(9) Combatting Percept Adaptation to Intracortical Microstimulation in Humans

Introduction: Without tactile sensation, even simple, everyday tasks become nearly impossible [1]. Intracortical microstimulation (ICMS) in the human somatosensory cortex (S1) can restore tactile sensations by using electrical stimulation to activate sensory neurons in the brain that would normally respond to touch [2]. Typically, ICMS trains consist of unmodulated single-channel stimulation with a constant amplitude and frequency. These stimuli evoke vivid tactile percepts originating from the participants’ own hands. Unfortunately, the perceived intensity of these sensations can rapidly decrease during stimulation, falling below the perceptual threshold in tens of seconds [3].

Methods: Two microelectrode arrays were implanted in both the motor and somatosensory cortices of three participants with tetraplegia as part of a clinical trial. We designed two ICMS encoding schemes leveraging biological principles: biomimetic ICMS, which mimics natural spatiotemporally modulated neural activity, and interleaved ICMS, which takes advantage of overlapping sensory fields to distribute stimulation across multiple electrodes. For biomimetic ICMS, we co-modulated frequency and amplitude with a large but brief onset and offset transient stimulation bursts. For interleaved ICMS we used four electrodes with overlapping projected fields and cycled through each electrode using parameters that were above threshold for each of the four electrodes. To test percept resiliency, participants either watched a clock face and were asked to report how long the sensation lasted or used a tablet to continuously report the stimulation intensity. Stimulation durations were randomly chosen from 1 to 180 s intervals, and evoked neural activity was recorded. Detection thresholds were collected before and after each testing session.

Results: Preliminary results show that unmodulated ICMS led to rapid adaptation, biomimetic stimulation increased the perceived duration, and interleaved ICMS completely eliminated percept adaptation (see Fig. 1, n = 7 electrodes). In fact, sensations from interleaved ICMS could be reliably felt for over a minute.

Conclusion: These experiments demonstrate that both biomimetic and interleaved ICMS encoding schemes greatly extend the perceived sensation duration compared to traditional unmodulated stimulation. These improvements likely arise from dynamic and distributed stimulation patterns, which may reduce neural adaptation. Notably, biologically inspired ICMS overcomes a critical limitation for the practical use of ICMS in restoring sensation, providing reliable and uninterrupted sensory percepts for neuroprosthetics. Future experiments will expand testing to additional electrodes and participants, while future analysis will explore neural correlates underpinning percept adaptation.