Influence of Feedback Parameters on Performance of a Vibrotactile Balance Prosthesis

Abstract

We investigated the influence of feedback conditions on the effectiveness of a balance prosthesis. 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 several different combinations of angular position and velocity of body sway in the sagittal plane. Control tests were performed with no tactor activation. Body sway was evoked in subjects with normal sensory function by rotating the support surface upon which subjects stood with eyes closed. Body sway was analyzed by computing root-mean-square sway measures and by a frequency-response function analysis that characterized the amplitude (gain) and timing (phase) of body sway over a frequency range of 0.017 to 2.2 Hz. Root-mean-square sway measures showed a reduction of surface stimulus evoked body sway for most vibrotactile feedback settings compared to control conditions. However, frequencyresponse function analysis showed that the sway reduction was due primarily to a reduction in sway below about 0.5 Hz, whereas there was actually an enhancement of sway above 0.6 Hz. Finally, we created a postural model that accounted for the experimental results and gave insight into how vibrotactile information was incorporated into the postural control system.

A Trunk Support System to Identify Posture Control Mechanisms in Populations Lacking Independent Sitting

“…When investigating mechanisms of posture control, it is beneficial to evaluate trunk responses across multiple frequencies because the influences of specific neural and biomechanical mechanisms are most evident across specific frequencies[19,22,25,26,28]. One example of a frequency-dependent neural mechanism can be seen in data from infants and children with CP.…”

“…In addition, the PRTS waveform was only 21.78 s so that many repetitions of the waveform (typically ranging from 8-12) could be delivered to subjects during each test. Repeating waveforms is important to accurately estimate posture responses[22-26], detect adaptation[22,25], and correlate posture with behavior coding for categorization within each waveform[15]. …”

“…Stimulus-response posture data has also been combined with parametric system identification methods to identify abnormal scaling between stimulus and torque generation [29] and to distinguish torque generation across intrinsic stiffness, reflexes, and sensorimotor integration [19,22,25,26,28]. Because poor sensory integration, muscle weakness, or improper torque scaling can all play a role in impaired sitting, obtaining stimulus-response research data may lead to major breakthroughs in understanding detailed posture impairments.…”

Application of Optimization Methods to Predict Performance of a Vibrotactile Balance Prosthesis

“…Experimental transfer function data used for the current study were obtained in earlier experiments [5]. Test subjects gave their informed consent to participate in an approved study.…”

“…We performed subsequent experiments [5] in subjects with normal sensory function to determine if tactor feedback provided by other weighted combinations of sway position and velocity might improve the performance of the prosthesis by reducing the transfer function gains across the entire bandwidth of frequencies. The ratio of sway position to sway velocity feedback was varied from 100% position and 0% velocity feedback (P100V0 condition) to 0% position and 100% velocity feedback (P0V100 condition) in increments of 25%.…”