Depth Perception

Question 1

Name the Binocular Depth Cues

Answer 1

• Retinal disparity
• Convergence

Question 2

Name the Monocular Depth Cues

Answer 2

• Motion Parallax
• Accommodation
• Pictorial
• Angular Declination below the Horizon
• Kinetic Depth

Question 3

Name the Pictorial Depth Cues

Answer 3

• Relative Size
• Familiar Size
• Linear Perspective
• Texture
• Interposition
• Clairy
• Lighting & Shadow

Question 4

Describe the Relative Size as a depth cue with an example

Answer 4

• Objects that produce the smaller retinal image size is perceived as farther away
• Relative size is an important depth cue when viewing a scene that includes objects whose sizes can be compared to each other
• Not useful for determining the plane’s distance
  
  • There are no objects to which it can be compared (against the blue sky)

Because we have no reference, we assume that the 2 balloons are of the same size

Question 5

Describe the Familiar Size (Size Constancy) as a depth cue with an example

Answer 5

• Used when viewing objects of known size
• In judging its distance, we would probably assume it is the size of a typical plane (size constancy)
• If it produces a small retinal image, the plane would be seen as far away

Question 6

Describe the Linear Perspective as a depth cue with an example

Answer 6

• Related to relative size
• Can be experienced by looking down a long corridor or railroad track
• The retinal image size of the rail separation in the background, however, is smaller than the foreground, creating an illusion of depth
  
  • Although all aspects of photograph are physically the same distance from the eyes

Question 7

Describe the Texture Gradient as a depth cue with an example

Answer 7

• Densely packed objects that produce smaller retinal image sizes are perceived as more distant
  
  • i.e., distance objects are seen as smaller and densely packed

• Bricks in the background appears more densely packed than those in the foreground

Question 8

Describe the Ariel Perspective (Clarity) as a depth cue with an example

Answer 8

• A form of interposition/clarity
• Unobstructed objects in photographs are perceived as closer than those that are obscured by Haze
• Example:
  
  • Fog, rain, smoke, and smog may act as interposing elements-causing them to appear more distant and contributes to the sense of depth

Question 9

Describe the Interposition as a depth cue with an example

Answer 9

• Occurs when the view of scene is partially obstructed
• A nearer object occludes the distant object creating a sense of depth

• Church is interposed between the viewer and the mountains (perceived as lying distant to it)

Question 10

Describe the Lighting and Shadow as a depth cue with an example

Answer 10

• When light falls on an object -> casts a shadow
• Shadow is interpreted as falling behind the object, consequently, a sense of depth is created

Question 11

Name the nonpictorial depth cues

Answer 11

• Kinetic Depth Effect
• Angular Declination below the horizon
• Motion parallax
• Accommodation

Question 12

Describe the Kinetic depth effect

Answer 12

• Shape from motion
• Kinetic depth effect is stimulated by matlab graphics
  
  • Two random “wires” of 7 angles are in the movie
• Rotating wire (2D)-(differential motion) gives rise to 3D percept
• Looming: rapid expansion (or shrinking) in the size of any given image

Question 13

Describe the Angular declination below the horizon with an example

Answer 13

• Monocular observer standing upright in an empty room viewing an object at 10 feet
• Despite the absence of pictorial depth cues, the observer can correctly judge the object’s distance
• The object makes and angle with the horizon referred to as angular declination below the horizon
  
  • The visual system uses this angle to determine object distance

Question 14

Describe Emmert’s Law

Answer 14

• The principle that the perceived size of an afterimage is proportional to the distance of the surface on which it is projected
  
  • The larger the afterimage, the farther away it is
• Afterimage illusions

Question 15

Describe Motion Parallax with an example

Answer 15

• A kinetic monocular depth cue that results when a moving observer fixates on an object while noticing the relative motion of surrounding objects
• Relative motion provides information regarding relative distance
• Demonstration:
  
  • Place 2 index fingers directly in line with each other in front of the right eye
    
    • One at 15 cm, another at 30 cm
    • Close your left eye
  • Fixate on distant finger while moving your head sideways
    
    • Near finger appears to move opposite to the direction of your head (against motion)
  • Fixate on near finger while moving your head sideways
    
    • Distance finger appears to move in the same direction of your head (with motion)

Question 16

Describe how motion parallax is useful depth cue in clinical examination

Answer 16

• Has useful clinical application when viewing ocular structures with a monocular instrument
• Consider a small dot like opacity that is observed during monocular ophthalmoscopy
  
  • If the clinician fixates on the iris and moves her head sideways:
    
    • The opacity will show against motion when located ANTERIOR TO THE IRIS
    • With motion when the opacity is located POSTERIOR TO THE IRIS

Question 17

Describe how accommodation is a monocular depth cue

Answer 17

• During accommodation = power of the lens increases
• Although the signal for accommodation contains information that could be used to determine the distance of viewed objects, the extent to which this information is utilized is not known
• Hypothesis: level of innervation of accommodation could provide distance information
• Studies have shown accommodation is not utilized much to judge distances
• Accommodation seems to be a weak cue to depth perception

Question 18

What are the binocular depth cues?

