Mind the gap: the temporal discrimination threshold of tactile sensation from implanted peripheral nerve stimulation.

backgroundThe ability to reliably detect the temporal order, overlap, and gaps between multiple tactile stimuli is crucial for making sense of our haptic interactions with complex environments. One important property of temporal processing of touch is the temporal discrimination threshold (TDT), which measures the shortest gap between two sequential stimuli required for them to be perceived as separate events. While the temporal properties of natural somatosensation are well understood, much less is known about the temporal properties of sensation elicited by peripheral nerve stimulation (PNS) delivered via implanted neural interfaces. Improving our understanding of the TDT of PNS-evoked sensations is crucial for the development of effective neuroprostheses.

methodsThis study was conducted with two individuals with unilateral upper limb loss implanted with 16-contact extraneural electrodes around the median, ulnar, and/or radial nerves. To measure TDT, we used an ascending staircase paradigm commonly used for clinical assessment of TDT. We investigated the effects of stimulation intensity, pulse train duration, and frequency on the TDT for five electrode contacts in total across the participants.

resultsWe found that the TDT significantly decreased with increasing stimulation intensity and frequency. We also found that the TDT of PNS pulse trains was significantly lower than the TDT of individual pulses. However, the duration of the stimulation pulse train did not have a consistent effect on TDT. For both participants, TDT values for implanted PNS were comparable to the values previously seen in the literature for electrocutaneous stimulation.

conclusionSince PNS intensity and frequency are routinely adjusted to shape sensory experiences in neuroprosthetic applications, our findings highlight the need for coordinated control of stimulation timing alongside modulation of other PNS parameters. This coordination is essential to enable accurate timing perception, which is critical for functional upper limb tasks, such as detecting object slippage, manipulating objects, and maintaining grasp force. Therefore, our results have important implications for optimizing the delivery of sensory feedback in neuroprosthetic systems. Trial registration: This study was prospectively registered on ClinicalTrials.gov (#NCT04947462) on 06/23/2021.