5.8 Perception of depth, size, and motion is guided by internal and external cues

Question 1

Depth perception

Answer 1

• We are able to perceive depth in the two-dimensional patterns of photographs, movies, etc. But we are still able to distinguish these two-dimensional images from our three-dimensional world because some depth cues are apparent in two-dimensional images, while others are not
• There are several types of depth cues, including binocular depth cues and monocular depth cues

Question 2

Binocular depth cues

Answer 2

• These are available form both eyes together and are present only when viewing the three-dimensional world
• They provide internal cues about how far away something is

Question 3

Monocular depth cues

Answer 3

These are available from each eye alone and provide organizational information that can be used to infer depth

Question 4

Motion depth cues

Answer 4

These emerge when we move through space and depend on relative changes to visual input with motion

Question 5

Binocular disparity

Answer 5

• One of the most important cues to depth perception is binocular disparity (or retinal disparity).
• This cue is caused by the distance between humans’ two eyes.
• Because each eye has a slightly different view of the world, the brain has access to two different but overlapping retinal images.
• The brain uses the disparity between these two retinal images to compute distances to nearby objects.
• The ability to determine an object’s depth based on that object’s projections to each eye is called stereoscopic vision.

Question 6

Convergence

Answer 6

• A binocular depth cue related to binocular disparity is convergence
• This term refers to the way that eye muscles turn the eyes inward when we view nearby objects
• To focus both eyes on a close object reqiures the eyes to converge more than if the object is far away
• The brain knows how much the eyes are converging through feedback from eye muscles and uses this information to perceive distance

Question 7

Downside of binocular disparity

Answer 7

• Although binocular disparity is an important cue for depth perception, it is useful only for relatively close objects.
• An object that is very far away will create little binocular disparity and require minimal convergence
• Monocular depth cues are effective with only one eye and can distinguish the relative distance of even faraway objects

Question 8

Pictorial depth cues

Answer 8

• Occlusion
• Relative size
• Familiar size
• Linear perspective
• Texture gradient
• Position relative to horizon

Question 9

Occlusion

Answer 9

A near object occludes (blocks) an object that is farther away

Question 10

Relative size

Answer 10

Far-off objects project a smaller retinal image than close objects do, if the far-off and close objects are the same size

Question 11

Familiar size

Answer 11

Because we know how large familiar objects are, we can tell how far they are by the size of their retinal images

Question 12

Linear perspective

Answer 12

Seemingly parallel lines appear to converge in the distance

Question 13

Texture gradient

Answer 13

As a uniformly textured surface recedes, its texture continuously becomes denser

Question 14

Position relative to horizon

Answer 14

• All else being equal, objects below the horizon that appear higher in the visual field are perceived as being father away.
• Objects above the horizon that appear lower in the visual field are perceived as being farther away

Question 15

Diagram of motion parallax

Answer 15

Question 16

Motion parallax

Answer 16

• Arises from the relative speed with which objects move across the retina as a person moves
• Because our view of objects closer to us changes more quickly than does our view of objects that are farther away, motion provides information about how far away something is

Question 17

Size perception

Answer 17

• The farther away the object is, the smaller its retinal image
• To determine an object’s size, the visual system needs to know how far away the object is
• Most of the time, enough depth information is available for the visual system to work out an object’s distance and thus infer how large the object is

Question 18

Ames box

Answer 18

• These constructions present powerful depth illusions
• Inside the Ames boxes, rooms play with linear perspective and other distance cues
• One such room makes a far corner appear the same distance away as a near corner

Question 19

Ponzo illusion

Answer 19

• Another example of a size/distance illusion
• The common explanation for this effect is that the monocular depth cues make the two-dimensional figure seem three-dimensional
• You perceive the two parallel lines in the center as if they are at different distances away from you and thus different in size when they are actually the same size

Question 20

Motion perception

Answer 20

• Motion is generally detected by the relative movement of visual information
• If you view one object, say, a billiard table, as a frame of reference, and a billiard ball changes its position on the table, that is a cue the ball is moving
• Subtle changes in static visual images that are presented in rapid succession can also give the illusion of movement. Movies are made up of still-frame images, presented one after the other to create the illusion of motion pictures. This phenomenon is based on stroboscopic movement, a perceptual illusion that occurs when two or more slightly different images are presented in rapid succession. The apparent motion illusion demonstrates that the brain, much as it fills in gaps to perceive objects, also fills in gaps in the perception of motion.

Question 21

Motion afereffects

Answer 21

• When motion is detected in an environment, specialized neurons in a secondary visual area of the brain respond to the orientation of movement
• Motion aftereffects provide evidence that motion-sensitive neurons exist in the brain
• Motion aftereffects occur when you gaze at a moving image for a long time and then look at a stationary scene. You experience a momentary impression that the new scene is moving in the opposite direction to the moving image.