Orientation of the body response to galvanic stimulation as a function of the inter-vestibular imbalance

We proposed to study and quantify the anteroposterior component, on top of the lateral one, of the body sway induced by different configurations of ga lvanic vestibular stimulation (GVS) in order to advance the understanding o f the orientation of the response. Four stimulation configurations were use d in two separate experiments: monaural, binaural, and opposite double mona ural in the first experiment (11 subjects); monaural and double monaural in the second (13 subjects). The postural response of the subjects, standing with their eyes closed, to the stimulus (0.6 mA, 4 s) was assessed by measu ring the displacement of the center of pressure (CoP) using a force platfor m. As usual, binaural GVS induced a strictly lateral deviation of the cente r of pressure. The opposite double monaural condition induced a similar lat eral sway to that obtained in the binaural mode, although with a very diffe rent stimulation configuration. Monaural GVS induced an oblique, stereotype d deviation in each subject. The anteroposterior component comprised a forw ard deviation when the anode was on the forehead and a backward deviation w hen the anode was on the mastoid. The lateral component, directed towards t he anode as in the binaural design, was twice as large in the binaural than in the monaural mode. The second experiment showed that double monaural st imulation elicited an anteroposterior deviation (backwards when the anode w as on the mastoids and forwards when it was on the forehead) that was equiv alent to the addition of two complementary monaural configurations. The pre sent results show that monaural stimulation activates one side of the vesti bular apparatus and induces reproducible, stereotyped deviations of the CoP in both the anteroposterior and lateral plane: Secondly, binaural GVS appe ars to result from the addition of two complementary monaural stimulations. Lateral components of the response to each stimulation, being in the same direction, are summed, whilst anteroposterior components, being in opposite directions, cancel each other out. The opposite happens when both labyrint hs are polarized in the same way, as in the double monaural configuration. We suggest that the orientation of the response to GVS is a function of the imbalance between right and left vestibular polarization, rather than a fu nction of the actual position of the electrodes.