REDUCTION IN EXCITABILITY OF THE AUDITORY-NERVE FOLLOWING ACUTE ELECTRICAL-STIMULATION AT HIGH STIMULUS RATES - III - CAPACITIVE VERSUS NON-CAPACITIVE COUPLING OF THE STIMULATING ELECTRODES

Safe electrical stimulation of neural tissue is typically achieved usi ng charge-balanced biphasic current pulses, which are designed to mini mize the generation of direct current (DC) and the production of harmf ul electrochemical products. However, due to the kinetics of the charg e injection process, neural stimulators must also use capacitive coupl ing or electrode shorting techniques, to ensure DC levels are minimal. Previous studies have reported a reduction in excitability of the aud itory nerve following acute simulation at high rates and intensities. Elevated levels of DC were reported in these studies despite using cha rge-balanced biphasic pulses and electrode shorting. The present study was designed to investigate the extent to which DC contributed to the se stimulus induced reductions in auditory nerve excitability. Adult g uinea pigs were bilaterally implanted and unilaterally stimulated for two hours using charge-balanced biphasic current pulses and stimulus r ates of 200, 400 or 1000 pulses/s (pps) at a stimulus intensity well a bove clinical levels (0.34 mu C/phase). DC levels were controlled usin g either electrode shorting, or electrode shorting with capacitive cou pling. Electrically evoked auditory brainstem responses (EABRs) were r ecorded before and periodically following the acute stimulation. It wa s found that the extent of reduction in the EABR amplitude was a funct ion of stimulus rate. While there was little change in the EABR follow ing stimulation at 200 pps, significant post-stimulus reductions in th e EABR amplitude were observed at stimulus rates of 400 and 1000 pps d uring the three hour post-stimulus monitoring period. Stimulation usin g capacitively coupled electrodes, which eliminated all DCs, showed re ductions in EABR amplitudes similar to those observed following stimul ation using electrode shorting alone. While there was no significant d ifference in tie extent of reduction in EABR amplitudes for capacitive coupling versus electrode shorting at stimulus rates of 200 pps (P>0. 05) and 300 pps (P>0.05), there was a significant difference at 1000 p ps (P<0.001). The present findings indicate that the major component o f the stimulus induced reductions observed in auditory nerve excitabil ity appear to be associated with stimulus induced neuronal activity, a lthough elevated levels of DC (>2.5 mu A) can also contribute to these changes. However, although statistically significant, the effects of DC are very small compared to the effects of high rate, high intensity stimulation per se. (C) 1998 Elsevier Science B.V.