Comparison Between Structure and Function of Human and Animal Eyes - Assignment Example

Comparison between Structure and Function of Human and Animal Eyes Introduction Vision is an integral part of humans and other animals. Human and animals share a common kingdom animalia; however, as one goes down the taxonomic key to the species level, individual characteristics differ greatly, including their vision. The anatomy and location of the eyes exhibit slight differences despite their common use for vision. Experiment shows that humans are one of the species with the best eyesight as compared to other organisms. Nevertheless, an organism’s visual system is optimized to its environment and survival needs (MVOH, 1997, p.1). This study analyzes the similarities and differences between the structures and functions of eyes of humans and dogs, eagles, fish, chameleons, and snakes. The aspects of visual function that include visual acuity, color discrimination, motion and the depth perception, contrast sensitivity and brightness perception will also be described. Description of terms Visual acuity refers to the acuteness or clearness of vision in detecting small objects (Dragoi and Tsuchitani, 2015) depending on the sharpness of the retinal focus, sensitivity of the nervous system, and the interpretive ability of the brain. It is measured quantitatively by the evaluating the ability to identify black spots on a white background at a standardized distance. Color vision is the ability to detect the differences in the wavelengths of light that differ in normal and color-blind eyes (Dragoi and Tsuchitani, 2015, p.1). Some animals are partially color-blind but exhibit superior vision in other aspects (Preston, 2014, p.1). Depth perception is the visual capability to see the world in three dimensions relative to the object distance. It occurs when the eye contrasts objects with their background that appear to be at different depths. Depth can be classified as monocular or binocular cue depending on the vision achieved through the exploitation of parallax. Contrast sensitivity occurs from the distance of the observer’s point of view and the angle of perspective (Grossberg, 2014, p.2). Distant objects appear hazy while closer objects appear clear due to the differences in the viewer’s points of view. Anatomy of the human eye The human eye is adapted to sense different colors and lights of various intensities and adjust according to brightness. Color and light vision involves the simultaneous interaction between the two eyes and brain. When light from the object hits the eye, it is focused by the cornea and lens elements that form transform it to an inverted image. The image successfully transverses the aqueous humor, crystalline lens, gelatinous vitreous body, and vascular ad neuronal layers before it is focused on the surface of the retina. Rod and cone cells found in the retina detect the image, translate the light into a series of electrical signals, and transmit the signals to the brain (Spring, Fellers and Davidson, 2015, p.1). Figure 1: Structure of the human eye: https://en.wikipedia.org/wiki/Lens_%28anatomy%29 The retina is the light sensitive part of the eye that is located at the back of the eyeball. It contains cones that provide color perception and detailed sight, and rods that detect motion and provide vision in dim light. Rod cells contain the photo-pigment rhodopsins that have peak sensitivity to blue-green light. Images formed in the visual spectrum by rod cell stimulation appear relatively unsharp with shades of grey. Rod vision is scotopic or twilight and vertebrates utilize it to detect potential preys and predators using shape and motion. Cones are linked to neurons within the retina. They are sensitive to color but have limited sensitivity to light as compared to rods that are more in concentration. Color vision is induced by stimulating cone cells. The intensity of the light that impacts on the three photo-pigments of cone receptors determines the color that is displayed as a mosaic (spring, Fellers and Davidson, 2015, p.1). Human and Animal Vision Humans are trichromate while animals are dichromate. Humans have three types of photoreceptors known as cones that are color-sensitive cells that function in bright light to distinguish blue, red, and green colors. Rods detect small amounts of light to allow them to perceive objects in the dark (Preston, 2014, p.1). Most animals have two types of photoreceptors that render them partially color-blind. However, some animals have four photoreceptors that enable them to perceive ultraviolet and polarized light waves oscillating on the same plane (Preston, 2014, p.1). Most birds are tetrachromats with four types of cone cells that enable them to perceive red, blue, green colors and ultraviolet light. The structure Smaller animals such as the earthworm have hundreds of light sensitive cells known as eyespots around the head and tail that