Low frequencies improve intensity discrimination for electrocutaneous artificial sensory feedback

honors thesisCollege of EngineeringBiomedical EngineeringJacob A. GeorgeThe current standard of care for those living with upper-limb loss is unsatisfactory, with up to 50% of amputees abandoning their prostheses, citing unintuitive use and a lack of sensory feedback as critical factors. Electrocutaneous stimulation uses electrodes on the skin to evoke tactile sensation. Stimulation of reinnervated afferent nerves can be used to provide amputees with natural, intuitive somatosensory feedback. Conveying the magnitude of tactile stimuli is an essential characteristic of natural touch. Tactile stimuli that vary in other properties can be judged on a single intensive continuum. Accordingly, the modulation of stimulus frequency results in differences in perceived magnitude of touch. Here, we are exploring the ability to discriminate between stimulation frequencies to restore sensory feedback with sensorized prostheses. Participants received stimulation through a custom-fabricated stimulation pad placed on the palm or residual limb. We measured the just-noticeable difference (JND) to describe how well electrocutaneous stimulation can convey the magnitude of tactile stimuli. The JND is defined as the minimum change in stimulation frequency that can be identified correctly 75% of the time. We quantified the JND using a two-alternative forced-choice paradigm in which stimulation pulse frequency was varied, and participants were asked to determine which of the two pulse frequencies felt stronger. Weber's law states that the JND is proportional to the absolute magnitude of that stimulus and that the percent change relative to the absolute magnitude (known as the Weber fraction) is consistent across stimulus magnitude (i.e., the more intense a stimulus, the greater the change must be to be detected). Here, we show that Weber's law does not iii hold true for at least electrocutaneous stimulation, where the Weber fraction is much smaller at lower stimulus frequencies (10% and 13% change needed at 25 Hz and 50 Hz, respectively, vs 28% change needed at 75 Hz vs 34% change needed at 100 Hz). This suggests that the number of perceivable sensory gradations may be closer to 42 (based on the variable Weber fractions across frequency), which is triple the previously estimated 14 (based on a static Weber fraction of 0.34 obtained at 100 Hz). In contrast, leading invasive methods of stimulation yield an estimated 36 and 15 gradations, respectively. This indicates that low-cost, non-invasive electrocutaneous stimulation constitutes an effective alternative. These results also help deepen the understanding of tactile perception across frequency, where lower frequencies may provide supplemental temporal cues that aid discrimination. These results can help facilitate the implementation of electrocutaneous stimulation as artificial sensory feedback. Improved discrimination of tactile features will benefit neural prostheses in real-world tasks that rely on somatosensory feedback. Its implementation will offer improved patient outcomes for those experiencing upper-limb loss and begin to restore the complete sensory experience of an intact hand. Electrocutaneous stimulation may also constitute a valuable rehabilitation tool for stroke patients and can be used to convey social touch information in telepresence and augmented or virtual reality