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Vision is the perception of objects based
on the light that they emit or reflect.
Human eyes can only detect visible light -
a narrow range of electromagnetic radiation,
roughly from 400 to 750 nm in wavelengths.
The eye consists of optical components, which
work like a camera, capturing and focusing images;
and neural components that convert these images
into nerve impulses and send them to the brain.
The main optical components are the cornea,
the lens, and the iris. The cornea and the
lens refract light and focus the image on
the retina. The iris acts as an aperture,
it controls the amount of light that enters
the eye by adjusting the size of the pupil.
The neural components are the retina - a
light-sensitive tissue lining the inner
surface of the eye, and the optic nerve. Light
is absorbed by photoreceptor cells in the retina.
The optical information is then passed through
several cell layers, where it is converted into
action potentials and sent, via the optic
nerve, to the visual cortex of the brain.
The fovea is the central part of the retina
where the sharpest central vision is achievable.
The optic disk, where the optic nerve
leaves the eye, has no photoreceptor cells.
It corresponds to the blind spot in the
visual field. If an object falls on that spot,
it would generate no visual information. However,
instead of leaving a black hole in the vision,
the brain fills it in with visual
information from around the object.
The major photoreceptor cells of the retina
are rods and cones. Rod cells are responsible
for night vision. They can detect dim light,
but provide low-resolution images and cannot
differentiate colors. Cones function in bright
day light. They detect colors, and provide high
resolution details. There are 3 kinds of cones
named after the color that they absorb best:
red, green and blue. A color is perceived based
on proportions of signals coming from these cones.
Color blindness occurs when a person
lacks a certain kind of cones.
The ability of photoreceptor cells to detect light
is due to their light-receptor
molecules, called visual pigments.
It’s rhodopsin in rods, and iodopsins in
cones. These molecules consist of 2 components:
a protein called opsin, and a vitamin A-derivative
called retinal. The retinal component is
identical for all visual pigments, but the opsin
is different for rods and each type of cones.
Different opsins absorb different wavelengths,
allowing detection of different colors.
In the dark, there is a so-called
dark current in photoreceptor cells.
This is due to the presence of cGMP, which
permits a constant influx of sodium. The cells are
depolarized, they release the neurotransmitter
glutamate at the synapse with bipolar cells.
The retinal exists in 2 conformations:
cis and trans. In the dark,
the cis-form is bound to opsin, keeping
it inactive. As the retinal absorbs light,
it changes to trans-form and dissociates from the
opsin, which now becomes an active enzyme. The
enzyme degrades cGMP, sodium channel closes, dark
current stops and so does glutamate secretion.
The drop in glutamate tells the bipolar
cells that light has been absorbed.
The information is then transmitted
to ganglion cells - the only cells
within the retina that generate action
potentials and send them to the brain.
On average, each ganglion cell receives
signals from over a hundred of rods. This
degree of convergence is at the basis of the high
sensitivity of rod cells. A dim light produces
only a weak signal in a rod, but together,
hundreds of these signals converge and become
one strong signal acting on a single ganglion
cell. However, as the signal comes from a large
area of the retina, the image resolution is poor.
The cones have a much lower degree of convergence.
The fovea in particular has only cones and
no rods, and each cone conveys signal to one
ganglion cell. Because one ganglion cell receives
input from a very small area of the retina, this
setup produces high resolution images. But high
resolution comes with low sensitivity, because
each cone must be stimulated with a signal
strong enough to generate action potentials
in the ganglion cell. This also explains
why there is no color vision in dim light.
There exist other cell types that form
connections between photoreceptor cells,
or bipolar cells. They detect changes in light
intensity within an image and provide additional
information about contrast and edges of objects.
Some of the ganglion cells also absorb light
directly, but not for the purpose of forming
images. They transmit information about light
intensity to the brainstem, as part of the reflex
that controls pupil size; and to the hypothalamus,
as input for the sleep-wake cycle.
The bipolar cells are first-order neurons,
and ganglion cells are second-order neurons. The
axons of ganglion cells form the optic nerve.
The 2 optic nerves from the 2
eyes converge at the optic chiasm.
Here, the medial half of nerve fibers from
each eye cross to the other side of the brain.
Most of the fibers then continue to the thalamus
and synapse with third-order neurons, whose axons
project to the primary visual cortex. Some fibers
take a different route: they terminate in the
midbrain and are responsible for pupillary light
reflex and accommodation reflex, among others.
Note that objects in the left visual field
are perceived by the right side of the brain,
which also controls motor responses of the
body’s left side – the same side as the objects.
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