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Recovery of the Response of Sensory Fibers to the Second of a Pair of Peripheral Nerve Stimuli

Neural interfaces that stimulate the peripheral nerves have the potential to provide sensory feedback from artificial hands. Many neural interfaces are now being developed that allow for multi-channel stimulation of nerves. It is widely accepted that the electric fields generated by two or more contacts on a neural interface can interact. However, this has previously not been examined in the context of sensory feedback prostheses. Here, we aimed to investigate these interactions and the recovery dynamics of the sensory fibers. A multi-channel cuff electrode was implanted on the sciatic nerve of a rat. It comprised four rings (1 mm apart), each containing four circumferentially arranged electrodes. Temporally-patterned pairs of electrical stimuli were delivered through all 120 combinations of electrode pairs. Compound action potentials, elicited by stimulation of the sciatic nerve, were measured with two pairs of hook electrodes placed on the L4 dorsal root. We find that regardless of the relative position of the two electrodes on the cuff, at an interval of 0 ms, the CAP response is facilitated. At all other intervals, an inter-stimulus interval of even 5 ms was not enough for the response to the second stimulus to fully recover. This observation suggests that overlapping regions of nerve were stimulated. Examining only the intervals where the CAP did not fully recover, we noticed that if the electrodes lay longitudinally, that is, along the nerve, the CAP recovery was significantly impaired, compared to when the electrodes were in any other relative position. The observed space- and time-dependent interactions advocate for further controlled neuroscience studies in parallel to translational work on closed-loop prosthesis control.