Page 365 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 359

2 CURRENT RETINAL IMPLANT TECHNOLOGIES

Retinal prostheses aim to restore visual capacity lost due to degener-
ation of the photoreceptor cells in the retina. The photosensitive cells con-
stitute the outer nuclear layer of the retina and include two functional types
of cells: rods and cones (Fig. 2A). The rods, positioned primarily in the
peripheral areas of the retina, are very sensitive to light and can be triggered
by a single photon (Hecht et al., 1942). Therefore, at low light levels, for
example, at night, visual signals are primarily initiated by the rod photore-
ceptors. The cone photoreceptors are located primarily at the macula (center
of the retina) and account for high acuity day vision and color experiences.
Cones require a significantly higher number of photons in order to produce
a signal, and can be distinguished into three different types according to their
pattern of response to short, medium, and long wavelengths in the visible
light range (Hurvich, 1981; Lennie and D’Zmura, 1987). The average
human retina contains about 4.6 million cone cells, with peak foveal density
2
of 199,000cones/mm , 92 million rod cells, and about 1.07 million ganglion

Fig. 2 Schematic representation of (A) organization of cells in the retina and (B) retinal
device implantation strategies.