Rotatory evoked cortical potentials in normal subjects and patients with unilateral and bilateral vestibular loss

Rotatory evoked late potentials following clockwise and counterclockwise rotational acceleration were measured in subjects with normal labyrinthine function and in patients with unilateral and bilateral vestibular loss. In the normal subjects the threshold of the evoked cortical responses ranged between 4 degrees/s2 and 7 degrees/s2, independent of the direction of rotation. For patients with unilateral vestibular loss thresholds were found to be dependent on the direction of rotation. The difference in threshold between clockwise and counter-clockwise rotation was at least 5 degrees/s2. The threshold for rotational accelerations in the direction of the healthy labyrinth was higher for patients with compensated unilateral losses. Those patients who still exhibited lesion-associated nystagmus manifested a higher threshold in the direction of the defective labyrinth. In those patients with bilateral vestibular loss, thresholds were found to be independent of the direction of rotation and were of the order of 20 degrees/s2. The results indicate that the late potentials reflect the interaction of somatosensory and vestibular stimulation. The difference in threshold (approx. 15 degrees/s2) between the normal subjects and patients with bilateral vestibular loss suggests that the vestibular component predominates at low accelerations, with the somatosensory component assuming significance at higher acceleration levels.