Task 6 Three dimensions

Question 1

Cue approach to depth perception

Answer 1

Explains how we get from the flat image on the retina to three dimensional perception of the scene

Question 2

Occlusion

Answer 2

Cue that one object is in front of another. This cue is learnt through previous experience with environment and once it is learnt, we automatically associate particular cues and depth –> We experience world in three dimensions

Question 3

Oculomotor cues

Answer 3

Cues based on our ability to sense the position

Question 4

Convergence (Oculomotor cues)

Answer 4

Inward movement of eyes that occurs when we look at nearby objects.
 Most effective cue of the two.

Question 5

Accomodation (Oculomotor cues)

Answer 5

Change in shape of lens that occurs when we focus on objects at various distances.
 These cues work over distances at close range (up to 2 meters).

Question 6

Monocular cues

Answer 6

Cues that work with one eye. These include accommodation, pictorial cues and motion-produced cues.

Question 7

Pictorial cues

Answer 7

Sources of depth information that can be depicted in a two-dimensional picture. These cues are stationary, meaning they are fixed.

Question 8

Occlusion

Answer 8

Occurs when one object (partially) hides another from view.
 Partially hidden object seen as being farther away.
 Does not provide info about object’s distance.
 Works at any range.

Question 9

Relative size

Answer 9

When two objects are of equal size, the one farther away will take up less of our field of view than the one closer.
 This cue depends on a person’s knowledge of physical sizes.
 Works at any range.

Question 10

Familiar size

Answer 10

Cue used when judging distance based on prior knowledge of size of objects.
 Most effective when other information about depth is absent.

Question 11

Texture gradient

Answer 11

Elements equally spaced in scene appear to be more closely packed as distance increases.
 Increasing fineness of texture as distance increases enhances perception of depth.

Question 12

Relative height

Answer 12

Objects with their bases closer to horizon are usually seen as being more distant. Works from 2m.

Question 13

Perspective Convergence

Answer 13

Cue experienced when looking down parallel lines that appear to converge in distance.

Question 14

Atmospheric perspective

Answer 14

Occurs when distant objects appear less sharp than nearer objects, with a slight blue tint. Works above 30m

Question 15

Shadows

Answer 15

Decreases in light intensity caused by blockage of light that provide info about location of objects.

Question 16

Motion produced cues

Answer 16

Cues that emerge when we start moving, that enhance our perception of depth. Both work at close and medium ranges (up to 20 meters).

Question 17

Motion parallax

Answer 17

Occurs when nearby objects appear to glide rapidly past us, but more distant objects appear to move more slowly.
 Example: Moving nearby cars appear to speed by in a blur, whereas those farther away appear to be moving slightly.
 Explanation: Image of objects closer to us move farther across the retina than that of objects farther away.

Question 18

Deletion and Accretion

Answer 18

As an observer moves sideways, some things become covered (= deletion), and others become uncovered (= accretion).

Question 19

Binocular depth information

Answer 19

cue that relies on info from both eyes

Question 20

Stereoscopic vision

Answer 20

Ability to use binocular disparity as a cue to depth.

Question 21

Binocular disparity

Answer 21

Differences between two retinal images of same scene. Basis for stereopsis.
 Difference with monocular vision – When watching a 2-D movie, both eyes receive same image, so depth is indicated by monocular pictorial depth cues. When watching a 3-D movie, both eyes receive different images, so stereoscopic depth perception occurs

Question 22

Corresponding retinal points

Answer 22

Points on the retina that overlap if eyes are superimposed on each other.

Question 23

Horopter

Answer 23

Imaginary circle that passes through point of focus. Surface of zero disparity

Question 24

Non corresponding points

Answer 24

Images of objects that are not on horopter fall on noncorresponding points.

Question 25

Absolute disparity

Answer 25

– Degree to which these objects deviate from falling on corresponding points. It is determined by measuring angle between where corresponding point would be located and where it is actually located.

