LITERATURE REVIEW: EAGLE'S EYE

The eagle eye is among one of the strongest weapon in the animal kingdom, with eyesight estimated to be able to reach up to distances 4 to 8 times more than the normal average human eye, with an ability to spot a rabbit up to 2 miles. Although the eagle’s physical features differ from the human, the eyes though can be of the same size as those as humans. The eagle can also speed up to high speeds during chases, hence the ‘eagle eye’ is the best ability the eagle possesses to stay focus on the prey. The eye continuously adjust itself by making use of the ball curvature to keep a good focus point on the prey at ground level and by keeping an accurate perception throughout the attack. Perched from a branch high up, the eagle eye needs to be up in good conditions. The eyes have fierce looks and this is due to a bony ridge found above its eyes and the amount of bare skin found between its eyes. The ridge helps in protecting the eyes from protruding objects such as branches or preys which are fighting back to escape. As for the feathers, they usually do not grow on the area near the eyes.

The bird of prey also has an eye that can capture a wider range of color spectrum than the human eyes can, thus allowing them to be able to differentiate small changes in coloration and shades which helps for a better eye resolution and clarity which in turn helps during the hunting. Other studies have shown that the eagles have an eyeball which can be larger than the brain in term of weight and size although it is the same size as the human eye. It was also found that the back side of the eye is flatter than the human one hence allowing for the better vision of the bird. But there's no way to know what these extra colours, including ultraviolet, look like. Suppose we wanted to describe the colour of a tomato to someone who was born blind. We would not be able do it. We cannot even guess what they are subjective sensation of ultraviolet light is. Young eagles cannot spot fishes in water as their eyes have not fully developed the anti-reflection error. As they grow older, this error is corrected and the eagles can start the hunt. Another reason for why their parents must keep them until they are fully grown. However, the eagle eye cannot operate as it does during day time.

The eagle’s retina fovea has more of cone cells and rod cells, with a density of one million cells per millimetre square as compared to 200, 000 for the human eyes. Eagles possess a second fovea which enables better and sharper vision, and also has three eyelids. The phenomenon of an eagle turning its head almost 270 degrees, is attributed to the fact that the eye also turns unlike humans, as the eyes are tightly locked inside the socket.

Eagles tears are also known for moistening the eyes as they contain the chemical Isozymes which protects against salt water and bacteria, thus preventing infections. The iris has a pale yellow colour, compared to darker colours for the humans, but both entities have the white area called sclera, but in the case of the eagle, it is hidden beneath the eye lid. The pecten is also a very important aspect of the eagle’s eye, as it helps in nourishing the retina by keeping a healthy pressurised flow of fluid, absorbs light to minimise reflection error, helps in perceiving motion creates protection from sunlight and senses magnetic fields.

Most eagles do not suffer from any vision problems, as the eagle would starve to death from not being able to hunt and that is why ladies and gentlemen we do not see blind eagles. But researchers at the University of Maryland, have cooked up a vision experiments for the eagle. One screen displayed a zebra stripped pattern and the bird gets a treat for landing on it. The scientists have varied the width of the stripes to test the eagles’ acuity as the check for the distance the eagles begin to veer in the correct direction. 

According to William Hodos, a distinguished professor emeritus at the University of Maryland who has studied the visual acuity of birds since the 1970s, two eyeball features confer eagles' sharper vision. First, their retinas are more densely coated with light-detecting cells called cones than human retinas, enhancing their power to resolve fine details just as higher pixel density increases the resolving power of cameras.

Second, they have a much deeper fovea, a cone-rich structure in the backs of the eyes of both humans and eagles that detects light from the centre of our visual field. Human fovea is a little shell or bowl, while in hawk or eagle it is a convex pit. Some investigators think this deep fovea allows their eyes to act like a telephoto lens, giving them extra magnification in the centre of their field of view.

Picture of Difference between eagle and human eye.

