PERCEPTUAL AND OCULOMOTOR EFFECTS OF NECK MUSCLE VIBRATION IN VESTIBULAR NEURITIS - IPSILATERAL SOMATOSENSORY SUBSTITUTION OF VESTIBULAR FUNCTION

Afferent cervical somatosensory input may substitute for absent vestib ular information as part of central vestibular compensation after unil ateral peripheral vestibular deficit. In order to determine the partic ular contribution of neck muscle spindles to the perception of body or ientation and to the oculomotor system, we measured (i) the subjective visual straight ahead (SVA) by psychophysical rests and (ii) the chan ges in eye position by video-nystagmography during unilateral stimulat ion of the posterior neck muscles by vibration (100 Hz). Twenty-five p atients with subacute unilateral vestibular lesion (vestibular neuriti s) and 25 controls participated in the study. Vibration elicited a hor izontal displacement of SVA towards the side of stimulation in all sub jects. Mean displacement (+/- SD) was 3.28 +/- 2.96 degrees for right- side and 3.45 +/- 2.93 degrees for left-side stimulation in controls. Muscle stimulation on the patients' lesion side induced a significantl y higher displacement (11.51 +/- 6.63 degrees) than contralateral stim ulation (3.04 +/- 2.95 degrees, P < 0.01, paired Student's t test). Th e mean difference during stimulation between the two sides in the pati ents was 8.02 +/- 5.52 degrees; in the controls, however it was only 0 .74 +/- 0.47 degrees (P < 0.001, Student's t test). This asymmetry inc reased gradually in patients over a period of weeks, reaching a maximu m at days 60-80 and declining thereafter: Video-nystagmography reveale d that ipsilateral stimulation in patients induced large horizontal ey e deviations of up to 25 degrees towards the side of the lesion (9.1 /- 7.6 degrees, n = 18). Contralateral stimulation induced only small shifts, which were within the range of controls. The correlation coeff icient between displacement of the SVA and change in eye position was high (r = 0.94, P < 0.0001), indicating that the shift of SVA is the p erceptual correlate of the directional change of gaze in space. This i nterpretation was supported by two control experiments in which the su bject was required to (i) indicate the subjective straight ahead by fi nger-painting with the eyes closed and (ii) adjust SVA when looking th rough horizontally reversing prisms. Vibration of neck muscles caused almost no displacement of the SVA when it was indicated by pointing wi th the eyes closed, but reversed the direction of the displacement if the subject wore reversing prisms. In summary, our data showed: (i) an increase in muscle spindle input following unilateral vestibular lesi on; (ii) this increase is asymmetrical, restricted to the affected sid e, and gradually builds up over weeks; and (iii) the perceived effects during vibration are secondary to changes in eye position rather than changes in cortical representation of body orientation. This is the f irst study to demonstrate a unilateral increase in somatosensory weigh t, which substitutes for missing vestibular input.