Complete information on the essential functions of human Eyes

The light ray first passes through the transparent cornea and aqueous humor, and enters the eye through the pupil. The size of the pupil varies according to lighting conditions: the less light present, the wider the pupil opening. These adjustments of the pupil are effected by the iris that contracts or expands to allow varying amounts of light to pass in. After entering through the pupil, the light rays passes through the lens, and vitreous humor. The lens helps us to focus on objects at varying distances. When we look at distant objects, the lens becomes thinner and flatter; when we look at distant objects, the lens becomes thicker and rounder. Once, the light ray passes through the lens, it is projected onto the retina at the back of the eyeball. The lens bends the lights rays in such a way that the image projected onto the retina is upside down and reversed. But the brain reverses this image again allowing us to see objects in proper positions.

The chemical composition of the rods and cones in the retina changes when energized by light waves and thereby neural impulses are created. These neural impulses are then transmitted to the bipolar cells, which in turn are communicated to the ganglion cells. Ganglion cells collect and summarize the visual information. Then, the optic nerves carry them to the primary visual cortex in the occipital lobe of the brain. At this point, the brain sees and interprets what eyes have collected. The brain integrates information received through both the eyes. The light waves from the right half of the visual field fall on the left half of the retina in both eyes, and the neural impulses produced therein are carried to the left occipital lobe of the brain. Likewise, light waves from the left half of the visual field fall on the right half of the retina in both eyes, and the neural impulses created therein are transmitted to the right occipital lobe of the brain. This is the way that information from both eyes is combined and we see a single and unified object.

Dark adaptation:

Dark adaptation is a function of the eye. It is a visual sensitivity, which increases as we move from bright light to an environment of dim illumination, such as a cinema hall. When we enter into a movie hall, we may at first find the hall very dark and may not locate our seats readily. But after some time we come to see everything. The process of adjusting to conditions of lower lighting is called dark adaptation. The dark-adapted eye is about 100,000 times more sensitive to light than a light-adapted eye. Rods and cones adapt at different rates. Cones acquire sensitivity quicker than the rods. Adaptation to brighter lighting conditions takes place much more rapidly: For example, after coming out of the movie hall, we may find extremely brighter and stray light around us, but very soon; such brightness dissipates as we adjust to it. The process is called light adaptation.

Visual acuity:

Our visual system has the ability to resolve finer details. We can focus our eyes to get a sharp picture of the finer details of the object. There are two types of visual acuity. The first is static visual acuity, which refers to the ability to discriminate different objects when they are stationery. The static visual acuity is measured when the eye doctor asks us to read the familiar chart of letters. The second kind that is dynamic visual acuity refers to observing details in objects in motion. The ability to resolve finer details of the objects decreases as the speed of the object’s image across the retina increases. The dynamic visual acuity is important for a cricket player taking a catch. Wearing eyeglasses by the cricket players improves visual acuity.

Eye movements:

The eyes are so made that they move inward and sideways to focus our gaze on objects. There are two types of eye movements. The version movements move both the eyes together in the same direction so that we can track moving objects. The vergence movements move the eyes so as to converge or diverge. The convergent movements are very crucial for perception of distance and depth.

Seeing Colors:

A person with normal color vision can discriminate up to 150 color differences across the visible spectrum (Bornstein & Marks, 1982). Different colors have different wavelengths. Our ability to perceive color depends on the eye’s transmission of different messages to the brain, when lights of different wavelengths stimulate the cones in the retina. The following theories explain how lights of different wavelengths are perceived as different colors.

The Trichromatic Theory of Color Vision:

The trichromatic theory was originally developed by a British Scientist Thomas Young and was later supported by German physiologist Hermann Von Helmholtz. This theory suggests that there are three different types of photoreceptor cells or cones in the retina each of which is maximally sensitive to a particular range of wavelength in light. As a result, some of these cones are sensitive to red light, some to green, and some to blue. Red, blue, and green are the three basic color systems. We see other colors when two different types of receptors are stimulated simultaneously. The perception of yellow, for example, would result from the simultaneous stimulation of receptors for red and green. The perception of white would result from the simultaneous stimulation of receptors for red, green and blue. Likewise, we see different colors from different combinations of the three basic colors.

The Opponent-Process Theory of Color Vision:

The opponent-process theory suggests that there are three different color receptors, one of which is stimulated by green and inhibited by red, a second is stimulated by yellow and inhibited by blue, and the third is activated by achromatic stimuli, which can perceive differences in brightness from light to dark or white-black type. This theory is called opponent-process theory because these combinations are opponent systems. For example, when receptors for red are active, receptors for green remain inactive. Similarly, when blue member is active, yellow member remains inactive. These processes, either singly or in combination, mediate our experiences of different color perception.

The Retinex Theory of Color Vision:

The retinex theory was developed by Land (as cited in Das, 1998), which suggests a very convincing explanation of color vision. This theory proposes that visual pigments in the eye are light-sensitive molecules. Each of these responds to a relatively wide band of light. There are four types of visual pigments. One type of pigments is sensitive to light and dark and is found in rods. The other three are found in cones. Each of them responds to different, but overlapping frequencies of light waves. Since different sensitivities of the cones overlap, it is erroneous to speak red-green or blue-yellow system. Red, blue, or green, etc. are subjective experiences. It is the person who interprets visual stimulus as red, blue, or green. They are not the properties of light waves. It is, however, not the final word about our experience of color vision. Further research is being carried out to explain our experiences of color reception in a more scientific way.