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Broadly, there are two types of eyes, simple and compound; which can then be sub-divided into several classifications. The various types of eyes differ in visual quality; with simple eyes generally performing better than compound. Nevertheless, compound eyes are found in far more species, suggesting that visual excellence may not be a dominant factor in optical sensing for certain animals. The human eye is a simple eye, as it has a single optical system which creates an image that is examined by several photoreceptors; this image is then interpreted by the brain.
Various animals have specified eyes, which have evolved to aid in their survival. By contrasting them to the simple eyes present in humans, variations in structure and function can be identified.
The Human Eye
The structure of the human eye is well adapted for its function and purpose for survival. It's a spherical multi-layer structure located anterior of the cranium in the orbital sockets. The first layer of the eye is the cornea and constitutes 16% of the eye's outer coat. The transparent cornea is the eye's principle refractive surface. The sclera, a white opaque layer made of collagen fibres makes up the rest of the outer layer. The two outer layers are rigid structures which protect the more delicate tissues.
The iris, ciliary body and the choroid make up the uveal tract. The iris is an opaque structure containing melanin pigment; consequently light only enters the eye through the pupil. The size of the pupil can be varied by the sphincter and dilator to control the amount of light that enters the eye. The ciliary body aids in accommodation of the lens, which alters the refractive power of the lens. The lens, contributes to about a third of the refractive power of the eye. The rest of the uveal tract is comprised of the choroid, which contains an extensive capillary network and an intense melanin pigment. The capillaries supply the retina with an essential blood supply for the photoreceptors; while the melanin limits the scattering of light by absorbing the light that would corrupt the image.
Within the eye there are three chambers; the anterior, posterior and vitreous chamber, which are filled with vitreous fluid that provides the various tissues with nutrients. The retina is located at the rear of the eye, and is comprised of photosensitive cells which detect light and process the information and transmits this to the brain via the optic nerve.
The eye structure of some animals has been adapted to enhance certain abilities which are essential for their survival. Certain birds, reptiles, amphibians and fish have a nictitating membrane, which is a transparent membrane similar to an eyelid that protects, cleanses and moistens the eye without obscuring vision.
Birds of prey like vultures have forward facing eyes for increased binocular vision, as in humans. Comparatively, birds that are hunted usually have side facing eyes which are adapted for monocular vision; this provides an increased range of vision. Birds of prey have foveae which contain considerably more cones than humans, along with a lens shaped fovea; this significantly increases their distance vision. They also have two foveae; a temporal one which increases binocular sharpness, and a central one which increases monocular sharpness. These structural changes are particularly useful when hunting and has been well adapted to for their survival.
Mechanisms of vision in humans compared with that of various animals.
The process of visualisation can be divided loosely into four processes; optical, transduction, physiological and conscious awareness. The optical stage involves the image being focused on the retina; the transduction stage involves the photoreceptors absorbing the photons and converting them to electrical signals; the physiological stage involves the analyses of the signals and conscious awareness involves the active awareness of the presence of the image. Only photons absorbed by the retina can be used to form an image; consequently, the geometry and optics of the eye is paramount. Although the nature of the environment is vital in the quality of the image, the geometry is essential in image construction.
Sensitivity and resolution are important aspects of image formation. Sensitivity can vary based on the type of light source being used. Images formed by a point source forms an airy pattern on the retina due to diffraction at the entrance of the pupil; the smaller the pattern the brighter the image. The number of photons incident is proportional to the square of the pupil diameter; therefore eyes with large pupils are more sensitive to point sources. For extended light sources the number of photons incident if proportional to the diameter of the pupil, but inversely proportional to the focal length. Consequently, the size of the pupil is irrelevant if the focal length is altered accordingly.
