Spatial Distribution of Cortical Responses Elicited by Different Stimulating Protocols With Optic Nerve Stimulation in Rabbits

Abstract

Purpose: : Optic nerve stimulation with penetrating microelectrode array has been suggested as a possible way for visual prosthesis. The purpose of this study is to investigate the spatial distribution of electrically evoked potentials (EEPs) at visual cortex elicited by different stimulating protocols to optimize the spatial resolution of optic nerve stimulation.

Methods: : Craniotomy was performed on the rabbit occipital to expose the contra-lateral visual cortex to the operated eye. Then orbital surgery was performed to expose the optic nerve. For electrical stimulation, linearly configured three platinum-iridium wire electrodes insulated by Teflon were inserted into the optic nerve. Monopolar stimulation mode was performed between one electrode inserted into the optic nerve as a stimulating electrode and one steel needle electrode inserted into sclera as a return one. Bipolar stimulation mode was performed between one pair of the three electrodes inserted into the optic nerve. Charge-balanced cathode-first biphasic current stimuli with different strength were applied to the optic nerve via the stimulating electrodes. The EEPs were recorded with a 16-channel silver-ball electrode array at the visual cortex.

Results: : The experimental results showed that the spatial profile of EEPs decayed faster as the current strength increased. The full width at half height (FWHH) of the EEP spatial extend in visual cortex by 100uA bipolar stimulation was 5.2 ± 1.2mm, which was bigger than that by monopolar stimulation(4.3 ± 0.7mm). The FWHH elicited by 100uA bipolar stimulation increased from 4.9 ± 0.7 to 6.2 ± 1.1mm while the inter-electrode distance increased from 0.5 to 1mm. The spatial distributions of multi-channel EEPs by bipolar stimulation were similar to the combination of those by monopolar stimulation via the corresponding electrodes, depending on the distance between the two electrodes.

Conclusions: : Our study shows that the inter-electrode distance of the bipolar stimulation is a critical parameter to realize the localized stimulation to increase the spatial resolution in the visual prosthesis based on optic nerve stimulation with penetrating electrodes.

Keywords: electrophysiology: non-clinical • optic nerve • visual cortex