Challenges in Realizing a Chronic High-Resolution Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 129

Challenges in Realizing a Chronic
High-Resolution Retinal Prosthesis
Wentai Liu 1 , Mohanasankar Sivaprakasam 1 , Guoxing Wang 1 , Mingcui
Zhou 1 , James D. Weiland 2 , and Mark S. Humayun 2
1 Department of Electrical Engineering, University of California
2 Department of Ophthalmology, University of Southern California
Introduction
Electrical stimulation has been proposed by several research works as a means
of restoring vision in blind patients whose vision is impaired due to retinitis
pigmentosa (RP) and age-related macular degeneration (AMD). RP affects the
rods (used in night vision) first and then the cones (used in ambient daylight
levels). AMD results from abnormal aging of the retinal pigment epithelium and
retina. Persons with AMD will start to have distorted vision and eventually lose
most of the vision in the central 30 . RP is a collective name for a number of
genetic defects that result in photoreceptor loss. In both diseases, the vision is
impaired due to the damage to the photoreceptors that convert photons to neural
signals [1]. Post-mortem evaluations of retina with RP or AMD have shown that
a large number of cells remain healthy in the inner retina compared to the outer
retina [2]. Further, electrical stimulation of humans with RP and AMD results in
the perception of light; so the neural cells can be activated, providing the hope
of restoring lost vision in blind persons [3]. A chronic implant with 16 electrode
sites on the retina in three blind patients has yielded promising results [4]. After
being implanted with the prosthetic device, the patients were able to detect
motion of a white bar (up, down, left, or right), detect a rectangular object, count
objects, discriminate the orientation of two white bars in an 'L' configuration as
to where the corner of the L was positioned, and discriminate between a dessert
plate, a coffee cup, and a plastic knife [5]. The success rate of these simple
visual tasks differed between subjects and the difference is attributed possibly to
the age and the number of years of blindness. The results of acute and chronic
studies have encouraged several research and development efforts for realizing
chronically implantable, high-resolution retinal prostheses [6].
One implementation of such a retinal prosthesis is shown in Figure 7.1. An
external camera, which preferably can be worn on the glasses, captures the
image in the field of vision of the patient. This image is processed through
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