The Visual Receptors
Once light reaches the retina, we leave the domain of optics and enter that of neuro-physiology, because it is at the retina that the physical stimulus energy is transduced into a neural impulse. The retina contains two kinds of receptor cells, the rods and the cones; the names of these cells reflect their different shapes (Figure 4.25). The cones areplentiful in the fovea, a small, roughly circular region at the center of the retina; but they become less and less prevalent at the outer edges of the retina. The opposite is true of the rods; they’re completely absent from the fovea but more numerous at the retina’s edges. In all, there are some 120 million rods and about 6 million cones in the normal human eye.
The rods and cones do not report to the brain directly. Instead, their mes-sage is relayed by several other layers of cells within the retina (see Figure 4.24). The receptors stimulate the bipolar cells, and these in turn excite the ganglioncells. The ganglion cells collect informa- tion from all over the retina, and the axons of these cells then converge to form a bundle of fibers that we call the opticnerve. Leaving the eyeball, the optic nerve car- ries information first to the lateral geniculatenucleus in the thalamus and then to the cor-tex (Figure 4.26). (Notice that this pathway resembles the one for audi-tory signals, which go from the ear to a different section of the geniculate nucleus and then to the cortex.)
This anatomical arrangement requires a space at the back of each eyeball to enable the axons of the ganglion cells to exit the eye on their way to the thalamus. These axons fill this space entirely, leaving no room for rods or cones. As a result, this region has no photoreceptors and is completely insensitive to light. Appropriately enough, it’s called the blind spot (Figure 4.27).
Rods and cones differ in their structure, number, and placement on the retina; they also differ in their function. The rods are the receptors for night vision; they operate at low light intensities and lead to achromatic (colorless) sensations. The cones serve day vision; they respond at much higher levels of illumination and are responsible for sensations of color.
Why do we need two types of photoreceptors? The answer is clear when we consider the enormous range of light intensities encountered by organisms like ourselves as we go about our business during both day and night. In humans, the ratio in energy level between the dimmest stimulus we can detect and the brightest we can tolerate is roughly 1:100,000,000,000. Natural selection has allowed for this incredible range by a biological division of labor—so we have two separate receptor systems, one for vision in dim light and the other for vision in bright light.
The enormous sensitivity of the rods comes at a price: The same traits that make the rods sensitive to low levels of light also make them less able to discriminate fine detail. As a result, acuity—the ability to perceive detail—is much greater in the cones. This is the major reason why we point our eyes toward any target that we’d like to perceive in detail. This action positions our eyes so that the image of the target falls onto the fovea, where the cones are most closely packed and visual acuity is greatest.
Be aware that the differences between rods and cones also create situations in which we want to rely on the rods. That’s why it’s sometimes helpful to look at something “out of the corner” of your eye. Sailors and astronomers have known for years that when you’re trying to find a barely visible star, it’s best not to look directly at the star’s loca-tion. By looking slightly away from the star, you can ensure that the star’s image falls outside of the fovea and onto a region of the retina that’s dense with the more light-sensitive rods. This strategy limits the ability to discern detail; but, by relying on the rods, it maximizes visual sensitivity to faint stimuli.
Rods and cones can also be distinguished in one further way—their chemistry. Inside each photoreceptor is a photopigment, a light-sensitive chemical pigment that allows the transduction of light energy into a neural signal. When light enters the receptor, the light energy changes the chemical form of the photopigment, setting off a chain of events that ultimately leads to an electrical signal. In this way, the light energy is translated into the electrochemical language of the nervous system. Inside the receptor, the pigment itself is then reconstituted so that it will be ready to react with light again when the next opportunity arises.
Rods and cones contain different photopigments. The rods contain rhodopsin, a pig-ment that breaks down more readily in response to light than the cone pigments do.
Rhodopsin is part of the reason that rods can function at lower light levels. There are three different cone photopigments, and each cone contains one of the three types. The differ-ences among the three pigments are crucial to the cones’ ability to discriminate colors—a topic we’ll turn to shortly. Rods, which contain just one pigment, are sensitive to differ-ences in brightness (white versus gray, or a strongly illuminated red versus a weakly illuminated one); but they cannot discriminate among different hues. So, for example, the rods will respond in exactly the same way to a patch of red and an equally bright patch of blue. In effect, this response makes each of us nearly “color blind” at the visual periphery—that is, rather poor at telling colors apart if they fall on a retina position far enough from the fovea so that the position contains mostly rods and very few cones.