Electrically elicited visually evoked potentials (eVEPs) in Argus^® II prosthesis wearers

Abstract

 

Purpose

 

To investigate whether electrically elicited visually evoked potentials (eVEPs) can be used to establish input-output characteristics and predict subjective threshold of electrical stimulation in Argus^® II retinal prosthesis wearers.

 

Methods

 

We recorded eVEPs in three subjects while systematically varying stimulus level. Subjects provided feedback by rating the brightness and size of the perceived flashes of light (‘phosphenes’). Input-output functions were generated using eVEP amplitude and latency based of the first two positive peaks (P₁ and P₂). Correlation was determined using linear regression, followed by an F-test on the slope. eVEP thresholds were defined as the amplitude equaling 4 times the standard deviation (4×SD) of the eVEP waveform. We also investigated the effect of stimulating different retinal locations, the maximal feasible pulse rate, and adaptation (‘fading’).

 

Results

 

P₁ and P₂ amplitudes significantly increased as a function of subjective percept in all three subjects (linear regression and F-test, P< 0.05). Only 1 out of 3 subjects showed a significant decrease of eVEP latency with stimulus level and rating (P< 0.05). P₂ amplitude yielded accurate predictions of subjective threshold in all three subjects (table 1). Stimulating macular electrodes resulted in higher eVEP amplitudes and shorter latencies compared to more peripheral electrodes (RM ANOVA and Tukey’s post hoc test, P< 0.05), while subjective ratings were not different (P> 0.05). At pulse rates above ²/₃ Hz, eVEP waveforms became distorted and amplitudes declined. Subjective phosphene brightness decreased over time, which was reflected in P₁ amplitude (linear regression and F-test, P< 0.01), but not in P₂ amplitude (P> 0.05).

 

Conclusions

 

The eVEP P₂ amplitude is a robust measure for generating input-output relationships and is a fairly accurate predictor of subjective threshold. Pulse rates of up to ²/₃ Hz can be used for eVEP recordings. Retinal location affects eVEP amplitudes and latencies irrespective of subjective percept, which has to be taken into account when using the eVEP clinically. We envision that eVEPs may become a diagnostic and monitoring tool that can find important use as an objective measure for rehabilitation purposes.

 

 

Subjective threshold and interpolated eVEP P₁ and P₂ thresholds.

 

Keywords: 508 electrophysiology: non-clinical • 755 visual cortex