The Visual System, part II (OLI)


Perceiving Color

Trichromatic Color Theory


In his important research on color vision, Hermann von Helmholtz (1821–1894) theorized that color is perceived because the cones in the retina come in three types. One type of cone reacts primarily to blue light (short wavelengths), another reacts primarily to green light (medium wavelengths), and a third reacts primarily to red light (long wavelengths). The visual cortex then detects and compares the strength of the signals from each of the three types of cones, creating the experience of color. According to this Young-Helmholtz trichromatic color theory, what color we see depends on the mix of the signals from the three types of cones. If the brain is receiving primarily red and blue signals, for instance, it perceives purple; if it is receiving primarily red and green signals, it perceives yellow; and if it is receiving messages from all three types of cones, it perceives white.

The different functions of the three types of cones are apparent in people who experience colorblindness—the inability to detect either green and/or red colors. About 1 in 50 people, mostly men, lack functioning in the red- or green-sensitive cones, leaving them only able to experience either one or two colors.

Example of Test for ColorblindnessPeople with normal color vision can see the number 42 in the first image and the number 12 in the second (they are vague but apparent). However, people who are colorblind cannot see the numbers at all. 

Opponent-Process Theory

The trichromatic color theory cannot explain all of human vision, however. For one, although the color purple does appear to us as a mixing of red and blue, yellow does not appear to be a mix of red and green. And people with colorblindness, who cannot see either green or red, nevertheless can still see yellow. An alternative approach to the Young-Helmholtz theory, known as the opponent-process color theory, proposes that we analyze sensory information not in terms of three colors but rather in three sets of “opponent colors”: red-green, yellow-blue, and white-black. Evidence for the opponent-process theory comes from the fact that some neurons in the retina and in the visual cortex are excited by one color (e.g., red) but inhibited by another color (e.g., green) as shown in the following figure.

One example of opponent processing occurs in the experience of an afterimage. If you stare at the flag on the left side of the figure below for about 30 seconds (the longer you look, the better the effect), and then move your eyes to the blank area to the right of it, you will see the afterimage. When we stare at the green stripes, our green receptors habituate and begin to process less strongly, whereas the red receptors remain at full strength. When we switch our gaze, we see primarily the red part of the opponent process. Similar processes create blue after yellow and white after black.

AfterimageThe presence of an afterimage is best explained by the opponent-process theory of color perception. Stare at the flag for a few seconds, and then move your gaze to the blank space next to it. Do you see the afterimage?

Watch the video below on aftereffect.

The tricolor and the opponent-process mechanisms work together to produce color vision. When light rays enter the eye, the red, blue, and green cones on the retina respond in different degrees, and send different strength signals of red, blue, and green through the optic nerve. The color signals are then processed both by the ganglion cells and by the neurons in the visual cortex. [1]

Did I get this

For each statement, select which of the two theories of color vision it fits with best.

Perceiving purple is a result of receiving messages from two types of cells: those that perceive red and those that perceive blue.
Young-Helmholtz OR opponent-process
The Young-Helmholtz trichromatic theory indicates that if the brain is perceiving information from primarily the red and blue cones, purple is what is perceived.


Afterimages caused by staring at a blue image then looking away makes you perceive yellow.
Young-Helmholtz OR opponent-process
The Opponent-Process theory is based on two color contrasts (red-green and blue-yellow) and black-white contrast. Yellow is opposite of blue on the color wheel and this theory would explain seeing the opposite color as an afterimage of its opposite.


Many people who are colorblind cannot distinguish red from green.
Young-Helmholtz OR opponent-process
The Young-Helmholz Trichromatic theory is based on three types of color receptors (blue, green, and red). The different functions of the three types of cones are apparent in those who experience color blindness (the inability to detect either green and/or red colors).


Neurons in the retina are excited by one color but inhibited by another color.
Young-Helmholtz OR opponent-process
The Opponent-Process theory deals with color contrasts and how some neurons in the retina and visual cortex are excited by one color (i.e., blue) but inhibited by another color (i.e., yellow).


There are three types of cone cells, one for each primary color.
Young-Helmholtz OR opponent-process
The Young-Helmholz Trichromatic theory is based on three types of color receptors, short waves (blue), medium waves (green), and long waves (red).

Perceiving Form


One of the important processes required in vision is the perception of form. German psychologists in the 1930s and 1940s, including Max Wertheimer (1880–1943), Kurt Koffka (1886–1941), and Wolfgang Köhler (1887–1967), argued that we create forms out of their component sensations based on the idea of the gestalt, a meaningfully organized whole. The idea of the gestalt is that the “whole is more than the sum of its parts.” Some examples of how gestalt principles lead us to see more than what is actually there are summarized in the following table.

