Towards a Retinal Prosthesis Using a Silicon Retina

Abstract

Abstract: : Purpose: A partial visual ability is expected to be restored in blind patients of retinal degenerative diseases using a retinal prosthesis to stimulate electrically the survived neurons. The purposes are: 1) Developing an analog integrated circuit (silicon retina) emulating a function of the retinal circuit to be utilized as an image processing unit of the retinal prosthesis; 2) Studying spatio-temporal properties of ganglion cell responses to transretinal current stimuli to design a stimulating electrode array. Methods: The silicon retina was designed using analog CMOS integrated circuit technology. Ganglion cell responses to light and current stimuli were extracellularly recorded by an 8x8 planar electrode array. The current stimuli were transretinally applied to the retina using a tungsten electrode inserted into the outer retina and one of the planar electrodes underneath. Results: 1) A two-dimensional silicon retina with 40x46 pixels was fabricated. It mimics two classes of light-induced responses found in the inner retina: one is a sustained response with a center-surround antagonistic receptive field and another is a transient response with a wide receptive field. The silicon retina can perform a real time image processing in indoor illumination. 2) Physiological experiments showed two typical classes of electrically induced response of ganglion cells. One showed relatively constant latency of 1.5 to 4.5 msec with respect to the current amplitude which were suggested to be activated by direct stimulation of ganglion cell(n=5). Another showed a variable latency of 3.5 to 71.5 msec(n=51). The responses with relatively long latency in this class suggested indirect excitation induced by neurotransmitters released from presynaptic neurons. In most cases, the ganglion cells within about 300 microns from the stimulation site were excited by a stimulation charge around threshold. Axon excitation by the transretinal current stimulus showed a relatively low threshold but with a low prevalence of about 10% among all the induced responses. Conclusions: The silicon retina fabricated in the present study is applicable to the image processing unit of retinal prostheses. The spatial resolution of excited ganglion cells was about 300 microns around threshold when applied transretinally. The low prevalence of axon excitation may be due to its dependence on the orientation of external electrical field. The present study provides a possible design of the retinal prosthesis using the silicon retina.

Keywords: 554 retina