Systems for identifying design constraints on tactile prosthetic design Open Access
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The term embodiment refers to the sense of ownership of a body part. Although prosthetics can restore some function following limb loss, true embodiment remains elusive. This may be due to the absence of natural sensory feedback. Accordingly, new prosthetic systems that use electrical stimulation to provide this feedback hope to increase the sense of prosthesis embodiment.To understand the input conditions necessary to induce embodiment, we have designed a system to induce embodiment effects using concurrent visual and tactile stimuli. A subject’s real right hand and both upper arms are draped, and an artificial hand is placed on the table in front of the subject with the wrist end covered by the drape, creating a visual illusion that the artificial hand is connected to the subject’s arm. Two stimulators are used to synchronously tap the hidden right hand and the visible artificial right hand, creating a “rubber hand illusion” . Each stimulator consists of a programmable stepper motor (ISM-7411E, National Instrumentation Corporation) driving a displacement sensor (SG-DVRT-8, LORD MicroStrain Sensing Systems). To study the physiological response to the illusion, two temperature sensors were constructed, each using a voltage divider (one 2 kΩ thermistor from DigiKey and 1 kΩ resistor) attached to the middle phalanx of the index finger on both hands to monitor limb temperature. The temperature was recorded for five minutes of baseline (visual illusion only), five minutes during synchronous tapping (RHI), and five minutes after tapping stopped (post-RHI). Embodiment of the artificial hand was then assessed using a standard RHI questionnaire and a proprioceptive drift assay, which registered the subject’s perceived hand location.The questionnaire responses of the subjects indicated that two of the three experienced the RHI. The subjects who experienced the RHI displayed proprioceptive drift, toward the artificial (visible) hand. Limb temperature has been shown to reflect the extent of prosthesis embodiment . To control for temperature fluctuations, the temperature ratio of the RHI (right hand) to the control hand (left hand) was calculated. For the two subjects experiencing the illusion, we observed transient dips in the hidden/visible hand temperature ratio that were not time-locked to any particular stimuli.In summary, the system successfully induced the RHI for two of three subjects, as evidenced by questionnaire results and proprioceptive drift. In future work, EEG signals will be recorded during the experiment and additional temperature sensors will be used to decrease the thermal variability.