VOR RESPONSE DUE TO SAFE DIRECT CURRENT STIMULATION AND PULSE-FREQUENCY MODULATED STIMULATION AND THE EFFECTS OF VARYING ELECTRODE COMPOSITION

The vestibular system functions by exciting neurons to increase their spike rate and by inhibiting neurons to decrease their spike rate, depending on the direction of movement of the head. A multichannel vestibular prosthesis (MVP) uses pulsatile frequency modulated (PFM) stimulation to stimulate vestibular nerve afferent fibers found in the semicircular canals (SCCs). This leads to effective encoding of head movements in the excitatory direction, or towards the implanted prosthesis, but less effective encoding of head movements in the inhibitory direction, or away from the implanted prosthesis. Safe direct current stimulation (SDCS) has shown Yu Zheng’s work to inhibit neural activity by delivering direct current (DC) to the target neural population. [1] This study aimed to replicate and confirm that PFM when delivered along with DC was able to encode head movements in the inhibitory directions. The MVP was used to deliver PFM to a specific SCC through platinum-iridium (Pt-Ir) wire, and DC was delivered through microcatheter tubes made from agar and 0.9% NaCl solution. Setting the baseline rate (BSR) to 60 pulses/second, the MVP then modulated the pulse rate below the BSR at a frequency of 2 Hz to stimulate afferent fibers. DC was applied at an amplitude of anodic 100 µA. To assess the ability of PFM and DC to encode inhibitory head movements, the left eye was monitored and the electrically-evoked vestibulo-ocular reflex (VOR) response was measured using video-oculography recording (VOG). Using a DC power supply (Keithley Model 6220), the results showed that DC did not show a significant improvement in encoding inhibitory head movements when used with PFM. In replicating the preliminary results, DC power supply AM2200 was used, and the results matched what the preliminary data showed. A thorough test of the Keithley and AM2200 was conducted in a salt bath to measure each current source’s performance. It was found that data acquired using an AM2200, which appeared to show that DC improved MVP performance for inhibitory motion, was confounded by the fact that the AM2200’s supposedly DC current output had a current disruption for every pulse delivered by the MVP, effectively causing an action potential at every pulse. It was concluded that the preliminary data was due to this equipment error. This study also examined the safety of using DC as a form of stimulation. The tests were done on 5 chinchillas. DC was being delivered via microcatheter tubes filled with an agar + NaCl gel, but after some time of delivering DC, VOR response would dramatically decrease. The control of using only PFM to stimulate afferent fibers showed the least decrease in VOR response over the course of the experiments (3 hours). When DC was delivered through tubes made from agar + artificial perilymph solution, there was less decrease in VOR response over the course of 3 hours, but more of a decrease compared to the control. Tubes made from agar + 0.9% NaCl solution showed the largest decrease in VOR response over the course of 3 hours. The ionic concentrations of Na+ and K+ in the agar gel used in the microcatheter tubes affects the time-course over which VOR response shows a decrease