Resolution is restricted by diffraction at the aperture and the size of the photoreceptors. The resolving power of the eye is a measure of the visual acuity, and is the smallest details that can be resolved. Aberrations are at a maximum when the diameter of the pupil is greatest. The radius of the airy pattern is inversely proportional to the aperture of the lens; and the smaller the pattern the brighter the image. Nevertheless, the smaller the aperture the greater the diffraction at the pupil. Therefore, the optimal diameter for maximum visual acuity is between these extremities. As the diameter of the pupil is reduced a smaller sized photoreceptor is required to analyse the visual image. The size of the cell cannot be decreased to below 1-2m because as the cells becomes smaller less light is transmitted and some of the light maybe absorbed by adjacent cells. Because the cones are responsible for resolution, for maximum visual acuity the eye needs to move to ensure that the photons are focused on the fovea where they are denser.
The avian lens is more ductile than the human lens, this allows for swift focusing. Certain birds such as diving birds, have adapted corneas which can change angle to allow for quicker transformation between water-based and air-based vision. To increase visual acuity birds tend to have larger eyes in relation to their heads than humans. A Starling's resolution is 2-3 times more efficient than humans, as its eye-to-head ratio is 15 times larger. Although Starlings' have side facing eye they are also able to rotate their eyes forward to increase their binocular vision.
In low light intensities the more photons that can be detected the better the quality of the image. The two major ways in which the number of photons detected can be maximised is dependent on the anatomy and physiology of the eye. The eye can be designed so that the greatest number of photons are focused onto the retina and then transmitted to the brain. Physiologically, the eye can be composed so that more photons are sampled. This can be done by increasing the area of vision, which decreases resolution, and also by increasing the length of time an image is sampled; this inadvertently decreases movement perception. The change in sensitivity of the photoreceptors, aid in the adaptation of the eye to varying light conditions; rods are more sensitive in dim light, while comes are more sensitive in brighter light.
Coloured vision allows for the distinction of light based on wavelength. The brain differentiates colour be comparing the responses to light from numerous cone receptors. The human eye is sensitive to light from wavelengths between 310 and 740 nm using three types of photoreceptors.
Most birds are tetrachromatic, with the ability to process UV light. This enables certain species to identify certain fruits which radiate UV light; in addition Kestrels hunt their prey by following the trail of excretion which reflects UV. Certain birds such as pigeons are pentachromatic with the ability to process a fourth range of wavelengths. The avian eye has been found to contain an abundance of cones specific to the absorbance of light at wavelengths near 570nm. This corresponds to the red and green part of the spectrum, which aids in the discrimination of various types foliage. Birds also contain coloured oil droplets which are located primarily in the cones. These oil droplets restrict the spectral sensitivity of the cone, particularly at shorter wavelengths, which sharpens hue discrimination but reduces sensitivity.
Figure 2 Average Absorbance spectrums for the avian eye.
Sarah R Pryke (2007) Reproductive Biology and Phylogeny of birds
The ability to perceive the distance of an object is an important feature of the visual system. This is done via various monocular and binocular cues. Monocular cues include; relative size, interposition, linear perspective, aerial perspective, light and shade, motion parallax, texture gradient, blurring, accommodation and familiar size. While binocular cues include; convergence and stereopsis. The main cues which affect the perception of depth are motion parallax, accommodation, familiar size and convergence. Motion parallax allows an observer to determine distance in relation to their position as they move. Observing a stationary object while moving gives information on its position in relation to the observer. Familiar size is the use of memory to determine the distance of an object by comparing it to what we know to be the actual size of the object Accommodation is when the ciliary muscles alter the size of the lens to change the focal length; the kinaesthetic sensation is interpreted by the visual cortex to determine depth. Convergence also uses kinaesthetic sensations of the extraocular muscles to determine the distance of an object based on the angle of convergence when viewing the object.
In pigeons, their side facing eyes provide relatively limited binocular vision but increased monocular vision. As a result they rely on motion to judge distance by moving their heads back and forth. A pigeon has a binocular range of 20-30 degrees and monocular range of 300 to 340 degrees.