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 semicircular canal effects of galvanic vestibular stimulation with increasing current strengths on eye movements and the perception of verticality.

Methods: We measured (1) 3-dimensional eye movements, (2) subjective tilt of the peripheral visual field, and (3) subjective tilt of a central vertical line in 12 healthy subjects during galvanic vestibular stimulation. A rectangular, unipolar binaural electric current was applied to each subject's mastoid.

Results: Anodal stimulation of the right mastoid led to an ipsiversive tonic ocular torsion of up to 5.4°, to a contralateral tilt of both the peripheral visual field (1–9°), and a central vertical line (0.5–6.2°) increasing in amplitude with increasing current strengths applied. This reflects otolith stimulation. In most subjects, current strengths of 3 mA or more elicited a slight (horizontal-) torsional nystagmus (amplitude 1–2°) that was superimposed on static torsion. This reflects horizontal and vertical semicircular canal stimulation. A correlation was found in the amount of the 3 measured parameters and the strength of the applied current.

Conclusions: Thus, galvanic vestibular stimulation at low current intensities (1– 3 mA) preferably excites otolith responses, which increase with increasing current intensity. With higher current intensity above 3 mA, additional semicircular canal responses are elicited in the form of horizontal-rotatory nystagmus superimposed on static torsional deviations. The lack of a vertical deviation and nystagmus can be explained by the counterdirected vertical components of the anterior and posterior semicircular canal.