Retinal Prosthesis Dissertation - Student Home Pages

1.1.1 Sub retinal
The sub retinal approach is to replace the retina by an array of micro photodiodes
underneath the retina (the side adjacent to the choroid) to channel natural sunlight
[31-40], the concept is to replicate the processing of the retina. Essentially a
subretinal implant would be inserted into the space normally occupied by
photoreceptors in a healthy retina. The array of micro photodiodes then provides the
stimulation data to the retinal implant. Most, if not all, sub retinal approaches rely on
converting the colours of light to greyscale prior to stimulation [41]. This has an
inherent flaw as the biological retinal actually processes incoming colour [42] in
terms of four perceptions of colour namely red, green, blue and yellow and processes
those colours to target `red’ cones, `green’ cones and `blue’ cones. These cones in
turn propagate colour intensity of each of those planes of colour to specific retinal
ganglion cells arranged in a regular topographical arrangement. The seriousness of
this inherent flaw depends quite literally on your point of view inasmuch as to be
truly biomimetic i.e. to imitate nature, a retinal prosthesis should target cones; for
colour vision. However it seems to be perceived wisdom that achromatic targeting is
of adequate sufficiency. The technological limitations driving this approach have
been twofold: firstly electrode size; and secondly extra wiring required for the colour
approach. Until recently electrode size has not been small enough; in vivo, to
accommodate average parvocellular cell size of 10µm and in some cases average
magnocellular cells of 80µm [43] i.e. although in vitro electrode size of 10µm has
been utilised, typically electrode size in vivo has been typically 50µm or in some
case 100µm. As the foveola (fovea pit) extends to 200µm diameter in the centre of
the fovea centralis, where the predominance of cones governs visual acuity, it is
nonsensical to map those signals out to 50µm electrodes; for colour vision, [5-7, 9,
10, 12-14, 44-48] where the RGC axons are about 1µm in diameter [49]. In the
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