The Eyes and Visual Sensation
Light is one type of electromagnetic energy that travels via waves. Other examples of electromagnetic energy include x-rays, ultraviolet rays, and radio waves. The human eye is capable of seeing only a tiny portion of the electromagnetic spectrum, a portion known as visible light. The eye is a highly intricate structure with over 100 million receptors that emit neural impulses when stimulated by light. More specifically, they respond to the “wavelengths” of light. A wavelength is exactly what it implies: a measurement of the distance from the peak of one wave to the next. It is measured in nanometers (billionths of a meter), and what is called the visible spectrum is the range from about 400 to 700 nanometers. What you see as purples and violets are the shorter wavelengths; below that are the ultraviolets that you can't see without special equipment. The reds you see are the longer wavelengths, above which are the infrareds that you again normally can't see.
After some additional processing by other neurons in the eye, impulses from the receptors travel out the back of the eye via the optic nerve and eventually to the occipital lobes of the brain. In between is the optic chiasm, which is responsible for the partial exception to the opposite-side brain mapping noted in Chapter 4. Here, vision is “split” so that the left visual field of each eye is sent to the right occipital lobe and the right visual field to the left. The evolutionary significance of this may be that if one eye is lost, the entire visual field will still be projected to both occipital lobes from the other eye.
How the Eye Works
Light enters the fluid-filled eye through an outer surface called the cornea, which is like the outer glass of a camera lens. Next it passes through an aperture called the pupil, which is expanded or contracted by the iris to allow for the intensity of the light — again, much the way a camera works. The lens of the eye, however, works differently. The ciliary muscles control the lens, causing it to “accommodate” and thicken or flatten to bring objects near or far into focus — something that the glass lens of a camera can't do. Finally, the retina is the rear of the eye where the visual receptors are located. Problems with the eye's structures cause many correctable visual problems, including the following:
A cataract is a visible clouding of the cornea that limits the amount of light that can enter the eye, which greatly interferes with vision.
An irregularly shaped cornea constitutes an astigmatism, which interferes with focusing.
Myopia, or “nearsightedness,” occurs when the focal point is in front of the retina instead of directly on it; distant objects appear fuzzy.
Hyperopia, or “farsightedness,” occurs when the focal point is behind the retina; nearby objects appear blurred.
Presbyopia is a stiffening of the lenses that interferes with accommodation.
Glaucoma is a severe disorder in which the pressure of the fluid within the eye becomes too high and can cause damage to the retina.
The Retina and Its Light Receptors
The retina consists of several layers of interconnected neurons. The deepest layer contains the rods and cones that are the actual light receptors, and the mostly transparent layers above combine signals from these and pass them along to the optic nerve. Rods and cones are named in accord with their shape, and they perform quite different functions.
Rods are by far the most abundant of the 100 million or so receptors and they are sensitive to the intensity (brightness) of the incoming light. Thus, rods are predominantly active in low light, such as on a moonlit night, when you see things mainly in black and white and shades of gray. If you go from a brightly lit area to a dark one, such as when you enter a dark movie theater, you may have to fumble for a seat because it can take up to thirty minutes for your rods to adapt and reach their maximum sensitivity.
The time that is normally required for dark adaptation is why many vehicles — including military ones — have reddish or orange panel lighting. Rods aren't activated by the longer wavelengths of this light, so your eyes remain dark adapted, say, when you're driving down a lonely road at night.
The eye contains only a few million cones, which are concentrated in the fovea — the area of the retina that is the main focal point of the lens. Cones have a much higher threshold for light than rods do, so they are inactive in the dark. Given sufficient light, however, cones have the ability to differentiate wavelengths and are therefore responsible for color vision. There are three kinds of cones, each of which emits signals in response to different wavelengths of light; the interplay between these sets the stage for the experience of color.
You probably know that in low light, you can see an object more clearly if you look just to the side of it. This is because the center of the human fovea contains no rods — only cones. So when you look slightly away, more of the low-light detecting rods that surround the fovea are stimulated and your vision improves.
There is one part of the eye that completely lacks rods and cones. This area is known as the optic disk and it is where the fibers composing the optic nerve leave the back of the eye. Because there are no visual receptors on the optic disk, there is actually a very small hole known as a blind spot in your field of vision. So why don't you notice this miniscule hole in your vision? Researchers have suggested that the brain actually “fills in” the missing information by using visual clues such as pattern and color from the surrounding environment.
Light waves have three properties that are involved in all visual sensation and perception. Wavelength, as discussed earlier, determines hue, the technical term for color. Intensity determines brightness. And the “blend” of wavelengths determines saturation — relatively pure colors such as vivid greens or reds contain a narrow band of wavelengths, whereas softer colors such as pastels contain a broader mix. Each of these properties of light interacts to influence the colors you perceive.
The rudiments of how you experience color have been understood reasonably well since the nineteenth century. In essence, the three types of cones and the cells in the eye that they synapse with generate different rates and combinations of neural impulses. Via the optic nerve, these impulses go next to the thalamus for further processing and recombination by several kinds of specialized cells, and from there they enter the brain through the visual cortex for even more processing, which researchers are still trying to figure out.
Objects don't actually have color in a real sense. What you experience is wavelengths that are reflected from objects instead of being absorbed by them. The pigmentation of a bright yellow canary, for example, absorbs all wavelengths of light except those for yellow, which bounce off and enter your eyes.
Each cone in the eye is sensitive to certain wavelengths of light — some cones sense short wavelengths (blue), some detect medium wavelengths (green), and others detect long wavelengths (red). When a color other than blue, green, or red strikes the retina, it stimulates a combination of cones in order to produce the experience of that color. The most common form of color blindness, known as red-green color blindness, occurs when people have the normal blue sensitive cones, but only have either red or green cones instead of both. As a result, red and green appear to be the same color.
It is known, however, that the processing is based on complementary colors, which are pairs that when combined produce white or gray. According to the opponent-processing theory of color vision, there are four basic colors and two pairs of color-sensing neurons — a red-green pair and a yellow-blue pair. The two members of each pair oppose each other, so if the red-sensing neurons are stimulated, then the green-sensing ones are inhibited. Thus, the myriad colors we experience are not neurologically “canceled out” down the line.