Constant error of visual egocentric orientation in patients with acute vestibular disorder.

Errors of visual egocentric localization are well-known in patients with paresis of ocular muscles or paresis of conjugate gaze. In the present paper a series of patients with unilateral vestibular disorder disclosed a constant lateralization of the visual agocentre in the absence of any ocular paresis. The perceptual illusion is associated with an altered resting position of the eyes caused by the vestibular imbalance. The disturbance of visual, egocentric localization was revealed only after elimination of the visual frame of reference and the extent of lateralization of the visual egocentre was proportional to the degree of resting deviation of the eyes. Although the findings are of limited clinical importance they have a considerable theoretical interest. From the clinical point of view they may provide a basis for further understnading of the complex-perceptual illusions which may accompany disorders affecting central vestibulo-ocular connections. The results indicate that the perceptual effects are related to an altered central evaluation of the oculomotor programme and thus depend upon the operation of an 'efference copy'. This hypothesis is discussed with reference to earlier and current theories of visual localization.