Determining the effectiveness of a vibrotactile balance prosthesis.

We present a quantitative method for characterizing the effectiveness of a balance prosthesis based on tactile vibrators. The balance prosthesis used an array of 12 tactile vibrators (tactors) placed on the anterior and posterior surfaces of the torso to provide body orientation feedback related to the angular position and velocity of anterior-posterior body sway. Body sway was evoked in subjects with normal sensory function and in vestibular loss subjects by rotating the support surface upon which a test subject stood with eyes closed. Tests were performed both with (tactor trials) and without (control trials) the prosthesis activated. Several amplitudes of support surface stimulation were presented with each stimulus following a pseudorandom motion profile. For each stimulus amplitude, a transfer function analysis characterized the amplitude (gain) and timing (phase) of body sway evoked by the support surface stimulus over a frequency range of 0.017 to 2.2 Hz. A comparison of transfer function results from the control trials of normal subjects with results from tactor trials of vestibular loss subjects provided a quantitative measure of the effectiveness of the balance prosthesis in substituting for missing vestibular information. Although this method was illustrated using a specific balance prosthesis, the method is general and could be applied to balance prostheses that utilize other technology.