enable it to sense light and darkness. Insects such as butterflies have compound eyes composed of separate units of light-sensitive cells known as ommatidia (MVOH, 1997, p.1).they have five different cones that help them perceive more colors than humans. Bees have UV vision that helps them to distinguish special patterns on flower petals and identify where nectar is present. Spiders, lizards, and sea stars have multiple eyes for adaptation reasons. An eagle’s eye has superior visual acuity that can only be matched by a human eye combined with binoculars that enable it to see details and discern distant objects. Cats and owls are known to have good night vision that is attributed to their bigger eyes, larger pupils, and a tape turn. Rattlesnakes and vipers have low resolution color vision during the day but plenty of rods that help them to perceive light at night and also, enhance sensitivity to infrared light. Comparison Human and Dog Vision Dogs have poor color vision because their retina has one tenth cones concentration; hence, they cannot perceive as many colors as humans. Unlike humans who are trichromate, they are dichromate with only two different kinds of cones that can only differentiate two colors; blue-violet and yellow, but not red from green. They can distinguish between shades of gray but cannot recognize red, yellow, orange, and green. Their retina is rod-dominated thus allowing them to see clearly in darkness. They have superior night vision and better motion visibility than humans. Dogs barely rely on colors; instead, they utilize other cues such as texture, smell, brightness, and position. For instance, dogs perceive the intensity and position of green and red traffic lights to define the right time to cross a street road. The position of a dog’s eye determines the field of view and the depth of perception. Eye location varies with dog species in the sense that; prey species tend to have eyes located on the side position of their heads to increase the field of view for approaching predators. On the other hand, predator species have frontal eyes set close together at 20degree angle to increase the field of view and peripheral vision of the dog. Generally, dogs have a wider field of vision than humans. Binocular vision is achieved when the fields of view for each eye overlaps and is necessary for predators as a survival tool to aid jumping, leaping, and catching. Figure 2: Structure of dogs eye https://www.petinsurance.com/healthzone/pet-articles/pet-health/~/media/All%20PHZ%20Images/Article%20images/Cherry%20Eye%20diagram.ashx Dogs have less visual acuity as compared to humans. Humans with normal eyesight have 20/20 vision such that they can distinguish objects located 20feet. Dogs have 20/75 vision, meaning; their visual clarity for an object located 20feet away is similar to that of a human standing 75feet away. Dogs recognize objects through motion and smell more often than they notice objects using the sense of hearing. Old and blind dogs navigate their environment more comfortably than humans with visual impairment. Human and Fish Vision Marine animal’s vision is different from that of humans in many aspects for adaptability reasons to their environment even though the human eye emulates fish eye by secreting saline water to keep the eye drenched. Water reduces the intensity of light that reaches the fish eye and therefore, fish eyes are adapted to low light intensity. They are more sensitive to blue light that remains in the visible light spectrum when the sun disappears. They have bulging eyes because the lens tends to bulge through the pupil to allow periscopic vision. The cornea mimics the refractive index of water to collect more light from a wider field of view. They have similar vision field to that of humans. The streamlined body of fish enables them to circumvent friction from water that can end up damaging their eyes. Fish perceive polarized light (Horváth and Varjú, 2004). Figure3: Fish and human eyes anatomy https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcShi78u46cYEBlOJSoBxnxWartMgiwPvbSrEi_J6tNqFyMVR_6l Fish eyes have more rods than cones. The rods have more light pigment than cones are therefore more sensitive to light. Their sensitivity is increased by rod convergence to one bipolar cells, a phenomenon called summation. There is no summation in cones hence the combined effect of light on rods assists in stimulating bipolar cells. Human and Eagle Vision Eagle’s eyes have enhanced perceptibility compared to human eyes due to two discerning eyeball features that confer the eagle with a sharper vision. Their retina is densely coated with cones and rods; about one million light-sensitive cells per square millimeter retinal surface while humans have 200,000 cells. This accounts for their better visual acuity, sharp eyesight and high discerning ability (Discovery Foundation, 2014, p.1). They can see four to five times further and clearer