AUDITORY AND VESTIBULAR SYSTEMS NEUROREPORT 0959-4965 & Lippincott Williams & Wilkins Vol 11 No 17 27 November 2000 3897 The effects of galvanic stimulation on the human vestibulo-ocular reflex Mikael Karlberg, 1,2,CA Leigh McGarvie, 1 Ma˚ns Magnusson, 2 Swee T. Aw 1 and G. Michael Halmagyi 1 1 Neuro-otology Department, Royal Prince Alfred Hospital, Camperdown, Sydney, 2050 NSW, Australia; 2 Department of Oto- rhino-laryngology, Lund University Hospital, SE-221 85 Lund, Sweden CA,2 Corresponding Author and Address Received 5 September 2000; accepted 28 September 2000 We studied the effects of 5 mA bilateral or unilateral, bipolar or monopolar, galvanic stimulation on the horizontal vestibulo- ocular reflex (hVOR) in six normal subjects during 0.01, 0.05, 0.1, 0.5 and 1 Hz yaw rotations and in two subjects during high-acceleration, low-amplitude yaw head rotations (head impulses). Bipolar galvanic stimulation induced horizontal nystagmus in all subjects and an asymmetry of the hVOR only during rotations below 0.1 Hz. Monopolar stimulation had no significant effect. The findings suggest that in humans galvanic stimulation affects those primary horizontal semicircular canal neurons that mediate the hVOR via indirect pathways through the velocity storage mechanism. NeuroReport 11:3897–3901 & 2000 Lippincott Williams & Wilkins. Key words: Adult; Electric stimulation; Eye movements/physiology; Galvanic; Human; Labyrinth; Male; Vertigo; Vestibule/physiology INTRODUCTION Transmastoid galvanic currents can stimulate the vestibu- lar system in humans. Such galvanic vestibular stimulation (GVS) can induce nystagmus [1–3], tonic ocular torsion [3– 5], a tilt of subjective visual vertical [4] and postural shifts [6,7]. These effects are presumably due to changes in the tonic activity of vestibular neurons. So far no studies have addressed the effects that GVS might produce in the dynamics of the vestibular neurons. GVS acts on the post-synaptic spike trigger zone of primary vestibular neurons and not on vestibular hair cells [8]. Cathodal currents increase and anodal currents decrease neuronal activity [8]. The sensitivity of a vestib- ular neuron to GVS is determined by its discharge properties so that irregularly firing neurons are more sensitive than regularly firing neurons [8]. In the squirrel monkey bilateral monopolar anodal GVS which silences irregularly firing neurons has no effect on the gain of the hVOR during 0.2–4.0 Hz rotations or during rapid head accelerations [9,10], but decreases nystagmus slow phase eye velocity by 30–50% during velocity steps [10]. The effect of GVS on hVOR dynamics in the squirrel monkey seems to depend on the frequency of rotation and it has been suggested that GVS should affect the VOR during sinusoidal rotations only at 0.1 Hz and below [10]. We studied in normal human subjects the effects of continuous 5 mA bilateral or unilateral, bipolar or mono- polar, GVS on the hVOR in response to 0.01, 0.05, 0.1, 0.5 and 1 Hz sinusoidal rotation and during high-acceleration, low-amplitude head rotations (head impulses) [11]. MATERIAL AND METHODS Subjects: We tested six healthy, male subjects (mean age 34 years, range 24–44). All subjects gave informed, written consent and were studied using techniques approved by the local ethics committee. Galvanic stimulation: We used surface electrodes of 600–900 mm 2 that were individually cut from adhesive electrosurgical plating (3M) and placed over each mastoid using electrode paste (Redux Creme, Hewlett-Packard) and adhesive tape. An electrode was placed on the skin over vertebra C7. A custom-designed current stimulator was used to deliver a current of 5 mA from a battery source. The current level was adjusted manually with a potenti- ometer. Each change in current level was made over 5 s. Each subject was tested during six different modes: (1) Baseline, no GVS; (2) bilateral bipolar GVS (cathode on left mastoid, anode on right mastoid in three subjects, cathode on right mastoid, anode on left mastoid in three subjects); (3) unilateral bipolar cathodal GVS (cathode on left mas- toid in three subjects and on right mastoid in three subjects and anode over vertebra C7); (4) unilateral bipolar anodal GVS (anode on left mastoid in three subjects and on right mastoid in three subjects and cathode over vertebra C7); (5) bilateral unipolar cathodal GVS (cathode on both mastoids and anode over vertebra C7) and (6) bilateral unipolar anodal GVS (anode on both mastoids and cathode over vertebra C7). In order to decrease any effects of adaptation, three subjects had the stimulations in the sequence from mode 1 to mode 6 and three subjects in the order 4, 5, 6, 1, 2, 3. To enable the recordings of all subjects