SAP - Visual Perception Lesson

Learning Target:

Explain the role of top-down processing.

AP psychology course and exam description, effective fall 2020. (n.d.). https://apcentral.collegeboard.org/media/pdf/ap-psychology-course-and-exam-description.pdf

Color Vision

2 Major Color Theories: Trichromatic Theory and Opponent Process Theory There are two major theories that explain how we see color. Each can explain some aspects of the process, but neither can explain all aspects of color vision.

Trichromatic Theory

The Trichromatic Theory states that we have three varieties of cones in our retinas. Cones that detect red, long wavelengths, cones that detect green, medium wavelengths, and cones that detect blue, short wavelengths. Any given cone will be overly sensitive to one of the wavelengths and slightly to others. Other colors on the spectrum are achieved by stimulating a combination of the cones, creating millions of color combinations.

According to this theory, people who are colorblind lack cone receptor cells for one or more of these primary colors. The most common type of color blindness is red/green. This is when someone cannot discern between the two colors. Their blue-sensitive cones are normal, but their other cones are either red-sensitive or green-sensitive, not both. This creates a situation in which red and green both look the same.

The image below illustrates a common test for colorblindness called Ishihara. A number is projected amongst many different colored dots. What number do you see?

Opponent Process Theory

While the Trichromatic Theory explains many aspects of color vision, it is unable to explain what is called the afterimage effect (a visual experience that occurs after the original source of stimulation is no longer present). Afterimages can, however, be explained by the Opponent Process Theory of Color.

In this theory, there are four basic color combinations divided into two pairs of color-sensitive neurons. Red/green, yellow/blue, and then black/white which absorbs and reflects all light. If one of the colors in the pairing is stimulated, the other becomes dormant. So, if red is stimulated, green will remain dormant. However, due to sensory adaptation, if the color that is being stimulated is removed, we will see its opposite.

Afterimage: If one of the color pairing is stimulated, the other becomes dormant. So if red is stimulated green will remain dormant. However, due to sensory adaptation if the color that is being stimulated is removed we will see it's opposite.

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