Suprachoroidal electrical stimulation: effects of stimulus pulse parameters on visual cortical responses

Objective. Neural responses to biphasic constant current pulses depend on stimulus pulse parameters such as polarity, duration, amplitude and interphase gap. The objective of this study was to systematically evaluate and optimize stimulus pulse parameters for a suprachoroidal retinal prosthesis. Approach. Normally sighted cats were acutely implanted with platinum electrode arrays in the suprachoroidal space. Monopolar stimulation comprised of monophasic and biphasic constant current pulses with varying polarity, pulse duration and interphase gap. Multiunit responses to electrical stimulation were recorded in the visual cortex. Main results. Anodal stimulation elicited cortical responses with shorter latencies and required lower charge per phase than cathodal stimulation. Clinically relevant retinal stimulation required relatively larger charge per phase compared with other neural prostheses. Increasing the interphase gap of biphasic pulses reduced the threshold of activation; however, the benefits of using an interphase gap need to be considered in light of the pulse duration and polarity used and other stimulation constraints. Based on our results, anodal first biphasic pulses between 300-1200 mu s are recommended for suprachoroidal retinal stimulation. Significance. These results provide insights into the efficacy of different pulse parameters for suprachoroidal retinal stimulation and have implications for the design of safe and clinically relevant stimulators for retinal prostheses.