Biomedical Engineering Reference
In-Depth Information
image and stimulates the brain directly with electrical signals, either by penetrating into
or placing electrodes on the surface of the optic nerve or the visual cortex.
Sensory substitution prostheses based on ultrasound technology have been available
for many years because that technology was really the only type that was portable and
reasonably low cost. In the last decade, however, the advent of low-cost cameras based on
charge coupled devices (CCD), and more recently complementary metal oxide semicon-
ductor (CMOS) arrays has spurred the development of camera-based systems.
Scientists knew as early as 1918 that touching electrodes to the visual cortex of
conscious patients produced spots of light (phosphenes), and by the 1940s researchers had
established the concept of artificial electrical stimulation of the visual cortex. In the late
1960s, British scientist Giles Brindley experimented with a system that placed electrodes
on the brain's surface. When specific areas of the brain were stimulated using relatively
high currents, the blind volunteers all reported “seeing” phosphenes that corresponded
approximately to where they would have appeared in space.
Encouraged by this work, the National Institutes of Health (NIH) supported research
to develop and deploy an interface based on ultrafine wire (25 to 50
m in diameter)
densely populated with electrode sites that could be implanted deep into the visual cor-
tex. This innovation decreased the current density required, compared with Brindley's
original design, and was therefore less damaging to the surrounding tissue. New elec-
trode technology was developed that could be safely implanted in animals to electrically
stimulate, and passively record, electrical activity in the brain. These efforts produced
significant advances in neurophysiology, with the publication of hundreds of papers in
which researchers attempted to develop electronic interfaces to the brain.
By 1995, NIH researchers decided they were ready to proceed to human testing
of an intracortical visual prosthesis. A total of 38 electrodes, connected to fine wires
penetrating the scalp, were implanted in the brain of a 42-year-old woman. Although
blinded by glaucoma 22 years earlier, she was still able to sense phosphenes using the
electrical stimulation and eventually became adept at perceiving those dots under a variety
of stimulation patterns.
The state-of-the-art continues to advance, with more sophisticated retinal and brain
implants along with their associated electrode arrays and image processing networks in
production. However, it will be many years, if ever, before any system can approach the
resolution and dynamic range of human vision.
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7.2
ANATOMY AND PHYSIOLOGY OF THE VISUAL
PATHWAY
Light enters the eye through the cornea, a transparent dome on the front surface. The
cornea serves as a protective covering for the front of the eye and, because it is curved,
also helps focus light on the retina. After passing through the cornea, some of the available
light travels through the pupil, a hole through the iris. The iris is the eye's circular, colored
area that controls the amount of light that enters by dilating and constricting the pupil like
the aperture of a camera lens. This adjustment is controlled by the action of the pupillary
sphincter muscle and the dilator muscle.
Behind the iris sits the lens, which by changing its shape focuses light onto the retina.
Through the action of small ciliary muscles that surround it, the lens becomes thicker to