Eagle vision wouldn't change how we perform most daily activities, such as reading computer screens or the newspaper, or finding milk in a crowded refrigerator but how we perceive the world and use our eyes would certainly be different. It's perhaps easiest to consider our new powers in the context of how eagles use them: for hunting. 

On top of the ability to see farther and perceive more colours, we would also have nearly double the field of view. With our eyes angled 30 degrees away from the midline of our faces like an eagle's, we would see almost all the way behind our heads with a 340-degree visual field (compared to normal humans' 180-degree field); this would confer a clear advantage in hunting and self-defence.

With eagle eyes, we would swivel our heads constantly. To locate prey or any other object of interest in the distance, we would periodically turn your head to the side to sweep your fovea (telephoto lens) across your field of view. After spotting, what you're looking for in this manner, you'd redirect your head toward it and use stereoscopic vision  combining the viewpoints of both eyes to gauge distance to calibrate the speed of your approach.

Enhanced perception and hunting prowess would likely come with a few drawbacks, but birds probably have a greater proportion of their brain volume devoted to visual processing than other groups of animals. Now the question of what it comes at the expense of: most birds appear not to have a well-developed sense of smell or taste.It's more difficult to say how your more sophisticated cognitive processes would fare. "Birds have areas that seem to function like the cortex [the part of our brains responsible for memory, language and complex thought], but it's arguable. But in terms of their ability to solve problems and so on, they match what many mammals can do. Many birds have superb memory.

Maximizing our potential

Today’s scientist has used eagle eye properties to enhance visual abilities of human. If the human swapped their eyes for an eagle’s eye, the latter would be able to see an ant up a 10-story building. The objects in the line of sight would appear magnified, and everything would be brilliantly coloured rendered in an inconceivable array of shade. 

The physical properties of our eyeballs limit us to 20/10 or 20/8 vision at best. Natural vision that good is extremely rare, but research by David Williams, director of the Centre for Visual Science at the University of Rochester, and his colleagues may soon enable laser eye surgeons to achieve 20/10-or-better vision for a large percentage of patients, placing their visual acuity halfway between that of humans and eagles.

Williams and his colleagues used an instrument called a wavefront sensor to detect distortions in human vision. They shoot light into the eye and observe how it bounces back through hundreds of tiny lenses in the sensor. The aberrations in patterns created by those lenses serve as a map of the eye's mistakes. Customized surgical techniques are being developed to implement the results of patients' wavefront measurements, in order to correct their vision beyond 20/20.

References.

• Boothe, Ronald G. (16 November 2001). Perception of the Visual Environment. Springer. pp. 235–. ISBN 978-0-387-98790-3. 
• Dudley, Karen (1997). Bald Eagles. Weigl Educational Publishers. p. 10. ISBN 9780919879942. 
• Grambo, Rebecca L. (14 December 2003). Eagles. Voyageur Press. ISBN 978-0-89658-363-4. 
• Hutchinson, Alan E.; Silliker, Bill (1 April 2000). Just Eagles. Willow Creek Press. pp. 34–. ISBN 978-1-57223-277-8. 
• Potts, Steve; Ueblacher, Sigrid Noll- (2006). Wildlife of North America. Capstone,. p. 11-. ISBN 9780736884839. 
• Wood, Casey Albert (1917). The Fundus Oculi of Birds, Especially as Viewed by the Ophthalmoscope: A Study in Comparative Anatomy and Physiology (public domain ed.). Lakeside Press. pp. 90.
• "Human Vision Vs Eagle Vision - Insight Vision Center", Insight Vision Center, 2017. [Online]. Available: http://www.insightvisioncenter.com/human-vision-vs-eagle-vision/. [Accessed: 31- May- 2017].
• L. Science, "What If Humans Had Eagle Vision?", Live Science, 2017. [Online]. Available: http://www.livescience.com/18658-humans-eagle-vision.html. [Accessed: 31- May- 2017].