Biomedical Engineering Reference
In-Depth Information
Visual information is processed from the outer to the inner layers in a column-wise
process. That is, the activity of a ganglion cell is modulated by the photoreceptor and
bipolar neurons along a column from the outer to the inner retina. This ensures that the
visual map structure is preserved through all of the layers. In addition to this, horizontal
and amacrine cells integrate information across neighboring regions of visual space.
The fovea has the highest density of ganglion cells to photoreceptors—a ratio of two
to one—hence, this region has the highest visual acuity. The number of ganglion cells
per photoreceptor decreases with increasing radius from the fovea, and by the edge of
the macular region a large number of photoreceptors drives a single ganglion cell. At the
fovea, the receptive field size is of the order of a few minutes of arc, while even at the
edge of the macula the receptive field size can be as large as 5
◦
.
Most RGCs are optimally stimulated by a circular patch of light of one intensity
surrounded by an annulus of a different intensity. This takes the form of a patch of one
color surrounded by an annulus of another color for 80% of cells, or a light center and a dark
annulus or a dark center and a light annulus in about 8% of the cells. The remaining 12%
of retinal ganglion cells have an as yet uncharacterized function (Finn and LoPresti, 2003).
As shown in Figure 7-5, nerve signals travel from each eye along the corresponding
optic nerve to the back of the brain where vision is sensed and interpreted. The two optic
nerves meet at the optic chiasma, which is an area behind the eyes immediately in front
of the pituitary gland and just below the cerebrum. There, the optic nerve from each eye
divides to form the optic tract with half of the nerve fibers from each side crossing to the
other side and continuing to the subcortical targets at the back of the brain. Thus, the right
side of this subcortical region receives information through both optic nerves for the left
field of vision, and the left side of the brain receives information through both optic nerves
for the right field of vision. The center of these fields of vision overlaps.
Indications suggest that the fibers of the optic nerve are visuotopically organized with
the upper retina (lower visual field) represented along the dorsal side of the nerve, the
central retina along the lateral side, and nasal visual field along the medial side. However,
this seems to vary somewhat along the length of the nerve (Finn and LoPresti, 2003).
The axons of the retinal ganglion cells target three subcortical structures: (1) the
superior colliculi; (2) the pretectum; and (3) the lateral geniculate nucleus (LGN). The
vast majority (
90%) target the LGN. The superior colliculi and the pretectum are located
on the roof of the midbrain and are associated with saccadic eye movements and pupillary
>
FIGURE 7-5
Nerve fiber path to
the neural cortex
showing potential
situations for visual
prosthetic implants.
[Adapted from (Foley
and Martin, (2006).]
