Interphase gap as a means to reduce electrical stimulation thresholds for epiretinal prostheses

Abstract

Epiretinal prostheses are designed to restore functional vision to the blind by electrically stimulating surviving retinal neurons. These devices have classically employed symmetric biphasic current pulses in order to maintain a balance of charge. Prior electrophysiological and psychophysical studies in peripheral nerve show that adding an interphase gap (IPG) between the two phases makes stimulation more efficient than pulses with no gap. We investigated the effect of IPG duration on retinal ganglion cell (RGC) electrical thresholds in salamander retina, as well as phosphene perceptual thresholds in epiretinal prosthesis patients. In general, there was a negative exponential correlation between threshold and IPG duration. Durations greater than or equal to ~0.5 ms reduced salamander RGC thresholds by 20–25%. Psychophysical testing in five retinal prosthesis patients indicated that stimulating with IPGs can decrease phosphene perceptual thresholds by 10–15%. Results from Hodgkin-Huxley-type simulations of RGC behavior corroborated the observed behavior. Incorporating interphase gaps can reduce the power consumption of epiretinal prostheses and increase the available dynamic range of phosphene size and brightness.

Factors Affecting Perceptual Thresholds in a Suprachoroidal Retinal Prosthesis

“…While this result is well established in the cochlear implant literature, 54,55 recent reports with epiretinal stimulation 56,57 and our preclinical study using suprachoroidal stimulation 21 have also demonstrated the beneficial effects of using an IPG with biphasic pulses compared to pulses without any IPG. In fact, the study by Weitz et al 57 showed an improvement of~10% in perceptual threshold when using IPGs of ‡500 ls compared to no gap with the PW set to 450 ls per phase, a result very closely matched with what we found when using a PW of 500 ls per phase. The effect of PW alone on threshold is also well established; longer PWs have been shown to increase threshold charge per phase and decrease threshold current (strength-duration curve) with both cochlear implants 58 and retinal implants.…”

Improving the spatial resolution of epiretinal implants by increasing stimulus pulse duration

“…It is well known that short pulses stimulate RGCs through activation of voltage-gated sodium channels ( 22, 32 ). Though the mechanism underlying bipolar cell responses to electrical stimulation is less clear, modeling studies have implicated L- and T-type calcium channels ( 18, 33 ).…”

“…Remaining within this range would be difficult in clinical practice ( 14 ). Electrical thresholds among ganglion cell types can vary by threefold ( 22 ), and perceptual thresholds in implant patients rise steadily during repeated stimulation ( 32 ).…”

Electrical stimulation of cultured neurons using a simply patterned indium-tin-oxide (ITO) glass electrode

“…The stimulation threshold may be lower in more excitable cells, such as differentiated PC12 cells, and neurons (Homma et al, 2006;Simms and Zamponi, 2014). Previous studies have reported the stimulation threshold changes depending on the input current pattern in cells cultured on MEAs (Shepherd and Javel, 1999;Weitz et al, 2014). It might be possible that investigations into the influence of the input current may further optimize the cell responses observed in this study.…”

“…analyses of the interactions and connectivity among them (Brustein et al, 2003;Bock et al, 2011). In recent years, devices employing indium-tin-oxide (ITO) glass electrodes have been developed to study the electrical stimulation of specific neurons via calcium imaging (Behrend et al, 2011;Weitz et al, 2013Weitz et al, , 2014.…”