gms | German Medical Science

Objective: Retinal prostheses work by electrically stimulating residual retinal neurons, providing artificial input to the visual system in patients with severe photoreceptor loss. The resolution provided by retinal implants is still severely limiting for recipients. A major limiting factor is presumed to be a lack of control of the spread of activation within the retina in response to monopolar (MP) electrical stimulation. This study investigates the use of a novel technique called focused multipolar (FMP) stimulation as a way to restrict current spread in the retina. Unlike MP stimulation, FMP stimulates all electrodes simultaneously with weighted positive and negative currents in order to better control excitation in the retina.

Methods: Normally-sighted cats (n=4) were implanted with a suprachoroidal electrode array containing 42 stimulating electrodes. Neural activity was recorded from the contralateral visual cortex to in response to MP and FMP retinal stimulation at varying currents. FMP weights were determined by measuring the impedances between the electrodes and calculating the current required to cancel out voltage spread from other electrodes.

Results: When measuring the cortical selectivity of neural responses (n=55 retinal electrodes), FMP stimulation showed a trend of higher selectivity compared to MP stimulation, however this was not significant. The lowest cortical thresholds for each stimulating electrode were found to be on average higher for FMP compared to MP stimulation. However, when inferring the spread in the retina using voltage recordings off the retinal array, FMP stimulation showed reduced voltages in regions surrounding the central stimulating electrode.

Discussion: FMP stimulation is a promising stimulation technique to reduce the spread of activation for a retinal prosthesis and possibly increase resolution. Further analysis is needed to understand why cortical selectivity measures do not show reduced spread of excitation in the brain.