Retinal Prosthesis Dissertation - Student Home Pages

3.6 Current Retinal Implants
Commonality exists between sub retinal [195, 196] and epi-retinal approaches in
terms of stimulus pulse requirements to be delivered by the implants. The difference
between these two approaches is that the sub retinal implant is meant to substitute
for retinal processing within the outer retina whereas the epi-retinal implant bypasses
both the inner and outer retina and uses signals derived from a camera to produce the
stimulus pulsing.
3.6.1 Retinal structure
There are five major classes of neuron within the retina which can be considered to
lie within the seven layers of the following representation (Figure 1) of a cross
section of the retinal architecture; namely cones and rods, horizontal, amacrine and
retinal ganglion cells (RGCs). The outer retina extends from the pigment epithelium
to the boundary of the inner nuclear layer whilst the inner retina encompasses the
inner nuclear layer, the inner plexiform layer and the ganglion cell layer. The
synaptic layers housing ion channels are the outer plexiform layer (OPL) and the
inner plexiform layer (IPL). A sub retinal implant (section 1.1.1) [197] would
occupy the space of photoreceptors in a healthy retina i.e. adjacent to the choroid
(the retinal pigment epithelium (RPE) lines the choroid), the photodiodes taking the
place of photoreceptors and the stimulatory electrodes impinging at the boundary of
the inner nuclear layer and typically but not exclusively stimulating bipolar cells. An
epi retinal implant (section 1.1.2) [111] is tacked to the inner limiting membrane of
the retina at the axon side of the ganglion layer reached from the inside of the
eyeball.
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