Effects of Stimulus Parameters on Phosphene Appearance in Epiretinal Prostheses

Abstract

Purpose : The shapes of phosphenes elicited by epiretinal prostheses have been shown to depend on stimulus parameters as well as the retinal location of the stimulating electrode. While previous studies have individually documented some of these effects, there is not yet a single model that can predict phosphene appearance across a wide range of electrical stimuli. To fill this gap, we conducted a meta-analysis across multiple modalities of patient data to comprehensively document and model the effects of stimulus amplitude, frequency, and pulse duration on phosphene appearance.

Methods : We first aggregated data from four psychophysical (phosphene drawings, size and brightness ratings) and one electrophysiological study (retinal ganglion cell activation maps) to investigate the effect of stimulus amplitude, frequency, and pulse duration on phosphene size, brightness, and streak length across 7 different Argus I/II users (Second Sight Medical Products) as well as 15 wild-type rat retinas. We then developed a simple phosphene model using a simulated map of retinal nerve fiber bundles combined with a few simple equations constrained by our combined dataset. We evaluated per subject how well the model predicted phosphene area, orientation, and ellipse major/minor axis length on held-out electrodes and compared the results to previous models.

Results : Phosphenes grew brighter (r=0.64, p<0.01), larger (r=0.46, p<0.001), and less elongated (r=-0.14, p<0.05) with increasing amplitude, grew brighter (r=0.77, p<0.001) with increasing frequency, and grew brighter (r=0.85, p<0.001), larger (r=0.85, p<0.001), and less elongated (r=-0.84, p<0.001) with increasing pulse duration (Table 1). When fit to data from individual patients, the model accurately predicted brightness as a function of amplitude (R²=0.91) and frequency modulation (R²=0.97) as well as size for amplitude (R²=0.97). The model also generalized to brightness ratings from new subjects (R²=0.61).

Conclusions : Through an analysis of psychophysical and electrophysiological data, we modeled the effects of various stimulus parameters on phosphene appearance in epiretinal prostheses. Overall this work is an important step towards predicting visual outcomes across a wide range of stimuli.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.

 

Table I: Effects of stimulus parameters on phosphene appearance
References: 1. Horsager et al., 2009; 2. Greenwald et al., 2009; 3. Nanduri et al., 2012; 4. Weitz et al., 2015; 5. Nanduri 2011