Factors Affecting Perceptual Thresholds in a Suprachoroidal Retinal Prosthesis

Abstract

With suprachoroidal stimulation, anodic-first pulses with a monopolar return are most efficacious. To enable high rates, an appropriate combination of pulse width and interphase gap must be chosen to ensure low thresholds and electrode voltages. Electrode-retina distance needs to be monitored carefully owing to its influence on thresholds. These results inform implantable stimulator specifications for a suprachoroidal retinal prosthesis. (ClinicalTrials.gov number, NCT01603576.).

The Appearance of Phosphenes Elicited Using a Suprachoroidal Retinal Prosthesis

“…4,5,27,28 In brief, the percutaneous connector and subcutaneous remote return electrode were implanted behind the ear (Fig. 1D), with the connector anchored to the temporal bone using titanium screws.…”

“…Stimulation was delivered at stepped charge levels above predetermined perceptual thresholds, measured using an iterative staircase procedure described previously. 4 The pulse phase width was fixed to a nominated value for the duration of each experiment, and the current amplitude was adjusted to deliver the required charge levels. A decibel (dB) scale was used to define stimulation levels relative to threshold, as it has been found that perceived intensity can be described as a power function of stimulation amplitude for an epiretinal visual prosthesis.…”

“…However, these advantages come at the expense of increased distance between the electrodes and the retinal stimulation targets, which can increase the charge levels required to elicit phosphenes and limit spatial resolution. 4,29,30 Our suprachoroidal array has been shown clinically to be stable over the first year of implantation in three profoundly visionimpaired patients with retinitis pigmentosa (RP), 5 and we have been able to reliably measure perceptual thresholds within safe charge density limits using a wide range of stimulus parameters in all three patients. 4 The main goal of this study was to investigate the appearance and recognizability of phosphenes elicited using our prototype suprachoroidal retinal prosthesis in these three patients with profound vision loss due to RP.…”

“…4,29,30 Our suprachoroidal array has been shown clinically to be stable over the first year of implantation in three profoundly visionimpaired patients with retinitis pigmentosa (RP), 5 and we have been able to reliably measure perceptual thresholds within safe charge density limits using a wide range of stimulus parameters in all three patients. 4 The main goal of this study was to investigate the appearance and recognizability of phosphenes elicited using our prototype suprachoroidal retinal prosthesis in these three patients with profound vision loss due to RP. Tasks were performed to assess phosphene shape, size, temporal properties, location, overlap, and recognizability, as well as the influence of stimulation parameters on percept appearance.…”

Are long stimulus pulse durations the answer to improving spatial resolution in retinal prostheses?

“…Weitz et al propose that a preferred way to improve spatial resolution is to modify the stimulation waveform, principally through increasing the pulse duration (leading phase only) of the predominantly used biphasic square-wave pulse in order to eliminate axonal activation. Clinical trials of retinal prostheses have most often used charged-balanced biphasic square-waves of around 0.5 msec duration per phase (7,8), or monophasic 1 msec pulses subretinally (2), with rates typically around 5 Hz (2,7), but ranging from 20 to 400 Hz in the case of suprachoroidal implantation (8,9).…”