Figure 4. Galvanic stimulation to evoke responses in the three cardinal planes.

A, the net angular acceleration vector (α) evoked by bipolar binaural galvanic stimulation with the cathodal electrode on the right is directed posteriorly and inclined 18 deg upward from Reid's plane (RP). B, to generate a vestibular signal of roll (lateral) relative to the vertical gravitational thrust vector (T), the subject faces forward with the head tilted slightly backwards so that Reid's plane is inclined 18 deg down and back. C, to generate a signal of pitch (sagittal) relative to T, the same backward head tilt is maintained while turning the head 90 deg relative to the feet. D, to generate a signal of yaw (horizontal) the head is bent forward so that Reid's plane is inclined 72 deg down and forward. Thus, during standing, postures B and C will evoke lateral and sagittal sway, respectively, whereas D will not produce a balance response but will create a sense of horizontal rotation. E, F and G show the directions of perceptual and balance responses for the three head postures with the body supine.