Prosthetic Vision Simulation in Fully and Partially Sighted Individuals - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 72

the leading cause of blindness among individuals over age 50. About 15 million
more Americans are pre-symptomatic. For RP there is currently no effective
treatment or cure. Early symptoms of this retinal degeneration include night
blindness and increasing loss of peripheral vision. These symptoms most often
begin in the first three decades of life and are followed by a progressive loss
of vision over several decades, often leading to complete blindness. [2]. The
disorder is inherited and about 100,000 people in the US are affected. Worldwide
one in every 4000 persons is afflicted with RP.
Blindness caused by these disorders is due to failure of the outer retina. i.e. the
photoreceptors and retinal pigment epithelium (RPE) layers. Secondary nerve
cell layers throughout the retina appear to remain viable for a period of years
or even decades [3]. Recent studies by Marc et al. show that deprivation of
input from the photoreceptor layer leads to rewiring in the nerve cell layer [4].
This plasticity results in new connections of the secondary neurons which may
extend up to several hundred micrometers, corresponding to approximately 1 in
visual space. This has an important implication for prosthetic vision: Either the
rewiring of the secondary neurons has to be stopped as early as possible or the
wearer of a visual prosthesis may at best see only a phosphene pattern with size
and spacing on the order of 1 .
A prosthesis could replace the function of the degenerated photoreceptors by
stimulating the remaining secondary retinal neurons, the optic nerve fibers, or
the visual cortex. Over the past few years, several groups have shown that it is
possible to stimulate the inner retina with small electrodes, producing a sensation
of points of light called phosphenes. The location of the stimulated area on the
retina and the visual field location where the elicited phosphene is perceived
match closely [5, 6].
In the experiments reported here, we assumed that the wearer of a retinal
prosthesis will be able to see a regular field of fuzzy dots. With this assumption
in mind, we modeled prosthetic vision by convolving camera images with a very
low-resolution spatial grid filter, in real time, and presenting the results to sighted
volunteers. The experiments included tasks in which vision had to guide motor
activities (hand-eye coordination). Subjects' performance under these conditions
of extremely poor vision was recorded.

Methods

The experiments were performed during an internship of Matthias Walter at the
Lions Vision Center, Department of Ophthalmology, Johns Hopkins University,
Baltimore, MD, USA.

Equipment
The headset in this study is a modified Low Vision Enhancement System (LVES;
no longer in production) head-mounted display (HMD). It has a 36 × 48 field of