Visual acuity is the eye's ability to
distinguish two points that are very close to each other.
This ability depends on many factors, but especially on the
precision of the eye's refraction
and the ratio
of cones to rods at a given location on the
retina.
THE EYE
Functionally,
the eye can be compared with a camera, and the retina
with photographic film. The purpose of the camera is to
focus an image that is sharp and neither too dark nor
too light onto the film. The photographer uses the camera's
focus ring to bring the image into focus and its diaphragm
to ensure that the amount of light entering the camera
is just right for the sensitivity of the film being used.
Your eye does exactly the same thing, all day long, without your even being aware of it! Your cornea and lens provide the focus,
while the iris adjusts to let the optimal amount of light reach your retina. But your retina, with its many layers of neurons, is far more complex and sensitive than any photographic film. The two are similar, however, in that the image focused on both of them is inverted.
The main optical components of the eye are as follows. First comes the cornea, the transparent, slightly convex outer surface at the centre of the eye. The cornea does not have any blood vessels, so its takes its nutrients from the fluid behind it, known as the aqueous humour, as well as from the fluid in front of it, the tears, which are spread across your cornea when you blink your eyelid.
Next comes the pupil, the opening that lets light enter the eye and ultimately reach the retina. The pupil appears black because of the layer of black pigmented cells that line the back of the eye and absorb the light.
The diameter of the pupil is controlled by the iris, a circular muscle whose pigmentation gives the eye its colour and whose contraction lets the eye adapt continuously to changing light conditions. On a dark night, your pupils are big and black, because your irises open wide to let in as much as possible of the little light available. This reaction is called the pupillary reflex. You can observe it easily yourself, by watching your eyes in a mirror while you turn a nearby light on and off.
After passing through the pupil, the light goes on through the lens, which is suspended between the aqueous humour and the vitreous humour, the fluid that fills the inside of the eye.
The lens in turn focuses the light rays onto the retina, lining the back of the eye. The retina converts the image formed by the light rays into nerve impulses. The optic nerve, composed of the axons of the retina's ganglion cells, then transmits these impulses from the eye to the first visual relay in the brain.
THE TARGETS OF THE OPTIC NERVE
The axons of the retina's
ganglion cells collect in a bundle at the optic
disc and emerge from the back of of the eye to form the optic
nerve. The optic nerve is the pathway that carries the
nerve impulses from each eye to the various structures in the brain
that analyze these visual signals.
The optic nerves of the
two eyes emerge from their optics discs and intersect at the optic
chiasm just in front of the pituitary gland. In the
optic chiasm, some of the axons from the two retinas undergo decussation:
they switch sides to allow crossed processing of the visual
signals.
The axons from the nasal side of each retina cross sides in
the optic chiasm so that the left
half of the field of vision is perceived by the right cerebral
hemisphere, and vice versa. But because the visual information
that reaches the temporal side of each retina comes from the
opposite side of the visual field to begin with, the axons
from this side of the retina do not need to cross sides. Instead
they proceed straight ahead through the optic tract.
The vast majority of the nerve fibres in the optic tract project
to the lateral
geniculate nucleus (LGN) in the dorsal part of the thalamus.
The LGN is the main relay in the pathway to the primary visual
cortex. The projection from the LGN to the visual cortex is called
the optic radiation. Because damage at any point
along the pathway from the retina to the cortex results in some
degree of blindness, this is clearly the pathway through which conscious
visual perception takes place in human beings.
People whose primary
visual cortexes have been damaged consider themselves to
be blind and unable to discern anything in their visual
environment. But if you ask these people to "take
a chance" and point their finger at a dot of light
in space, they will point straight at this target. And
the data show that this result is not random. This phenomenon
is called blind vision.
Thus these people are still processing some visual information,
even though part of the neural pathways in V1 have been destroyed.
The mechanisms by which they do so may involve little understood
transfer pathways that bypass V1, as well as certain
subcortical visual nuclei. Some researchers also
believe that the dorsal
visual pathway plays a role in this phenomenon.
THE VARIOUS VISUAL CORTEXES
The image captured
by each eye is transmitted
to the brain by the optic
nerve. This nerve terminates on the cells of the lateral
geniculate nucleus, the first relay in the brain's
visual pathways. The cells of the lateral geniculate nucleus
then project to their main target, the primary
visual cortex. It is in the primary visual cortex
that the brain begins to reconstitute the image from the receptive
fields of the cells of the retina.
Also known as the striate cortex, or
simply V1, the primary visual cortex is located
in the most posterior portion of the brain's occipital
lobe . In fact, a large part of the primary visual cortex cannot
be seen from the outside of the brain, because this cortex lies
on either side of the calcarine fissure. This
fissure, however, is clearly visible in a sagittal section made
between the two cerebral hemispheres.
The primary visual cortex, with
its distinctive cell architecture, also corresponds to Area
17 described by the anatomist Brodmann in the early
20th century (link to Tool module from the sidebar to the left).
The primary visual cortex sends a large proportion of its connections
to the secondary visual cortex (V2),
which consists of Brodmann's areas 18 and 19. Though most of the
neurons in the secondary visual cortex have properties similar
to those of the neurons in the primary visual cortex, many others
have the distinctive trait of responding
to far more complex shapes.
The analysis of visual
stimuli that begins in V1 and V2 continues through two major
cortical systems for processing visual information. The first
is the ventral
pathway, which extends to the temporal lobe and
is thought to be involved in recognizing objects. The second
is the dorsal
pathway, which projects to the parietal lobe and
appears to be essential for locating objects.
Similarly to the other
sensory systems and the motor system, there is a correspondence
or "mapping" between the arrangement of the elements
of the visual field as they strike the retina and their arrangement
on the surface of the visual cortex. This mapping onto the
visual cortex is called retinotopy, because
it is the retina that serves as the reference for the cortical
maps of the various visual areas.
In retinotopic maps, the zone of greatest discrimination
in the retina—the fovea, a small area at its
centre—is represented by a disproportionately
large area on the cortex. The centre of the visual
field, covered by the fovea, occupies the entire
posterior portion of the primary visual cortex,
while the entire peripheral zone of the visual
field is analyzed in the remaining anterior portion.
