Galvanic vestibular stimulation in humans: effects on otolith function in roll Reto Zink*, Sven Steddin, Alexander Weiss, Thomas Brandt, Marianne Dieterich Department of Neurology, Klinikum Grosshadern, Marchioninistrasse 15, 81377 Munich, Germany Received 8 July 1997; received in revised form 7 August 1997; accepted 8 August 1997 Abstract The effects of unilateral galvanic vestibular stimulation on (1) ocular torsion, (2) subjective tilt of the peripheral visual field, and (3) subjective tilt of a foveal vertical line were measured in 12 healthy subjects. A rectangular, unipolar binaural electric current was applied to the subject’s mastoid. Anodal stimulation of the right mastoid led to an ipsiversive tonic ocular torsion (0.5–3.7°) and to a contralateral tilt of both the peripheral visual field (1–9°), and a foveal vertical line (0.5–6.2°). There was a correlation between the amount of the three measured parameters and the strength of the applied current. Static ocular torsion, central and peripheral visual tilts represent stimulus- induced tonic otolith imbalance between the two labyrinths. Thus, galvanic vestibular stimulation not only affects dynamic semicircular canal input but also static otolith input in the roll plane.  1997 Elsevier Science Ireland Ltd. Keywords: Galvanic stimulation; Subjective tilt of the visual field; Otoliths; Ocular torsion; Graviceptive pathways; Human In previous human studies it was shown that galvanic stimulation elicits direction-specific body sway [11,12,19], postural electromyographic responses in the arm and leg muscles [3], and nystagmic eye movements [1]. The aim of this study was to determine the differential effects of galvanic stimulation on tonic otolith function of the vesti- bulo-ocular reflex in the roll plane. A tone imbalance in the roll plane can easily be quantified in degrees of ocular tor- sion and perceived visual tilt [9]. Vestibular tone in roll plane is based on the bilateral input from both the semicir- cular canals (dynamic effects, nystagmus) and the otoliths (tonic effects, tonic ocular deviation). In the ‘normal’ upright position, the subjective visual vertical is aligned with gravitational vertical, and the axes of the eyes and the head are horizontal. This study was comprised of 12 subjects (8 males, 4 females, mean age 30.8 years, range 25–46 years) without any history of cochlear, vestibular or central nervous system disorders. Grass gold electrodes (5 mm in diameter) taped to both mastoid processes were used to apply a rectangular, unipolar binaural electric current (1.5–3.0 mA; direct cur- rent (DC)) between both mastoids seven times at 10 s inter- vals. At least two different current intensities were used, chosen individually so as not to cause any discomfort. Polarity was changed between the different test trials. Ocular torsion of both eyes was measured separately by (1) video recordings of the fundus with the laser scanning ophthalmoscope (Rodenstock, Germany) and by (2) video- oculography technique. Ocular torsion (in degrees) was cal- culated from 4–6 fundus photographs taken from the online video recording as the mean angle between a straight line through the papilla and fovea (papilla-fovea meridian) and the horizontal line (for details, see Dieterich and Brandt [4]). In addition, in three subjects three-dimensional video recordings [16] of each eye were made during stimulation periods (Fig. 1). For psychophysical measures of perceived tilts, the sub- ject sat with his/her head in an upright position in front of a half-spherical dome 60 cm in diameter. The surface of the dome extended to the limits of the observer’s visual field, and did not contain any gravitational cues. A central black target with a white line subtended 14° of visual angle at a distance of 30 cm to the eyes. This central test edge had to be adjusted after stimulation according to the perceived tilt during stimulation (central foveal tilt). Neuroscience Letters 232 (1997) 171–174 0304-3940/97/$17.00  1997 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(97)00610-1 * Corresponding author. Tel.: +49 89 70906119; fax: +49 89 70906101; e-mail: rzink@brain.nefo.med.uni-muenchen.de