Vibrotactile Stimulation for Object Stiffness Feedback Using Spatiotemporal Encoding

Restoring stiffness perception in prosthetic users through non-invasive methods remains a major challenge in haptic feedback research. This study evaluates a wearable vibrotactile stimulation system that conveys object stiffness information through two encoding strategies: Proposed Spatiotemporal Encoding and Circular Encoding, applied to two anatomical locations: upper-arm and forearm. Ten healthy participants completed structured trials of stiffness-discrimination involving vibrotactile cues corresponding to four stiffness categories. The results showed significantly higher classification accuracy (CA) and information transfer (IT) with the Proposed encoding strategy at both sites. The upper-arm-proposed configuration achieved peak performance (CA: 97.75%, IT: 1.84 b/s), whereas the forearm-circular strategy yielded the lowest (CA: 73.62%, IT: 0.86 b/s). NASA-TLX scores indicated a significantly lower mental workload for the proposed strategy, with the upper-arm feedback location providing superior perceptual clarity. A supplementary evaluation with a transradial amputee further demonstrated that the proposed encoding strategy remained interpretable, achieving classification accuracies above 85%. The classification accuracies over different conditions followed the same pattern as observed in healthy participants. These findings validate the importance of encoding geometry and stimulation site in designing effective haptic interfaces and support the feasibility of spatially distributed, non-invasive vibrotactile feedback for enhancing tactile perception in prosthetic applications.