Chronic thresholds for evoking perceptual responses in the rat sensory cortex

Stimulation of neural tissue for the remediation of brain and sensory deficits requires that stimulation paradigms are selected carefully to deliver the most stable, efficient and safest stimulation to evoke the desired therapeutic response. Here we asked two questions of a penetrating cortical prosthesis use to evoke sensory-guided behavior: 1) does the threshold charge required to evoke a behavioral response change over time? 2) what effect does changing the frequency of stimulation have on stimulus threshold? To answer these questions we implanted a 4-electrode array into the somatosensory (tactile) cortex of a Sprague Dawley rat. The threshold charge to evoke a behavioral response was measured weekly over a 9-week period. Stimulation frequencies of 50 or 200 Hz were used, while all other stimulus parameters were kept constant. Within a maximum current limit of 100 ?A and with a pulse width of 200 ?s, we reliably elicited a behavioral response on 2 electrodes. Over the 9 week implantation period there was an initial increase in threshold current at 4 weeks, followed by a decrease at week 5 post-implantation; by week 8 post-implantation, thresholds appeared to have stabilized. Although we could reliably evoke a response at both 50 and 200 Hz, the stimulus frequency of 50 Hz required on average a lower threshold charge to evoke a response.