Sensory Physiology 313- The rods provide black-and-white vision under
conditions of low light intensity. At higher light
intensity, the rods are bleached out and the cones
provide color vision. - The pigment epithelium has many functions that are
required by the photoreceptors in the retina.
a. The pigment epithelium phagocytoses shed outer
segments of the rods and cones, absorbs stray light,
and has many other important functions.
b. The photoreceptors cannot convert all- trans retinal
back into 11- cis retinal; this is done by the pigment
epithelium.
c. The 11- cis retinal is then moved back into the
photoreceptors, where it can associate with opsin to
regenerate the photopigment; this is known as the
visual cycle of retinal.
B. In the dark, a constant movement of Na^1 into the rods pro-
duces what is known as a “dark current.” - When light causes the dissociation of rhodopsin, the
Na^1 channels become blocked and the rods become
hyperpolarized in comparison to their membrane
potential in the dark. - When the rods are hyperpolarized, they release less
neurotransmitter at their synapses with bipolar cells. - Neurotransmitters from rods cause depolarization of
bipolar cells in some cases and hyperpolarization of
bipolar cells in other cases; thus, when the rods are in
light and release less neurotransmitter, these effects are
inverted.
C. According to the trichromatic theory of color vision, there
are three systems of cones, each of which responds to one of
three colors: red, blue, or green. - Each type of cone contains retinene attached to a
different type of protein. - The names for the cones signify the region of the
spectrum in which the cones absorb light maximally.
D. The fovea centralis contains only cones; more peripheral
parts of the retina contain both cones and rods.
- Each cone in the fovea synapses with one bipolar cell,
which in turn synapses with one ganglion cell.
a. The ganglion cell that receives input from the fovea
thus has a visual field limited to that part of the retina
that activated its cone.
b. As a result of this 1:1 ratio of cones to bipolar cells,
visual acuity is high in the fovea but sensitivity to low
light levels is lower than in other regions of the retina. - In regions of the retina where rods predominate, large
numbers of rods provide input to each ganglion cell
(there is a great convergence). As a result, visual acuity is
impaired, but sensitivity to low light levels is improved.
E. The right half of the visual field is projected to the left half
of the retina of each eye. - The left half of the left retina sends fibers to the left
lateral geniculate body of the thalamus. - The left half of the right retina also sends fibers to the
left lateral geniculate body. This is because these fibers
decussate in the optic chiasma.
3. The left lateral geniculate body thus receives input from
the left half of the retina of both eyes, corresponding
to the right half of the visual field; the right lateral
geniculate receives information about the left half of the
visual field.
a. Neurons in the lateral geniculate bodies send fibers
to the striate cortex of the occipital lobes.
b. The geniculostriate system is involved in
providing meaning to the images that form on the
retina.
4. Instead of synapsing in the geniculate bodies, some
fibers from the ganglion cells of the retina synapse in
the superior colliculus of the midbrain, which controls
eye movement.
a. Because this brain region is also called the optic
tectum, this pathway is called the tectal system.
b. The tectal system enables the eyes to move and track
an object; it is also responsible for the pupillary
reflex and the changes in lens shape that are needed
for accommodation.10.8 Neural Processing of Visual
Information 307
A. The area of the retina that provides input to a ganglion cell is
called the receptive field of the ganglion cell.
1. The receptive field of a ganglion cell is roughly circular,
with an “on” or “off ” center and an antagonistic
surround.
a. A spot of light in the center of an “on” receptive field
stimulates the ganglion cell, whereas a spot of light
in its surround inhibits the ganglion cell.
b. The opposite is true for ganglion cells with “off ”
receptive cells.
c. Wide illumination that stimulates both the center and
the surround of a receptive field affects a ganglion
cell to a lesser degree than a pinpoint of light that
illuminates only the center or the surround.
2. The antagonistic center and surround of the receptive
field of ganglion cells provide lateral inhibition, which
enhances contours and provides better visual acuity.
B. Each lateral geniculate body receives input from both eyes
relating to the same part of the visual field.
1. The neurons receiving input from each eye are arranged
in layers within the lateral geniculate.
2. The receptive fields of neurons in the lateral geniculate
are circular, with an antagonistic center and surround—
much like the receptive field of ganglion cells.
C. Cortical neurons involved in vision may be simple, complex,
or hypercomplex.
1. Simple neurons receive input from neurons in the lateral
geniculate; complex neurons receive input from simple
cells; and hypercomplex neurons receive input from
complex cells.
2. Simple neurons are best stimulated by a slit or bar of
light that is located in a precise part of the visual field
and that has a precise orientation.