# 13.6: Perceiving Motion

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Many animals, including human beings, have very sophisticated perceptual skills that allow them to coordinate their own motion with the motion of moving objects in order to create a collision with that object. Bats and birds use this mechanism to catch up with prey, dogs use it to catch a Frisbee, and humans use it to catch a moving football. The brain detects motion partly from the changing size of an image on the retina (objects that look bigger are usually closer to us) and in part from the relative brightness of objects.

We also experience motion when objects near each other change their appearance. The beta effect refers to the perception of motion that occurs when different images are presented next to each other in succession (see the Beta Effect and Phi Phenomenon note). The visual cortex fills in the missing part of the motion and we see the object moving. The beta effect is used in movies to create the experience of motion. A related effect is the phi phenomenon, in which we perceive a sensation of motion caused by the appearance and disappearance of objects that are near each other. The phi phenomenon looks like a moving zone or cloud of background color surrounding the flashing objects. The beta effect and the phi phenomenon are other examples of the importance of the gestalt—our tendency to “see more than the sum of the parts.”

#### BETA EFFECT AND PHI PHENOMENON

In the beta effect, our eyes detect motion from a series of still images, each with the object in a different place. This is the fundamental mechanism of motion pictures (movies).
In the phi phenomenon, the perception of motion is based on the momentary hiding of an image.

#### KEY TAKEAWAYS

• Vision is the process of detecting the electromagnetic energy that surrounds us. Only a small fraction of the electromagnetic spec- trum is visible to humans.
• The visual receptor cells on the retina detect shape, color, motion, and depth.
• Light enters the eye through the transparent cornea and passes through the pupil at the center of the iris. The lens adjusts to focus the light on the retina, where it appears upside down and backward. Receptor cells on the retina are excited or inhibited by the light and send information to the visual cortex through the optic nerve.
• • The retina has two types of photoreceptor cells: rods, which detect brightness and respond to black and white, and cones, which respond to red, green, and blue. Color blindness occurs when people lack function in the red- or green-sensitive cones.
• • Feature detector neurons in the visual cortex help us recognize objects, and some neurons respond selectively to faces and other body parts.
• The Young-Helmholtz trichromatic color theory proposes that color perception is the result of the signals sent by the three types of cones, whereas the opponent-process color theory proposes that we perceive color as three sets of opponent colors: red-green, yellow-blue, and white-black.
• The ability to perceive depth occurs through the result of binocular and monocular depth cues.
• Motion is perceived as a function of the size and brightness of objects. The beta effect and the phi phenomenon are examples of perceived motion.

#### EXERCISES AND CRITICAL THINKING

1. Consider some ways that the processes of visual perception help you engage in an everyday activity, such as driving a car or riding a bicycle.
2. Imagine for a moment what your life would be like if you couldn’t see. Do you think you would be able to compensate for your loss of sight by using other senses?

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This page titled 13.6: Perceiving Motion is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Kate Votaw.