By Catherine A. Sanderson, Amherst College
Humans rely most heavily on their sense of vision to understand and interpret the world. We can receive and process visual information with remarkable speed. A professional baseball player can hit a baseball four-tenths of a second after a pitcher throws it from 60 feet away. Read on to know more about the inner workings of our sense of vision.
Sense of Vision: Highest in Order?
When we’re in a movie theater, we see the words coming out of the actors’ mouths on the screen in front of us, instead of hearing them coming from the speakers all around us. If we ride a virtual reality roller coaster that doesn’t actually move, we still brace ourselves, even though our body understands that we’re not actually in any danger of falling.
So, how does the vision sense work? What we see starts when light waves enter the eye through the cornea and then pass through the pupil and onto the retina. In the retina, light waves are converted by the process called transduction into neural impulses, thanks to two types of vision receptor cells, rods, and cones. Finally, the optic nerve carries these neural impulses into a specific part of the brain, the visual cortex.
Vision Is Affected by Both Nature and Nurture
Common vision problems such as nearsightedness—meaning the ability to see close-up objects normally but have difficulty seeing far-away objects—clearly run in families with most evidence pointing to the role of genetics.
But there’s also evidence that points to environmental factors. As of the year 2000, about 25% of the world’s population was nearsighted, or myopic. But estimates are that this will climb to about 50 % by the year 2050.
One reason for this predicted rise is the massive increase in the amount of time people spend staring at computers and screens. Conversely, children and people of all ages spend less time outside.
The Trichromatic Theory
One of the most remarkable aspects of human vision is our ability to see color. Except for the 2% of the population with color-blindness, our difference threshold for colors is so low that we can discriminate between some seven million different color variations. We know color is produced by the different wavelengths of light. But our understanding of the actual way in which color is perceived has changed over time.
The original theory of vision, known as the trichromatic or Young-Helmholtz theory of color vision, described color vision as resulting from three types of receptors in the cones of the retina: one sensitive to the color green, another to the color blue, and the third to the color red.
According to this theory, these three receptors stimulated in some combination produce all of the colors we can see in the world. But some critiques have been raised about the ability of this theory to explain why we see some combinations of colors clearly, but not others. For example, we can see greenish-blue or blueish-red, but we don’t see yellowish-blue or greenish-red.
This article comes directly from content in the video series Introduction to Psychology. Watch it now, on Wondrium.
The Opponent-process Theory
Our ability to see certain color combinations, and inability to see others, led to the development of the opponent-process theory of color vision. According to this theory, color vision results from three different receptor systems, which each contain two color opposites: red-green, blue-yellow, and black-white.
According to this theory, receptor cells can only detect the presence of one of the colors in a pair at a time. Some neurons are turned on by one color, such as red, but are then turned off by its color pair, green. This theory explains why you can’t see greenish-red because the opponent cells can only detect one of these colors at a time. This theory also explains red-green color-blindness; people are missing the red-green receptor system.
The opponent-process theory also explains a fascinating perceptual phenomenon known as the afterimage effect. You might have noticed how after staring intensely at an image for a period of time and then looking away, you see a brief afterimage in complementary colors after you look away.
Seeing Colors Where There’s None
One often-used demonstration of the afterimage effect involves staring intently at a small red dot in the middle of a version of the United States flag that features green and black stripes, and in the top left corner, black stars on a yellow background. This “green, black, and yellow” version of the flag is utilizing the complementary colors for red, white, and blue.
This demonstration involves staring intently at this flag for 60 seconds and then immediately turning the eyes to a piece of water paper or a white wall. If you’ve looked long enough, you should see an afterimage rectangle with colors like an actual American flag. Why?
Staring steadily at an image of complementary colors for 30 to 60 seconds causes the green, black, and yellow opponent cells to get tired. Then, when you shift your focus to a blank surface, those cells stop firing, and their opposing cells—red, white, and blue—instead are activated, creating the brief afterimage.
Both theories of color work together to fully explain color vision. Color processing occurs in two stages: first within the retina, in line with the trichromatic theory, different cells detect different wavelengths of light. But the opponent-process theory explains how these cells then connect to the nerve cells that determine how we actually perceive color at a neural level in the brain.
Common Questions about How Our Sense of Vision Works
One reason for this prediction regarding problems in our sense of vision is the amount of time people spend staring at computers and screens, which is massively increasing as time goes by.
The trichromatic theory of vision suggests that the colors our sense of vision understands are a combination of three types of receptors being stimulated. The three types of receptors are sensitive to green, blue, or red.
According to the opponent-process theory, the three different types of receptor systems each contain two color opposites. When we see one color for a prolonged amount of time, the cells that were firing that color get tired, so when we look at a blank surface, the cells of the other color start firing for a short amount of time, and our sense of vision experiences the afterimage effect.