Biomedical Engineering Reference
In-Depth Information
reflexes, respectively. Neither of these structures is associated with vision as such, so
neural prostheses generally target the LGN or the visual cortex.
The LGN is a small structure about 7
2 mm located on the ventral side of the
thalamus and is often considered a relay point receiving input from the RGC and passing
the same on to the cortex. However, this is a simplification as the LGN consists of six
distinct processing layers that are visuotopically organized so that the image (primarily
from the fovea) propagates through the structure undistorted.
Most of the axons from the LGN project to the gyri lining the calcarine fissure shown
in Figure 7-5. This 2500 to 3500 mm
2
region is known as the primary visual cortex or visual
area (V1), indicating that it is the first region of visual processing at the cortical level. Five
cortical areas have a similar laminar structure consisting of six layers arranged in planes
extending from the pia mater through to the white matter below. Their total thickness is
about 2 mm in human beings. Visual processing is performed in both a columnar and
a horizontal manner. In these layers of area V1, the circular sensitivity of the RGCs is
elongated into a long ellipse that leads to a preference for bars of a particular orientation.
The other areas—V2, V3, V4, and V5—indicate a somewhat hierarchical structure of
visual processing that is not completely accurate as significant feedback occurs between
the levels and areas. After area V1, the organization of the visual pathway becomes more
complex and optimal visual stimuli become less clear. From a biomechatronic perspective,
these regions of higher-level feature extraction are not well enough understood to be used
as sites for visual prostheses.
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7
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7.3
MAIN CAUSES OF BLINDNESS
The main causes of untreatable blindness are age-related macular degeneration (AMD), re-
tinitis pigmentosa (RP), accidents, and cancers. Other causes include glaucoma, cataracts,
and diabetic retinopathy, but these can be treated or sometimes even prevented if treatment
starts at an early stage.
AMD is characterized by a progressive loss of photoreceptor cells in the macula
and then of central vision. For unknown reasons the pigmented epithelium of the retina
degenerates, leading to a subsequent degeneration of the photoreceptor layer and fluid
leakage into the neural portions of the retina.
RP is an inherited disease characterized by loss of peripheral and night vision. Once
again, the cause of the problem is unknown, but it results in the degeneration of rod
photoreceptors. In extreme cases, sufferers are left with only cone cells and therefore have
tunnel vision.
Despite the degradation of the outer layers of photoreceptors in both of these diseases,
the inner cell layers are often partially preserved, which gives retinal implants the potential
of restoring some vision.
7.4
OPTICAL PROSTHETICS—GLASSES,
THERMAL IMAGERS, NIGHT VISION
As discussed at the start of this chapter, eyeglasses are the most common optical prosthesis,
followed closely by contact lenses. A third extremely common prosthesis is the plastic
lens that replaces the cloudy one in cataract sufferers. The lenses in most eyeglasses are
