Page 366 - Handbook of Biomechatronics

360 Lilach Bareket et al.

cells (Curcio and Allen, 1990; Curcio et al., 1990). Besides the photorecep-
tor layer, the retinal tissue is composed of two more layers of nerve cell bod-
ies and two layers of synapses (Fig. 2A). The middle nuclear layer of the
retina includes bipolar, horizontal, and amacrine cells and the outer layer
consists of RGCs. Between these three structures are two layers where
the neurons synapse, the outer and inner plexiform layers (OPL and IPL,
respectively) (Fig. 2A). The bottom layer is the retinal pigment epithelium
(RPE), which regulates nutrients and waste exchange. In a healthy retina,
the photoreceptor cells transduce light into biochemical signals that propa-
gate through the mid-layers up to the output neurons, the ganglion cells.
The axons of the RGCs converge into the ON that delivers the visual signals
to higher visual centers, the LGN and ultimately the visual cortex.
The most prevalent degenerative disorders of the retina are RP and
AMD, both leading to progressive reduction in vision where the typical out-
come is legal blindness (Ferris et al., 1984; Hartong et al., 2006; Wright et al.,
2010). RP may further decline to profound blindness (no useful vision)
(Hartong et al., 2006). AMD primarily affects people aged 60 and older.
Currently, approximately 170 million people live with AMD, of which 2
million are blind (Mariotti, 2012). With the aging of the global population,
this number is expected to rise to become 196 million by 2020 and 288 mil-
lion by 2040 (Wong et al., 2014). RP is a group of inherited pathologies,
afflicting approximately 1.5 million people worldwide 25% of RP patients
are legally blind. The majority of gene defects leading to RP are expressed in
rod photoreceptors or the RPE, in the peripheral areas of the retina
(Hartong et al., 2006). Initial RP symptoms include poor night vision,
peripheral vision loss followed by damage, and remodeling of foveal cone
cells. Further escalation leads to “tunnel vision” and in many cases, a com-
plete loss of vision. AMD mainly affects the cones around the fovea,
impairing the center of the visual field (Ferris et al., 1984).
To replace the function of the lost photoreceptive tissue, retinal prosthe-
ses apply electrical stimulation to the residual neural components of the ret-
ina. In order to capture the visual scene, retinal prostheses fall into two
categories. They either employ an external camera (Humayun et al.,
2012) or capture the visual scene via an array of photodiodes integrated with
the electrodes implanted in the vicinity of the retina (Stingl et al., 2015). The
camera can be mounted on spectacles worn by the patient. Because the loca-
tion of the camera is fixed, the perceived image is not coordinated with eye
movements (saccades and smooth pursuit) and with the direction of gaze
(Sabbah et al., 2014). To compensate for the lack of natural coordination,