Summary of Gestalt Principles of Form Perception

Perceiving Depth

Depth perception is the ability to perceive three-dimensional space and to accurately judge distance. Without depth perception, we would be unable to drive a car, thread a needle, or simply navigate our way around the supermarket. [1] Research has found that depth perception is in part based on innate capacities and in part learned through experience. [2]

Psychologists Eleanor Gibson and Richard Walk [3] tested the ability to perceive depth in 6- to 14-month-old infants by placing them on a visual cliff, a mechanism that gives the perception of a dangerous drop-off, in which infants can be safely tested for their perception of depth (see video below). The infants were placed on one side of the “cliff” while their mothers called to them from the other side. Gibson and Walk found that most infants either crawled away from the cliff or remained on the board and cried because they wanted to go to their mothers, but the infants perceived a chasm that they instinctively could not cross. Further research has found that even very young children who cannot yet crawl are fearful of heights. [4] On the other hand, studies have also found that infants improve their hand-eye coordination as they learn to better grasp objects and as they gain more experience in crawling, indicating that depth perception is also learned. [5]

The Visual Cliff



Depth perception is the result of our use of depth cues, messages from our bodies and the external environment that supply us with information about space and distance. Binocular depth cues are depth cues that are created by retinal image disparity—that is, the space between our eyes, and thus require the coordination of both eyes. One outcome of retinal disparity is that the images projected on each eye are slightly different from each other. The visual cortex automatically merges the two images into one, enabling us to perceive depth. Three-dimensional movies make use of retinal disparity by using 3-D glasses that the viewer wears to create a different image on each eye. The perceptual system quickly, easily, and unconsciously turns the disparity into three dimensions.

An important binocular depth cue is convergence, the inward turning of our eyes that is required to focus on objects that are less than about 50 feet away from us. The visual cortex uses the size of the convergence angle between the eyes to judge the object’s distance. You will be able to feel your eyes converging if you slowly bring a finger closer to your nose while continuing to focus on it. When you close one eye, you no longer feel the tension—convergence is a binocular depth cue that requires both eyes to work.

The visual system also uses accommodation to help determine depth. As the lens changes its curvature to focus on distant or close objects, information relayed from the muscles attached to the lens helps us determine an object’s distance. Accommodation is only effective at short viewing distances, however, so while it comes in handy when threading a needle or tying shoelaces, it is far less effective when driving or playing sports.






Although the best cues to depth occur when both eyes work together, we are able to see depth even with one eye closed. Monocular depth cues are depth cues that help us perceive depth using only one eye. [6] Some of the most important are summarized in the following table.

Monocular Depth Cues That Help Us Judge Depth at a Distance

Perceiving Motion



Creative artists have taken advantage of the cues that the brain uses to perceive motion, starting in the early days of motion pictures and continuing to the present with modern computerized visual effects. The general phenomenon is called apparent motion. One example of apparent motion can be seen if two bright circles, one on the left of the screen and the other on the right of the screen, are flashed on and off in quick succession. At the right speed, your brain creates a blur that seems to move back and forth between the two circles. This is called the phi phenomenon. A similar, but different phenomenon occurs if a series of circles are flashed on and off in sequence, though the flashing occurs more slowly than in the phi phenomenon. The circle appears to move from one location to the next, though the connecting blur associated with the phi phenomenon is not present.

It is not necessary to use circles. Any visual shape can produce apparent motion. Motion pictures, which use a sequence of still images, each similar to but slightly different from the one before, to create the experience of smooth movement. At the frame speeds of modern motion pictures, the phi phenomenon is the best explanation for our experience of smooth and natural movement. However, as visual artists discovered more than a century ago, even at slower change rates, the beta effect can produce the experience of a moving image.

Example of the Phi Phenomenon

Example of the Beta Effect


Did I get this


By Eadweard Muybridge, The Zoopraxiscope—A Couple Waltzing (image shows a phenakistoscope, not a zoopraxiscope)

Which phenomenon is illustrated above?
beta effect OR phil phenomenon
This is the phi phenomenon. You perceive motion because the objects are appearing and disappearing.

- What is the best explanation for the movement you see in the illustration above?
- This spinning wheel is called a Phenakistoscope. This demonstration certainly illustrates apparent motion, the experience of motion that the perceptual system produces from a sequence of still images showing in rapid succession. It is probably best thought of as an example of the phi phenomenon, where the gaps between the still images are filled in, though the distinction between the phi phenomenon and the beta phenomenon is not clear in this demonstration.



Which phenomenon is illustrated above?
beta effect OR phil phenomenon
This is the beta effect. You perceive motion based on the succession of images being presented.

- What is the best explanation for the movement you see in the illustration above?
- In this example, the red dots flash on and off fairly slowly. The usual experience is to see the back circle moving to the front, but not the strong experience of a blur between them, as occurs in the Phi Phenomenon. This is a great example of the Beta Effect. As with the phi phenomenon, the brain takes cues from the changing image and creates for our conscious mind the experience of movement.

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