than the average human eyesight because of the enhanced power of the retina to resolve finer details. Secondly, they have a fovea on the retina that is densely packed with cone and rod cells to optimize binocular vision and a second fovea that optimizes monocular vision on the sides. Eagle’s eyes are positioned in an intermediate position between the frontal and lateral placement while the human eyes are frontally placed with the two eyes in parallel. The frontal eye position gives humans good binocular vision but poor peripheral vision. Eagle’s ayes are 30degrees from the midline of the face thus providing 340degrees vision field. They have better peripheral view than humans (Lasikmd.com, 2013, p.1). Unlike the human eye, the eye an eagle is not spherical to provide a more focused visual field. The lens is pushed forward in order to magnify the image that is transmitted to the retina. The human retina is shell-like or bowl-like; however, the eagle’s fovea is like a convex pit. The deep fovea permits the eye to act like a telephoto that increases magnification. The lens can change shapes to allow the bird to focus the image that are further or near from different angles and the cornea changes to enable extreme precision when perceiving objects (Wolchover, 2012, p.1). https://encrypted-tbn2.gstatic.com/images?q=tbn: ANd9GcT1ELFVmNi0c50a_1tTBZSH_ Yo3JY6l9DjtRi8bFsEX2cODhyfpx3LBsFM Figure 3a: location of Eagles eye Figure 4b: Front location of human eye https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcTbaRWrcl-0KtIgu-8WF94UwOTEMxAPbcgbch1JDgvj3Z6H5zXJ The retina has hundreds of millions of cones and rods connecting to the brain to transmit light for interpretation. Eagles have higher and better spatial and temporal visual acuity (20/5 or 20/4) than humans for adaptation purposes as a high flying predator to locate mice that are crawling hundreds of feet below. Additionally, eagles have superior color vision that enables them to perceive colors more vividly than humans. They have strong ability to discriminate between multiple color shades and also, detect UV light from UV-reflecting urine trails of smaller prey. Basically, for an eagle, clarity is the key component for focusing near and far (Googlesites.com, 2014, p.1). Figure 5: Structure of Eagle’s Eye https://www.learner.org/jnorth/images/graphics/d-e/eagle_eagleeye.GIF Human and Chameleon Figure 6b: Human eye location and appearance https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcSPgsEp0vwnDuC4BQdXsIpNW4xwD2Pa_qd8eXJNu4-YxqKwenz_xw Chameleons have unique eyes that differ from that of humans in position, anatomy, and environmental adaptability. The eyes are located on opposite sides of the head. They rotate with high degree of freedom to attain 360degrees vision that aid to detect objects on opposite sides at the same time. They can transition between monocular and binocular vision. The eyes can move separately to achieve visual field of 180 degrees (Softschools.com, 2005). They have high visual acuity and can detect insects as small as 5-10cm. They are capable of detecting ultraviolet light. Humans have deep orbital sockets that hold the eyes in place while chameleons have thick, muscular lids surrounding the twin conical turrets and keep the eyeballs from falling. Chameleon views objects panoramically by switching between monocular and binocular vision. Their ability to switch between uncoupled and synchronous saccadic eye movements is a remarkable mechanism of protection, reflexes, and food gathering (Asknature.org, 2015). Figure 8: structure of chameleon and human eye http://archives.evergreen.edu/webpages/curricular/2011-2012/m2o1112/web/images/ov2.gif Conclusion Vision is insidious in the animal kingdom since; it is the sensory organ that is relied upon in finding reproductive mates, suitable food, shelter, and escaping from predators and danger (Erichsen and Woodhouse, 2015). The study of human and animal’s eyes show wild disparities in their mode of vision and anatomical features. Human vision is presented as sometimes ineffective (Erichsen and Woodhouse, 2015,) as in detecting infrared and ultraviolet lights, or overly sophisticated than that of felines (Rit-mcsl.org, 2015). This study is important to help researchers to find cure for human eye diseases and explain the adaptation of animals to their environment. The visual perception of living creatures is dependent on how they process light using their photoreceptors. Bibliography Asknature.org, (2015). Eyes give 360Ëš vision: chameleon. [Online] AskNature. Available at: http://www.asknature.org/strategy/f6b73865a35b39d2974e29905e8b1a8c [Accessed 27 Oct. 2015]. Bruce, V. and Green, P. (1985). Visual perception, physiology, psychology, and ecology. London: L. Erlbaum. Denning, D. (2015). [Online] Available at: https://www.ebiomedia.com/how-do-animals-see-underwater.html [Accessed 27 Oct. 2015]. 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