Modulation of human vestibular-evoked postural responses by alterations in load

The effects of body loading and unloading on human postural responses elicited by 1 mA bilateral, bipolar galvanic vestibular stimulation (GVS) were investigated. Subjects stood symmetrically, and in separate experiments were either loaded by 16, 33 and 50 % of their body weight with weights attached to the trunk, or unloaded by 10, 20 and 30 % using a whole-body harness that partially lifted the body but was free to translate horizontally. Randomised blocks of stimuli for each loading/unloading condition were compared to a non-loaded control condition. The rate of lateral reaction force development over the period 200-350 ms poststimulus increased in both legs with loading and decreased with unloading. The rate of force development was always larger from the leg on the side of cathodal stimulation. Vertical force responses were equal and opposite in the two legs, increasing on the side of the cathode and decreasing on the side of the anode. The rate of vertical force development over the period 200-350 ms after stimulus onset was increased with loading and decreased with unloading. In the frontal plane, the rate of head and trunk tilt in space was increased and decreased with loading and unloading, respectively. However, the relative rate of head tilt with respect to the trunk was not affected by loading conditions. These experiments provide further evidence that load-related afferent feedback influences the processing of vestibular information for the control of balance.