EFFECTS OF GALVANIC VESTIBULAR STIMULATION ON OTOLITHIC AND SEMICIRCULAR CANAL EYE-MOVEMENTS AND PERCEIVED VERTICAL

Objective: The aim of this study was to determine the otolithic and se micircular canal effects of galvanic vestibular stimulation with incre asing current strengths on eye movements and the perception of vertica lity. Methods: We measured (1) 3-dimensional eye movements, (2) subjec tive tilt of the peripheral visual held, and (3) subjective tilt of a central vertical line in 12 healthy subjects during galvanic vestibula r stimulation. A rectangular, unipolar binaural electric current was a pplied to each subject's mastoid. Results: Anodal stimulation of the r ight mastoid led to an ipsiversive tonic ocular torsion of up to 5.4 d egrees, to a contralateral tilt of both the peripheral visual field (1 -9 degrees), and a central vertical line (0.5-6.2 degrees) increasing in amplitude with increasing current strengths applied. This reflects otolith stimulation. In most subjects, current strengths of 3 mA or mo re elicited a slight (horizontal-) torsional nystagmus (amplitude 1-2 degrees) that was superimposed on static torsion. This reflects horizo ntal and vertical semicircular canal stimulation. A correlation was fo und in the amount of the 3 measured parameters and the strength of the applied current. Conclusions: Thus, galvanic vestibular stimulation a t low current intensities (1-3 mA) preferably excites otolith response s, which increase with increasing current intensity. With higher curre nt intensity above 3 mA, additional semicircular canal responses are e licited in the form of horizontal-rotatory nystagmus superimposed on s tatic torsional deviations. The lack of a vertical deviation and nysta gmus can be explained by the counterdirected vertical components of th e anterior and posterior semicircular canal. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.