Eyes are organs that detect light, and convert it to electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement. In higher organisms the eye is a complex optical system which collects light from the surrounding environment; regulates its intensity through a diaphragm; focuses it through an adjustable assembly of lenses to form an image; converts this image into a set of electrical signals; and transmits these signals to the brain, through complex neural pathways that connect the eye, via the optic nerve, to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.[1]Image-resolving eyes are present in molluscs, chordates and arthropods.
The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for theentrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to thesuprachiasmatic nuclei to effect circadian adjustment.
Overview
Complex eyes can distinguish shapes and colours. The visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception; in other organisms, eyes are located so as to maximize the field of view, such as in rabbits and horses, which have monocular vision.
According to the theory of biological evolution, the first proto-eyes evolved among animals 600 million years ago, about the time of the Cambrian explosion.The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in six of the thirty-plus main phyla. In most vertebrates and some molluscs, the eye works by allowing light to enter and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for colour) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals for vision. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye, and reducing aberrations when there is enough light.
The eyes of most cephalopods, fish, amphibians and snakes have fixed lens shapes, and focusing vision is achieved by telescoping the lens—similar to how a camera focuses.
Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360-degree field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images.
Possessing detailed hyperspectral colour vision, the Mantis shrimp has been reported to have the world's most complex colour vision system.Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.
In contrast to compound eyes, simple eyes are those that have a single lens. For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight. In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents–in this way the bearers can spot hot springs and avoid being boiled alive.
Stereoscopic Vision
Humans and many other animals have stereoscopic vision, in which they see an object with two eyes at the same time, with both eyes set in the same plane. This is not true of all animals: in most fishes each eye, located on opposite sides of the head, generates an independent image.This type of vision is important in depth perception, and animals that have it usually need the ability to judge distances nicely. The images formed by the two eyes and transmitted to the brain's visual cortex are not quite identical: the distance between the two eyes means that they see an object from slightly different angles. The principle is exactly the same as that used in the "rangefinder" mechanism of a camera, in which a rotating mirror is adjusted to make its image coincide with that viewed directly through the viewfinder, and the distance is read off from a scale calibrated to the angle of rotation. The brain does the same thing; it is capable of comparing the two images and judging the amount of parallax the "baseline" between the eyes has produced, and rendering a single integrated image in three-dimensional view.
Individuals who have lost the vision in one eye have to develop other mechanisms to compensate for the loss of parallax as a means to judge distances, and often have poor depth perception, regardless of whatever adjustments they make. The human ability in stereoscopic vision is closely related to their fine manipulation skills. In arboreal animals that move from branch to branch, for obvious reasons, there is also stereotactic visual ability.
The squirrel has eyes on opposite sides of its head, but the two produce visual fields that overlap. This is sufficient to permit a squirrel to leap from branch to branch. Many small birds that perch on tree branches and flit through shrubbery have a similar arrangement. Owls are the exceptions among birds; they are the only group of avians whose eyes are located in the same plane, and they have binocular vision in the same sense that flat-faced animals like primates do. Many birds have eyes on the sides of their heads, and are unable to view a single object with both eyes. The chicken and the pigeon are two good examples of this. Such birds must judge distance by moving their heads and viewing an object with each eye independently, deducing from the displacement how far away the object must be. (A human can get an idea of this effect by viewing an object at a fixed distance with each eye alternately.)
Monocular Vision
The term “monocular vision” can be used in two different ways. In the first sense, it refers tovisual perception where the eyes see independently, rather than acting as a pair as human eyes do. In the second sense, it is more properly termed monocular vision impairment, and refers to a person or animal who can only see out of one eye. This may be congenital or acquired, and can lead to impairments as a result of changes in visual perception.
Many animals see with monocular vision, including horses, sheep, and lizards. These animals often have eyes set far apart in their heads, allowing for a very wide range of vision. This is especially important for prey animals like horses and sheep, who need to be able to spot threats from as many directions as possible. The problem with this type of visual perception is the lack of depth perception. Because the eyes do not work in concert, it is harder to provide useful information about the distance from and between objects. This would be a disadvantage for predators like big cats, who usually have binocular vision.
There are other variations in visual perception between monocular and binocular vision, depending on the species. Eyes come in a variety of styles, so to speak, including eyes equipped for better night or color vision, as well as eyes capable of seeing in a wider range than the human eye. Bees, for instance, can see ultraviolet markings on flowers
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