Looking with a paralysed eye: adaptive plasticity of the vestibulo-ocular reflex.

When a patient with a peripheral monocular paresis is forced to look with the paretic eye, head movements induce the sensation of an unstable visual world. The patient behaves as if he had acute bilateral labyrinthine lesions. These symptoms are due to the lack of compensatory ocular movement and the patients complain that the visual objects move in the direction opposite to the head. The patients develop ataxia, nausea, vomiting and past pointing. The symptoms, however, are transient and consistently disappear after approximately 48 hours. The central adaptation to looking and seeing with the paralysed eye is associated with a plastic change of the VOR. This plastic adaptation is probably induced by the large retinal slip produced by the lack of compensatory movement of the eye and can be studied in the normally mobile eye in the dark. The psychophysical adaptation is probably generated by an efferent copy or corollary discharge of the vestibular system to the visual system that cancels the retinal error.