Objects on the left side of space will be seen on the right side of the retina of both eyes

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.

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.

Brodmann Areas

Seeing without knowing it : the strange phenomenon of blindsight

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 blindsight.

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 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.