserved caloric function on both sides. Testing wa s performed before, 1 week and 6-10 weeks after surgery. Patients were seated upright in an electrically powered car running on a linear tra ck. The car executed acceleration steps of 0.24 g, randomly to the lef t and right in the dark. The normal response consisted of a bidirectio nally symmetrical nystagmus with compensatory slow phases. Response as ymmetry of the slow-phase velocity of the desaccaded and averaged eye position signal was less than 13% in normals (n = 21). Before surgery, patients' responses were mostly symmetrical. Postoperatively, respons es were diminished or absent with head acceleration towards the operat ed ear in all patients, causing a marked asymmetry which averaged 56% after correction for spontaneous nystagmus. On follow-up, responses re gained symmetry. Thus, early after vestibular nerve section, a single utricle produces a normal LVOR only with ipsilateral head translation. Therefore, afferents for the LVOR seem to originate from the mid-late ral area of the macula, where hair cells are stimulated in their on-di rection during ipsilateral head translation. Compensation may depend o n recovery of the off-directional responses from lateral hair cells of the remaining utricle.