Question 26

Angle of disparity

Answer 26

Amount of absolute disparity. It provides information about object’s distance from horopter, with greater angles of disparity indicating greater distances from it

Question 27

Crossed disparity

Answer 27

Sign of disparity produced by objects in front of horoptor. Objects located in front of horopter appear to be displaced to the left in right eye, and to the right in left eye

Question 28

Uncrossed disparity

Answer 28

Sign of disparity produced by objects behind horoptor. Objects located behind horopter appear to be displaced to the right in right eye and to the left in left eye

Question 29

Relative disparity

Answer 29

Difference in absolute disparities of objects in a scene that remains the same as an observer looks around a scene.
 Example: When gaze is on woman, her image falls on observer’s foveas (= disparity is zero) and that of the man falls on noncorresponding points (= disparity). When gaze is shifted to man, his image falls on foveas (= disparity is zero) and that of the woman falls on noncorresponding points (= disparity).

Question 30

DISPARITY (GEOMETRICAL) CREATES STEREOPSIS (PERCEPTUAL)

Answer 30

Disparity is related to geometry, the locations of images on retina, and stereopsis is related to perception, the experience of depth created by disparity

Question 31

Stereoscope

Answer 31

Device for simultaneously presenting slightly different images to one eye and the other, producing an illusion of depth because it creates same binocular disparity that occurs when a person views the scene naturally.

Question 32

Random-dot stereogram

Answer 32

Stereogram made of large number of randomly placed dots, that contains no monocular cues to depth. Aimed at showing that disparity alone can result in depth perception

Question 33

Correspondence problem

Answer 33

Visual system must compare images of left and right retinas in order to calculate the amount of disparity.
 Visual system matches images on left and right retinas on the basis of specific features of objects.
 Still no answer as to how it works with random-dot stereogram

Question 34

PHYSIOLOGY OF BINOCULAR DEPTH PERCEPTION

Answer 34

Neurons have been found that respond best to binocular disparity.

Question 35

Binocular depth cells (disparity-selective cells)

Answer 35

Respond best when stimuli presented to left and right eyes create specific degree of absolute disparity.
 Primary receiving area – Neurons sensitive to absolute disparity.
 Temporal lobe and other areas – Neurons sensitive to relative disparity.
 Depth perception begins in primary visual cortex and extends to different areas in ventral and dorsal streams.

Question 36

Selective rearing

Answer 36

experiment by Blake and Hirsch where cats were reared by alternating vision between two eyes every other day during their first 6 months.
 Results: Showed that cats had few disparity-selective neurons and they were unable to use binocular disparity to perceive depth.
 Conclusion: Eliminating disparity-selective neurons eliminates stereopsis and therefore, disparity-selective cells are responsible for stereopsis.

Question 37

Microstimulation

Answer 37

a procedure in which small electrode is inserted into cortex and electrical charge is passed through electrode to activate neurons nearby.
 Procedure: Monkey observes random-dot stereogram while experimenters stimulate neurons in monkey’s cortex that were sensitive to particular amount of disparity. When neurons tuned to disparity different from that of images on retina, monkey shifted its perception of depth toward disparity signaled by stimulated neurons

Question 38

Holway and Boring experiment

Answer 38

Experiment aimed at demonstrating that we can misperceive size when accurate depth information is not present.
 Procedure: Observer changes diameter of comparison circle in left corridor to match her perception of the size of test circles presented in right corridor. Each test circle, presented separately, casts same image on retina and has visual angle of 1 degree.

Question 39

Visual angle

Answer 39

WHAT IS A VISUAL ANGLE?
Angle of object relative to observer’s eye that is determined by extending lines from person to the lens of observer’s eye.
 It depends on size of stimulus and on its distance from observer – When person moves closer, visual angle and size of image on retina become larger.
 Small object near and larger object far can have same visual angle (= thumb method).
 We perceive the sun and moon to be same size because we are unable to perceive their distance, therefore, we base our judgement on their visual angles.

