VESTIBULAR PERCEPTION IN SUBJECTS WITH CONGENITAL NYSTAGMUS

Subjects with congenital nystagmus (CN), 18 otherwise healthy and 3 wi th cochleo-vestibular disease, were assessed on a test of 'vestibular perception'. Subjects were seated on a motorised chair and tasked with returning themselves to a starting position, using a joystick control , in response to passive rightwards or leftwards displacements which h ad raised cosine velocity profiles of 30 degrees/s or 60 degrees/s and amplitudes varying from 30 degrees to 180 degrees. On average, normal subjects return to the starting position for displacements up to circ a 150 degrees and are slightly hypometric for larger displacements, un dershooting the start because of the low frequency characteristic of v estibular transduction. Eight of the healthy CN subjects were normal. Six had responses which were mildly hypometric in one direction. Three subjects were hypometric in both directions. One subject had mildly h ypermetric responses for one direction overshooting the start. The thr ee CN subjects with otological disease had marked hypometricity in one direction, presumed to be towards the affected side, comparable with patients with unilateral vestibular neurinectomies. The abnormal findi ngs in CN subjects are explained in terms of a reduction of 'velocity storage' in vestibular processing and mild asymmetries of vestibular t one. The results on the test clearly differentiate CN subjects with ot ological disease and help substantiate their diagnoses. A model based on the processing of vestibular ocular reflex eye movements with an ad ditional integrator for 'perception' of position is utilised to explai n the findings.