On sensory feedback in hand prostheses

Sammanfattning: Amputation of the hand implies the loss of the ability to grasp and the ability to "feel". The grasping function can be primitively restored using an active prosthesis. Multi­articulating electrically powered hands have recently made their way to the market and these hands provide enhanced grasping and gripping capabilities. However, these hands provide no direct and conscious sensory feedback to the user and there are ongoing research efforts in providing prosthetic hands with a sense of touch. There is no commercially available system for artificial limbs today that provides the user with conscious sensory feedback. This thesis presents the development and experimental studies of a new concept for providing users of prosthetic hands with a conscious sensory feedback. Previous studies have mostly relied on "sensory substitution" methods where sensory feedback is delivered in a different modality or to a different location on the body. By using modality matched sensory feedback, e.g. if pressure is sensed at a prosthetic finger then pressure is fed back to the amputee, the amputee would not have to learn to interpret the feedback signal (e.g. pressure coded as vibrations). Furthermore, many amputees experience a phantom "hand map" in their residual limb and when specific areas of this map is stimulated, the stimuli is perceived as coming from specific fingers in the amputated hand. By utilizing the phantom "hand map", amputees can be provided with modality matched sensory feedback, delivered to the "correct" body location and activating the correct location in the somatosensory cortex. In several studies, feedback actuators in the form of digital servomotors or air­mediated pressure bulbs were used to provide both non­amputees and amputees with sensory feedback. It was shown that amputees can learn to interpret sensory feedback on different locations and that the developed sensory feedback system would not interfere negatively with a myoelectric control system. Furthermore, it was shown that the use of modality matched feedback in a multi­site discrimination task yielded better results than modality mismatched sensory feedback. In conclusion, the studies indicate that this new concept for sensory feedback in hand prostheses can be useful in future artificial hands and enhance the sense of body ownership of the prosthesis.