Psychophysics of a suprachoroidal retinal prosthesis

Abstract

Purpose: The hypothesis was tested that electrodes implanted in the suprachoroidal space would produce phosphenes suitable for the representation of visual information in patients with profound vision loss.

Methods: Two patients with retinitis pigmentosa were implanted with a suprachoroidal electrode array having 17 600μ and three 400μ platinum electrodes and several return configurations. Pre-operatively, both patients had bare light perception and could not recognise shapes. A specially designed stimulator and psychophysics test setup were used to measure electrode impedance, threshold, dynamic range, perceived shape, size, position, and intensity of phosphenes produced by stimulating one electrode at a time. These data were included in phosphene maps suitable for encoding images or creating complex shapes by stimulating multiple electrodes in an interleaved fashion.

Results: Electrode impedances were around 20 kΩ for stimulation with biphasic current pulses with durations 500μs per phase and 500μs interphase gap. All electrodes produced phosphenes . Thresholds were lowest (down to 50 nC per phase) when the anodic phase preceded the cathodic phase of the pulse, the monopolar electrode configuration was used, and pulse rates of 200 to 500 pps were used. Dynamic ranges were limited by the maximum electric charge density per phase. Phosphenes varied from quite complex (including dark and light regions) for electrodes close to the fovea, and became simpler for more peripheral electrodes (such as grey cloudy convex shapes). Phosphene shape, size , and position did not vary much between electrode configurations (monopolar, common ground, hexagonal) or with pulse parameters. Pulse rates of 220 Perceived position of phosphenes in space varied with head position and eye gaze direction. Phosphenes tended to become larger and/or more intense as charge per phase was increased. Several phosphene maps using monopolar and common ground electrode configurations were constructed from the data and used to create small sets of recognisable stimuli representing numerals or letters of the alphabet. It was found that the order of stimulation of electrodes in an interleaved pattern affected the combined percepts.

Conclusions: A suprachoroidal retinal prosthesis with relatively large electrodes produces distinct phosphenes when stimulated in monopolar or common ground electrode configurations, and these phosphenes can be used to "paint" distinctive shapes.

Keywords: 713 shape, form, contour, object perception • 688 retina • 641 perception