Fig. 1

Experimental setup and protocol. a The prosthesis was commanded to close at different velocities (grasping forces) while the subjects perceived incidental sound (SND, blue), visual (VIS, red) or combined visual and sound (VIS + SND, green) feedback or vibration feedback (VIB, orange, in Exp II only). For the three incidental feedback conditions (SND, VIS, VIS + SND), the setup included a prosthetic hand grasping a wooden block. For SND, the subjects were prevented from seeing the prosthesis by a screen; during VIS, earphones played white noise to block the sound. For VIB, the setup included two C2 tactors, one strapped to the subjects’ left arm displaying the vibration that had to be interpreted, the other hanging freely in the air, producing a strong sound to block the sound of the first tactor, together with white-noise playing headphones. b For each experiment (I and II) and each feedback condition (except VIS + SND), there was a familiarization, reinforcement-learning, and testing phase. CV: Comparison value, SV: Standard value. 20:5:100 stands for 5% steps from 20 to 100%. See text for details