Chronic recruitment of primary afferent neurons by microstimulation in the feline dorsal root ganglia

Objective. This study describes results of primary afferent neural microstimulation experiments using microelectrode arrays implanted chronically in the lumbar dorsal root ganglia (DRG) of four cats. The goal was to test the stability and selectivity of these microelectrode arrays as a potential interface for restoration of somatosensory feedback after damage to the nervous system such as amputation. Approach. A five-contact nerve-cuff electrode implanted on the sciatic nerve was used to record the antidromic compound action potential response to DRG microstimulation (2-15 mu A biphasic pulses, 200 mu s cathodal pulse width), and the threshold for eliciting a response was tracked over time. Recorded responses were segregated based on conduction velocity to determine thresholds for recruiting Group I and Group II/A beta primary afferent fibers. Main results. Thresholds were initially low (5.1 +/- 2.3 mu A for Group I and 6.3 +/- 2.0 mu A for Group II/A beta) and increased over time. Additionally the number of electrodes with thresholds less than or equal to 15 mu A decreased over time. Approximately 12% of tested electrodes continued to elicit responses at 15 mu A up to 26 weeks after implantation. Higher stimulation intensities (up to 30 mu A) were tested in one cat at 23 weeks post-implantation yielding responses on over 20 additional electrodes. Within the first six weeks after implantation, approximately equal numbers of electrodes elicited only Group I or Group II/A beta responses at threshold, but the relative proportion of Group II/A beta responses decreased over time. Significance. These results suggest that it is possible to activate Group I or Group II/A beta primary afferent fibers in isolation with penetrating microelectrode arrays implanted in the DRG, and that those responses can be elicited up to 26 weeks after implantation, although it may be difficult to achieve a consistent response day-to-day with currently available electrode technology. The DRG are compelling targets for sensory neuroprostheses with potential to achieve recruitment of a range of sensory fiber types over multiple months after implantation.