Reducing spread of neural activation using focused multipolar retinal electrical stimulation in sighted and blinded cats

Abstract

Purpose : The spatial resolution provided by even the most advanced retinal prostheses is severely limiting for recipients. A major factor is presumed to be broad spread of activation within the retina in response to monopolar (MP) electrical stimulation. This study hypothesized that compared with MP stimulation of a single retinal electrode, focused multipolar (FMP) stimulation of all electrodes simultaneously with weighted positive and negative charges would result in a narrow electric field and reduced spread of neural activation.

Methods : Four cats were unilaterally blinded for a minimum of 14 weeks via intravitreal injection of adenosine triphosphate. Retinal structure and function was assessed using optical coherence tomography (OCT) and dark-adapted flash electroretinogram (ERG). Ten cats (6 normal-sighted and 4 blinded) were bilaterally implanted with suprachoroidal electrode arrays. In response to MP and FMP retinal stimulation at varying charge levels, voltage recordings were taken on surrounding retinal electrodes as a measure of the electric field spread and multiunit activity was recorded from the visual cortex to measure neural activation spread.

Results : FMP stimulation exhibited a narrower electric field through a significant reduction in retinal voltage spread (paired t-test p<0.001) as well as a reduced cortical activation spread (two-way ANOVA p<0.001) compared with MP stimulation in normal-sighted cats. In blinded eyes, OCT imaging showed photoreceptor degeneration in regions overlying the electrode array and a reduction in ERG a-wave responses by 50-85%. While FMP stimulation still showed reduced cortical activation spread when stimulating the normal-sighted eyes of the blind cohort, no such benefits were observed when the blind eyes were stimulated. Cortical thresholds for each stimulating electrode were found to be higher for FMP compared to MP stimulation (paired t-test p<0.001) in both sighted and blinded cats.

Conclusions : The study found FMP stimulation of the retina can constrain the electric field in two dimensions, and elicit a reduced spread of cortical activation but only in normal-sighted cat eyes. While these results question the effectiveness of FMP stimulation in improving resolution for retinal prosthesis recipients, psychophysical experiments are required to verify this notion.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.