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How The Eye Works


Eyes detect light, and send electrical impulses through the optic nerve to the visual and other areas of the brain. The functioning of the eye is often paralleled to the functioning of a camera, where external light is focused onto a light-sensitive medium. In the camera, this medium is film; in the eye, it is an array of visual receptors.

When light enters the eye it is refracted as it passes through the cornea. It passes through the pupil, and is further refracted by the lens. These refractions produce an inverted image onto the retina.

The retina contains photoreceptor cells, which are sensitive to light. These photoreceptor cells come in primarily two forms – rods and cones. Rods function well in dim light and provide black and white vision, while cones support daytime vision and the perception of colour. Neural signals from rods and cones are processed by other neurons in the retina. Outputs from the retina come in the forms of action potentials of retinal ganglion cells, whose axons form the optic nerve.

The optic nerve transmits visual information from the retina to the rest of the brain. The optic nerves from each eye meet and cross at the optic chiasm, which is located at the base of the hypothalamus. At this point, information from both eyes is combined, then divided based on visual fields. The information from the right visual field is processed by the left side of the brain, while information from the left visual field is processed by the right side of the brain.

From the optic chiasm, the information travels through the optic tract to the lateral geniculate nucleus (LGN) in the thalamus. The neurons in the LGN relay information to the primary visual cortex, in the occipital lobe. From here, information flows through a visual hierarchy, with each layer of the hierarchy representing more complex neural representations.

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