ap psych study guides

🤔  Unit 5: Cognitive Psychology

👶  Unit 6: Developmental Psychology

🤪  Unit 7: Motivation, Emotion, & Personality

🛋  Unit 8: Clinical Psychology

3.3 Visual Anatomy

#sensation

#perception

#retina

#pupil

#lens

⏱️  5 min read

written by

Dalia Savy

dalia savy


⚡️Visible Light Spectrum

To understand light, you should be familiar with the electromagnetic spectrum. Humans see visible light. The spectrum varies in wavelength from 400nm to 700m, purple to red. Wavelength is the distance between two peaks or two troughs of a wave. Wavelength determines hue, the dimension of color.

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Image Courtesy of Amazon.

You should also be familiar with the term intensity, which is the amount of energy in light. For example, if something is really bright, it has a high light intensity; if something is really loud, it has a high sound intensity. Intensity is determined by the amplitude of a wave, which is the wave’s height.

You may wonder, what are the other waves on the electromagnetic spectrum? Even though we cannot see these other waves, such as x-rays, other animals could see them. Interestingly enough, bees can see waves from 300-650nm, which means they can’t see the “color” red, but they can see UV rays.

You may also wonder why I put color in quotes. Color is actually a mental construct—objects don’t actually have color. The way that waves reflect an object creates what we perceive as color—our brain gives the object color.

👁 Parts of the Eye

Now we get down to the study of the 5 senses in-depth, beginning with sight! 

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Image Courtesy of Myers AP Psychology Textbook - 2nd Edition

1. First, light passes through the cornea, a thin tissue that protects the eye and bends light to provide focus.

2. Next, light passes through the pupil, a small opening controlled by the iris. The iris is a colored muscle that constricts or dilates based on light intensity. The more light exposure, the more the pupil constricts.

3. Behind the pupil is the lens. It focuses incoming light onto the retina as an upside-down image and changes the shape of light. This is called accommodation

4. Once the image is received on the retina, visual information begins to be processed. At the retina's receptor, cells convert light into neural impulses that travel to the brain via the optic nerve. The retina is a multilayer tissue that contains ganglion cells, bipolar cells, rods, and cones.

  • It is important to know that light first goes to the rods and cones, then to the bipolar cells, then to the ganglion cells, then to the optic nerve, and then finally to the thalamus and the visual cortex. The second the image hits the rods and cones, it becomes a neural impulse.

  • Rods are one of the two photoreceptors necessary to convert light into impulses. They detect black, white, and gray. They are also more light-sensitive and necessary for peripheral vision. 

  • Cones are the other photoreceptors, but they function well in daylight, detect color (RBG), and detect fine detail. While rods are found throughout the whole retina, cones are usually found closer to the fovea, the central focal point.

  • Note: memorizing the differences between rods and cones is essential for this unit!

5. In the visual cortex, cells that respond to specific features of a stimulus are found. These are called feature detectors (discussed in more detail below).

6. Note: Where the optic nerve leaves the eye is a blind spot, as a result of the absence of receptor cells there. 

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Image Courtesy of Myers' AP Psychology Textbook - 2nd Edition

As mentioned before, feature detectors were discovered by Hubel and Wiesel in the visual cortex. They pass information about stimuli (lines, shapes, edges) to other regions of the brain where supercell clusters (a team of cells) work to respond to the patterns. These cells activated on based on your environment. Supercell clusters are so specific that looking at an fMRI, you can tell exactly what the patient is looking at.

Since every stimulus has a million lines, shapes, and edges, how does our brain process all of this at once? Through parallel processing, we are able to divide a visual scene by these aspects and work on each aspect simultaneously. 

🌈 Theories of Color Vision

Remember the electromagnetic spectrum? We only see visible light, which corresponds to ROYGBIV. The longest wavelength corresponds to red and the shortest corresponds to purple. But how do we really see color?

Thomas Young and Hermann von Helmholtz came up with a theory that the retina contains three different color receptors, hence RBG (red, blue, green). This theory is called the Young-Helmholtz Trichromatic Theory, and they concluded that these three colors combine to produce the other colors we see.

People that are colorblind may lack one or two of these color receptors. Their vision may be monochromatic (one color; lacking two receptors) or dichromatic (two colors; lacking one receptor). Men are more likely to be color blind. Another psychologist, Ewald Hering, came up with the Opponent-Process Theory. He found that certain neurons could be turned “on” or “off” based on the wavelength of the incoming light.

Basically, once you stare at one color for a really long time, the afterimage of the color would be a completely opposite color. For example, stare at the following image for about a minute. It may seem completely opposite at first since the American flag is actually red, white, and blue, but after a minute, stare at a blank white piece of paper. What do you see?

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Image Courtesy of Study

Your brain “tires” the other color out and activates its opposite. This is why when staring at green, yellow, and black, the afterimage would be red, blue, and yellow.

The color pairs are: Red + Green

                             Yellow + Blue

                             White + Black 

👓 Common Sensory Conditions

Sometimes, we need glasses, and maybe one day when you go to the optometrist, they will say you have astigmatism. Here’s what this means:

If you are near-sighted, your vision is blurry when you look at something in the distance. This is because of too much curvature of the cornea which causes the image in front of you to be clearer than images in the distance. If you are far-sighted, there is too little curvature of the cornea, making the images in the distance seem much clearer.

Astigmatism distorts/blurs the image at the retina. It is caused by an irregularity in the shape of the cornea or lens.

🎥 Watch: AP Psychology - Visual Anatomy and Perception

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