Topic 4 - Visual Illusions

Question 1

Oculomotor cues

Answer 1

based on sensing the position of the eyes and muscle tension

• Ocular convergence
• Accommodation

Question 2

Ocular convergence

Answer 2

inward movement of the eyes when we focus on nearby objects

Question 3

Accommodation

Answer 3

change in the shape of the lens when we focus on objects at different distances

Question 4

Monocular depth cues

Answer 4

Only come from one eye

Pictorial cues and motion-produced cues

Question 5

Pictorial Cues

Answer 5

sources of depth information that come from 2-D images, such as pictures

Includes:
Occlusion
Relative height
Relative size
Familiar Size
Perspective convergence
Atmospheric perspective
Texture gradient
Shadows

Question 6

Occlusion

Answer 6

When one object partially covers another

Question 7

Relative height

Answer 7

objects below the horizon that are higher in the field of vision are more distant

Question 8

Relative size

Answer 8

when objects are equal size, the closer one will take up more of your visual field

Question 9

Familiar size

Answer 9

distance information based on our knowledge of object size

Question 10

Perspective convergence

Answer 10

parallel lines appear to come together in the distance

Question 11

Atmospheric perspective

Answer 11

distance objects are fuzzy and have a blue tint (on a clear day)

Question 12

Texture gradient

Answer 12

equally spaced elements are more closely packed as distance increases

Question 13

Shadows

Answer 13

indicate where objects are located

Question 14

Motion parallax

Answer 14

close objects in direction of movement glide rapidly past but objects in the distance appear to move slowly

Question 15

Deletion and accretion

Answer 15

objects are covered or uncovered as we move relative to them

Question 16

Binocular depth cues

Answer 16

Two eyes cues

Includes binocular disparity and stereopsis

Question 17

Stereoscopic depth perception

Answer 17

depth perception created by input form both eyes

Question 18

Strabismus

Answer 18

misalignment of the eyes

Question 19

Binocular disparity

Answer 19

difference in the images on the left and right retinas

• Corresponding retinal points - points on retina that would overlap - a surface called the horopter
• Noncorresponding points - images that are not on the horopter
• Absolute disparity - angle between noncorresponding points, angle of disparity
• Crossed disparity - eyes see object on the opposite side of visual field to side eye is on
• Uncrossed disparity - eyes see object on same side of visual field to side eye is on
• Relative disparity - difference between absolute disparity of two objects

Question 20

Steropsis

Answer 20

depth information provided by binocular disparity (perceptual)

Stereoscope uses two pictures from slightly different viewpoints (one to each eye)
- Random-dot stereogram - has two identical patterns with one shifted in position

Question 21

Correspondence problem

Answer 21

How does the visual system match images from the two eyes? – Matches may be made by specific features of objects

This may not work for objects like random-dot stereograms, A satisfactory answer has not yet been proposed

Question 22

Depth perception in other species

Answer 22

Animals use same cues

Frontal eyes needed for binocular disparity
Lateral eyes provide a wider view

Question 23

Experiment by Tsutsui et al.

binocular depth perception

Answer 23

• Monkeys matched texture gradients that were 2-D pictures and 3-D stereograms.
• Recordings from a neuron in the parietal lobe showed: >Cell responded to 2D pictorial cues, Cell also responded to 3D binocular disparity

Question 24

Binocular depth cells, disparity selective cells

Answer 24

These cells respond best to a specific degree of absolute disparity between images on the right and left retinas (have a tuning curve)

Question 25

Experiment by Blake and Hirsch

binocular disparity and depth perception

Answer 25

• Cats were reared by alternating vision between two eyes every other day for 6 months - no chance of binocular disparity
• Results showed that they: had few binocular neurons due to cortical plasticity and were unable to use binocular disparity to perceive depth

Question 26

Experiment by DeAngelis et al.

binocular disparity and depth perception

Answer 26

• Monkey trained to indicate depth from disparate images
• Disparity-selective neurons were activated by this process (in V1)
• Experimenter used micro-stimulation to activate different disparity-selective neurons
• Monkey shifted depth judgment to the artificially stimulated disparity

Question 27

Experiment by Holway and Boring

Perceiving size

Answer 27

• Observer was at the intersection of two hallways
  ○ A luminous test circle was in the right hallway placed from 10 to 120 feet away
  ○ A luminous comparison circle was in the left hallway at 10 feet away
  ○ On each trial the observer was to adjust the diameter of the test circle to match the comparison
  ○ Test stimuli all had same visual angle (angle of object relative to the observer’s eye)
  ○ Part 1 of the experiment provided observers with depth cues - Judgments of size were based on physical size
  ○ Part 2 of the experiment provided no depth information – Judgments of size were based on size of the retinal images (i.e., visual angle)

Question 28

Visual angle

Answer 28

Angle of an object relative to the observer’s eye

Question 29

Size constancy

Answer 29

Perception of an object’s size remains relatively constant
- This effect remains even if the size of the retinal image changes (or distance from the observer changes)

Size-distance scaling equation – S = K (R x D) - The changes in distance and retinal size balance each other

Question 30

Size-distance scaling - Emmert’s Law

Answer 30

Retinal size of an afterimage remains constant

• Perceived size will change depending on distance of projection (if you look at something further away, the afterimage will appear larger and vice versa)

Question 31

Muller-Lyer Illusion

Answer 31

Straight lines with inward fins appear shorter than straight lines with outward fins, lines are actually the same length

Misapplied Size Constancy Scaling - what works in 3D is misapplied in 2D
- Problems with this theory

Conflicting cues theory - perception depends on actual length of vertical lines in combination with overall length of the figure - compromise of perception

Question 32

Ponzo illusion

Answer 32

Horizontal objects are placed over railroad tracks in a picture, upper object appears larger
- Misapplied size constancy scaling

Question 33

Ames Room

Answer 33

Reasons:
Size-distance scaling - Observer thinks the room is normal, People would be at same distance – Person on the left has smaller visual angle (R) – Due to the perceived distance (D) being the same their perceived size (S) is smaller

Relative Size - Perception of size depends on size relative to other objects. – One woman fills the distance between the top and bottom of the room. – The other woman only fills part of the distance – Thus, the woman on the right appears taller

Question 34

Moon illusion

Answer 34

The moon appears larger on the horizon than when it is higher in the sky

Apparent-distance theory - horizon moon is surrounded by depth cues while moon higher in the sky has none - Horizon is perceived as further away than the sky - called “flattened heavens”

Angular size-contrast theory - the moon appears smaller when surrounded by larger objects, Thus, the large expanse of the sky makes it appear smaller