Galvanic Stimulation of the Vestibular System and Perception of the Vertical

Abstract

The gravitational vertical within a tilted visual field was visually estimated by 10 Ss during uni- and bilateral galvanic stimulation at 0.19 and 0.4 ma. through each of 6 cranial (mastoid-forehead) and 8 muscle (sternomastoid, trapezius, and hand) electrode combinations. Verticality estimates were significantly different among Ss and the cranial electrode combinations. Improved and impaired estimates of about ½ to 1° gain or loss were dependent on both electrode polarity and laterality of stimulation with respect to the direction of visual field tilt. Stimulation at the mastoid shifted the apparent vertical away from the anode and toward the cathode. These deviations were larger with anode and unilateral stimulation than with cathode and bilateral stimulation. In contrast, neck stimulation produced shifts toward the anode. Significant deviations occurred with mastoid stimulation below the threshold for cutaneous sensation. Results were discussed with reference to muscle tonus distribution in the body, vestibular-ocular reflexes, and their interactive effects as implicated in motor-sensory feedback basic to coordination of movement.