Development of an electroencephalogram (EEG) protocol to map electrically-elicited visual responses in blind patients implanted with the Argus II retinal prosthesis

Abstract

Purpose : Long-term visual deprivation often leads to cortical reorganization. It is unclear how electrically-elicited visual perception is represented cortically for blind people who use visual prosthesis devices. This study aims to develop a protocol using electroencephalogram (EEG) to evaluate the retinotopic mapping in users of the Argus II retinal prosthesis (Argus II).

Methods : Patients who were blind due to end-stage retinitis pigmentosa and were implanted with Argus II were invited to participate in this study. In our protocol we bypassed the video signal from the Argus II camera and delivered pre-programmed visual patterns from a computer to the epi-retinal electrodes. Based on fundus photos, we designed patterns to activate three electrodes that were located in the upper retina, corresponding to the lower visual field, or three in the lower retina that corresponds to the upper field. The brain activity was recorded using a 32-channel BioSemi EEG recording system. The duration of each trial was 45 seconds and consisted of 30 stimulation frames. Each stimulation frame lasted 33 ms. Twenty trials were recorded with patterns in the upper and lower retina each. The EEG signal was filtered and detrended to remove background noise and linear drift, synchronized with the onset of the stimulation, and averaged across stimulations. Waveforms elicited by stimulating the upper and lower retina were compared.

Results : Initial results from one subject revealed a visual-field dependent polarity reversal in a component with an onset near 40 ms and peak latency near 55 ms. This component was observed in occipito-parietal sites. The polarity reversal property, latency, and cortical distribution of this component are reminiscent of the C1 component observed in normally-sighted subjects. C1 is the earliest visual-evoked component and is thought to originate from retinotopically mapped visual areas.

Conclusions : Our EEG protocol provides a practical way to evaluate the retinotopic mapping that distinguishes different parts of the visual field in the early visual cortex of retinal implant users. Developing methods to measure the preserved mapping is important for understanding how blind patients utilize the spatial information provided by retinal implant.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.