Sinusoidal electrical pulse more efficiently evokes retinal excitation than rectangular electrical pulse in retinal prostheses

Retinal prostheses for blindness due to retinal photoreceptor degeneration stimulate the retina electrically to evoke a pseudolight sensation (phosphenes). Although rectangular pulses of electrical stimulation are commonly used in retinal prostheses, it remains unclear whether nonrectangular pulses are effective in evoking this response. Here, we conducted in vivo electrophysiological experiments to compare the effectiveness of sinusoidal and rectangular pulses. Biphasic sinusoidal pulses (cathodic-first) applied suprachoroidally and transretinally to the rat eye elicited larger field responses in the superior colliculus than did rectangular pulses. The threshold charge for the evoked response of the sinusoidal pulse was significantly lower than that of the rectangular pulse, suggesting that a sinusoidal pulse is more effective than a rectangular pulse in our retinal prosthesis. Because a sinusoidal pulse can evoke phosphenes with a small charge magnitude even if the electrode area is reduced, the charge density does not increase; thus, the pulse can stimulate the retina without causing tissue injury. Because the sinusoidal pulse allows us to reduce the electrode area, it is possible that phosphenes can be localized to a smaller area by limiting the range of stimulated retinal ganglion cells. Therefore, high resolution of retinal prostheses using the sinusoidal pulse can be expected.