Differences in Thresholds between Light and Dark Phosphene-Inducing Electrode Groups of the Intracortical Visual Prosthesis (ICVP)

Abstract

Purpose : The Intracortical Visual Prosthesis (ICVP) is a recently developed cortical implant that stimulates modular wireless floating microelectrode arrays (WFMAs) with the goal of restoring vision, including the perhaps primary visual cortex (V1) and surrounding higher cortical areas. This study aims to explore how distinctions within electrode groups, specifically those eliciting light phosphene versus dark phosphenes, contribute to variations in electrode thresholds.

Methods : Data from 2022 to 2023 encompassed a single subject's assessment sessions, involving 16-electrode Wireless Floating Microelectrode Arrays (WFMAs). Sessions tested various electrode group configurations (single, double, quad) using stimuli of 70 Hz, 100 Hz, 140 Hz, or 200 Hz pulse trains with 400 ms or 500 ms durations and cathodic phases of 50 µs, 100 µs, or 200 µs. The Wilcoxon rank-sum test analyzed differences in thresholds between light and dark phosphene-inducing groups. Histograms were generated for each phosphene group, depicting threshold distributions for single and multi-electrode configurations.

Results : The statistical analysis revealed significant differences within the dark phosphene group across different frequency transitions: 70 Hz to 100 Hz (p=0.041), 100 Hz to 140 Hz (p=0.121), and 140 Hz to 200 Hz (p=0.004). Similarly, within the light phosphene group, significant differences were observed in the transition from 100 Hz to 140 Hz (p=0.002) and a trend towards significance in the transition from 140 Hz to 200 Hz (p=0.187). When comparing light and dark phosphene groups, notable distinctions were identified at 70 Hz (p=0.010), 100 Hz (p=0.001), 140 Hz (p=0.003), and 200 Hz (p=0.044). These findings suggest that electrode groups inducing dark phosphene generally exhibit higher thresholds compared to those inducing light phosphene.

Conclusions : Based on the different thresholds, we conclude that dark phosphenes generally exhibit higher thresholds compared to their light counterparts. Even in the lower range of the histogram, particularly below 30 or 40 uA, the discernible difference becomes evident. In summary, the higher thresholds of dark phosphenes suggest a minor role at low stimulation levels. Yet, as we progress in refining advanced image processing techniques, recognizing their influence becomes crucial for optimizing visual prosthetic technologies.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.