Retinal Prosthesis Dissertation - Student Home Pages

Chapter 3 Concepts
An epiretinal implant has no light sensitive areas; unlike subretinal implants, but
typically receives electrical signals from a distant camera and processing unit outside
of the body. Electrodes in the epiretinal implant then directly stimulate the axons of
the inner layer ganglion cells that form the optic nerve. The epiretinal implant is
effectively a readout chip receiving electrical signals containing image information
from a camera and processing unit, and is electrically coupled to the ganglion cell
axons. This project will concentrate on the processing [177-182] unit outside of the
body but will be novel in the sense that it will use colour information of scenes
viewed as opposed to achromatic approaches taken in every other work to date.
Colour is an extra challenge as the RGC axon needs to be targeted in an individual
sense and as the RGC axon diameter is of the order of 1µm/2.5µm [49 {Schröder,
1988 #257]} use of 50µm/100µm electrodes is not an option as might be the case
with achromatic stimulation. Colour stimulation in that 10% of axons that are
devoted to the fovea is now viable due to the order of magnitude decrease in
electrode sizing [29]. A possible reason, among others, for the appearance of
coloured phosphenes in achromatic retinal prostheses may be this blanket coverage
of RGCs including those comparatively few expecting colour information as
opposed to the preponderance of nerve fibres stimulated by the rods. Another issue
with colour is three times the `wiring’ is needed for the same pixel size as is needed
for the achromatic approach. However targeting the high acuity region of the fovea
with its high concentration of RGCs; evolved to process colour from the cone
photoreceptors is the more biomimetic and natural approach. In other words we were
designed to see in colour not shades of grey.
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