In conventional electrical visual prostheses, current spread produces diffuse patches of response, whereas optical stimulation of gold nanorods bound to genetically modified photoreceptors has recently shown great promise. Here we demonstrate that Retinal Ganglion Cells (RGCs) can be optically stimulated directly via gold nanorods without the need for genetic modification. Single-cell responses from explanted rat retinae exposed to gold nanorods were recorded by patch clamping. Near infrared laser pulses of 100 and 500 µs evoked robust stimulation, whilst longer 200 ms pulses were biased towards OFF-type inhibition. Differential modulation of ON- and OFF-type RGCs is essential for future high-acuity prostheses.
Conventional electrically-stimulating retinal prostheses exhibit low stimulus resolution due to current spread, which precludes high-acuity vision. Optogenetic neuromodulation techniques offer a high stimulus resolution, and are uniquely well-suited to exploit the optical accessibility of the retina. However, such techniques often exhibit a low stimulus response rate, and may risk phototoxic damage during chronic applications. The present study uses combined optogenetic and electrical co-stimulation to reduce the current threshold requirements, thereby limiting current spread. A time delay in the electrical stimulus was found to improve stimulation efficacy, and response probability was increased during co-stimulation at higher pulse train frequencies.
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