Question 40

Size constancy

Answer 40

Fact that our perception of object’s size is relatively constant, even when we view object from different distances.
 Under conditions of poor depth information, size constancy decreases.
 Size constancy remains even if size of object in retina changes.

Question 41

Size distance scaling

Answer 41

Idea that perceiver combines the size of object formed on retina and its distance to perceive the actual size of an object.
 Equation – S = K ( R x D ).
 S  Object’s perceived size; K  Constant; R  Size of retinal image and; D  Perceived distance of object.
 As person goes farther away from us and size of image in retina (R) becomes smaller, our perception of person’s distance (D) becomes larger. Since it creates a balance, it results in perception of size (S) staying the same.

Question 42

Emmert’s law

Answer 42

The farther away an afterimage appears, the larger it will seem.
 Size of afterimage is determined by distance of surface against which afterimage is viewed.
 (R) remains the same, (D) increases and, therefore, (S) is larger.

Question 43

Misapplied size constancy scaling

Answer 43

Explanation for this effect proposing that mechanisms that help us perceiving in 3-D world sometimes create illusions when applied to objects drawn on 2-D surface.
 Fins on right line make it look like inside corner of room and fins on left line make it look like corner viewed from outside.
 (R) remains the same and (D) is larger, therefore, (S) is determined by perceived distance.

Question 44

Conflicting cues theory

Answer 44

Our perception of line length depends on (1) actual length of vertical lines and (2) overall length of figure.
 Overall length of right figure is larger so, the vertical line appears larger.
 Theory rejects idea that depth information is involved in determining illusions.

Question 45

Ponzo illusion

Answer 45

Illusion that causes two people of equal size to appear very different in size.
 Reason – Construction of room because it is shaped so that left corner is almost twice as far from observer as right corner, causing woman on left to have much smaller visual angle than one on the right.
 Size-distance scaling – (D) remains same, (R) smaller for woman on left, therefore, (S) is smaller.
 Relative size – Perception of size of two women is determined by how they fill distance between bottom and top of room.

Question 46

Moon illusion

Answer 46

Illusion in which moon on horizon appears much larger than when it is higher in the sky.
 Constant visual angle – Moon’s physical size and distance from Earth remain the same throughout the night.
 Apparent distance theory – States that moon horizon appears more distant because it is viewed across filled space of terrain, which contains depth information, and when it is higher in sky, it appears less distant because it is viewed through empty space, with little depth information.
 Horizon appears to be farther away than the sky overhead – (R) remains same, (D) is larger for moon on horizon and, therefore, (S) is larger.
 Angular size contrast theory – States that moon appears smaller when surrounded by larger objects.
 Other factors – Atmospheric perspective, color and oculomotor factors.

Question 47

Two eyes, to views: Brain and depth perception

Answer 47

Visual-image processing from eye to brain happens in stages
 Rudimentary features are extracted early on areas V1 and V2 before reaching next stages for more refined analysis.
 These include orientation of edges, direction of motion and color.
 Many pathways go back from stage to stage.
 V1 – Contains cells signaling disparity. Binocular disparity starts here.
 V2 – Contains cells that extract illusory contours. Contours process starts here.

Question 48

Correspondence problem

Answer 48

In binocular vision, problem of figuring out which feature of image in left eye should match with that of right eye.
 Brain solves this problem by recognizing forms and then comparing extended outlines of the forms, helping the brain to avoid false matches.

Question 49

Is comparison done before object boundaries are recognized or after?

Answer 49

 Stereopsis precedes detection of extended outlines and boundaries.
 Random-dot experiment – Square emerges only a result of a stereoscopic fusion, stereo matching must be a point-to-point measurement of displacement. Outline of square emerges from this comparison.
 Special circumstances – Sometimes form perception precedes stereopsis, which shows the flexibility of brain’s visual centers.
 It occurs when dots defining squares are uncorrelated in two eyes and thus, recognized separately.