Optimizing Vibrotactile Feedback for Sensory Substitution in the Thigh: Spatial Acuity and Frequency Characteristics

Amputation of a lower limb not only affects mobility but also interferes with sensory feedback, leading to an elevated risk of falls among individuals living with amputation. Sensory substitution, achieved through tactile displays embedded in transfemoral prosthetic sockets, presents a promising non-invasive solution to provide artificial sensation to users. However, for this approach to be effective, users must accurately perceive distinct combinations of vibrations, a capacity limited by their two-point discrimination ability. This study examined whether spacing two vibrotactile stimuli within the 20-30 mm range, on the thigh, enabled the perception of distinct points and whether vibration frequency affected spatial acuity. We defined the ability to perceive two distinct points as achieving at least a 75% accuracy in responses, and based on this criterion, we determined that the minimum distance required for two-point discrimination lies between 25 mm and 30 mm. Notably, our study revealed that spatial acuity was not altered when vibrating at either low (30 Hz) or high (150 Hz) frequencies, provided the vibrations were at the perceptual threshold. Lastly, our findings consistently favoured stimuli that were spaced out vertically over horizontal ones. These findings contribute to the improvement of tactile displays intended for sensory substitution in transfemoral prostheses.