Jan 30, 2019
by Dr. Michael Trapani
There are few body parts that engender the emotional response produced by the eyes. Vision is hugely important to everyone’s wellbeing, and just the thought of an injury to the eye is cringeworthy in the extreme. Loss of the sense of sight is at least debilitating and at worst, crippling. And yet, it’s easy to take the sense of vision completely for granted. Let’s "take a look" at the sense of vision and the marvelous organ that makes it possible.
The eye is, in fact is the window of the mind. When we look through the pupil, we observe the fundus, the deepest region of a hollow organ that is furthest from the opening. Inside the eye, we are seeing the tapetum lucidum, a reflective layer located behind the retina, and the associated retinal blood vessels. It is the tapetum that creates the tapetal reflex, or "eye-shine" that can be seen in flash photographs and in animal eyes under conditions of dark-ness. The tapetum lucidum is typically much more developed in animals and is often very brightly colored when compared to that of humans. It is particularly amazing that light signals are perceived when they pass through the retina, and then again when they reflect back off the tapetum! This visual signal amplification helps improve night vision.
Development of the tapetum is related to the particular species’ benefit from vision under low-light conditions. Dogs and cats have a well-developed tapetum lucidum, while in humans, the tapetum is developed enough to ruin flash photographs but not nearly well enough for us to stroll at night without a flashlight.
During an examination of the fundus, the retina itself is normally invisible because its elements are transparent. In the way that a coffee cup and a doughnut have the same topo-graphical shape, so is the retina, in fact, the surface of the brain. The entire retinal cup buds off the brain early in embryonic development and migrates to the future location of the eye. There, a complicated set of nine nerve cell layers form to perform immediate processing of light signals — long before any of the information is sent to the brain. The retina, for example, recognizes movement, identifies and enhances edges, and perceives color, leaving interpretation of these signals to the visual cortex. This arrangement eliminates the need for miles of nerve fibers and vastly speeds up visual processing.
The photoreceptor elements of the retina are composed of rods and cones. Rods detect motion and provide black and white images, particularly under low light conditions, while cones are responsible for central and color vision. Cones work best under medium and bright light. Differential color perception itself takes place because cone receptors are capped with one of four visual pigment filters that limit the color of light a given cone cell can perceive. The accumulated effect of these pigment filters (three-color vision in humans) is similar to the cyan - magenta - yellow mixture of color generation by an ink jet printer - but only for humans!
In the distant evolutionary past, our progenitors had four visual pigments (four-color vision), a condition still found in birds today. Prehistoric rodents, being nocturnal, degenerat-ed to lose two of their four visual pigments, resulting in two-color vision and a greatly limited ability to perceive color. Two-color vision is present in all modern mammals, with the exception of humans, apes, and some New World monkeys, whose ancestors re-evolved a third color pigment, making the leap from two-color to three-color vision. Modern birds, with their four-color vision, can see well into the ultraviolet spectrum. Many birds, who look dull and drab to humans, are very ornately decorated when seen by other birds! We humans cannot even begin to understand the complexity of color perceived with four-color vision.
The differences in color perception ability are determined by need and "life-style." Fruit-eating species, like apes, derive great advantage from the ability to perceive red colors. Nocturnal hunters, like cats, have a much greater benefit from improved black and white, low light vision, but less advantage from color perception. There is only so much room in the retina and the precious real estate occupied by light sensing cells must be apportioned in a way that creates the greatest advantage within the evolutionary niche occupied by each species.
There is so much more to discuss on this topic! We’ll talk more about the eye next month.
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