Answer 18

• Stereopsis
• Convergence

Question 19

How does stereopsis provide depth information to the visual system?

Answer 19

• The perception of depth that is produced by retinal disparity is referred to as stereopsis
  
  • It’s an important contributor to depth perception at near distances
• Less important when viewing far objects because the threshold for retinal disparity requires such objects to be separated by great distances
• Retinal disparity
  
  • Produces stereopsis only if it is sufficiently small to allow fusion
  • A binocular cue to the visual system
• If the disparity is large, the images fall on retinal positions that signal grossly different directions resulting in double vision or physiological diplopia
• Uncrossed disparity – images form nasal to the foveas (signaling that the ball is farther than fixation)
• Crossed disparity – images form temporal to the foveas (signaling that the ball is closer)
• You can experience physiological diplopia by holding (& fixating) at one index finger at 15cm from your nose and your other index finger directly behind it at arm’s length
  
  • Certain binocular cortical neurons are maximally responsive when a stimulus is located at a specific distance from the eyes
  • The same stimulus, when positioned at other distances, elicits a less vigorous neural response
  • By encoding disparity, these neurons may contribute to the physiological basis for stereopsis

Question 20

How does convergence contribute to depth perception?

Answer 20

• The eyes converge when we view near objects and diverge when we view distant objects
• The degree of convergence can potentially provide information regarding distance
• The way this information is incorporated into the conscious perception of depth, however, is unresolved

Question 21

Describe Perceptual Constancy

Answer 21

• The tendency to see familiar objects as having standard shape, size, color, or location regardless of changes in the angle of perspective, distance, or lighting
  
  • As it is assumed to be, rather than to the actual stimulus presented to the eye
  • I.e., stability in perception despite gross instability in stimulation

Question 22

Size Constancy

Answer 22

When people’s perception of a particular object’s size does not change regardless of changes in distance from the object, even though distance affects the size of the object as it is projected onto the retina

Question 23

Shape Constancy

Answer 23

When people’s perception of the shape of an object does not change regardless of changes to the object’s orientation

Question 24

Distance Constancy

Answer 24

Refers to the relationship between apparent distance and physical distance: it can cause us to perceive things as closer or farther away than they actually are

Question 25

Color Constancy

Answer 25

A feature of the human color perception system that ensures that the color of an object is perceived as similar even under varying conditions

Question 26

What are size illusions?

Answer 26

• Visual illusions – erroneous perceptions
  
  • There are many types
• Some result when pictorial depth cues are used to determine object size
• In spite of changes in the retinal image size, the apparent size of an object does not normally change with viewing distance
  
  • E.g., the size of an automobile at different distances
• Our visual system compensates for differences in retinal image size by taking into account the relative distance of an object -> size constancy
• When judgments of distances are erroneous, such as viewing a flat picture, size constancy fails to result in a size illusion

Question 27

Describe the Corridor Illusion

Answer 27

• Size constancy fails because monocular depth cues provide incorrect information regarding the relative distance
• Though the two balls are the same size and distance from the eyes, the top ball is perceived as larger and farther away

Question 28

Describe the Moon Illusion

Answer 28

• When viewed on the horizon, the moon appear larger than when viewed at its Zenith (overhead), even though the angular subtense is the same under both conditions
• Trees, houses, fields and other interposing objects cause the moon to be seen as farther away when viewed on the horizon, and hence it is made to look larger

Question 29

Describe the Muller-Lyer Illusion

Answer 29

• Both lines are equal size, but the line that appears to form an outgoing corner appears farther away than the line that appears to form an ingoing corner
• fMRI studies reveal the AREA OF CORTICAL ACTIVATION is related to the perceived size and not the physical size, suggesting that perceived size is encoded early at the earliest stages of cortical processing

Question 30

Describe the Clinical Considerations of Stereopsis and Visual Development

Answer 30

• Measurement of patient’s stereopsis provides important information regarding the status of the visual system
• Stereopsis is measured clinically by asking a patient to view a flat surface that has two identical figures separated by a small distance
  
  • Using polaroid or R/G glasses, only one object is presented to each eye
• The separation of the images is designed to produce retinal disparity, and this disparity results in stereopsis
  
  • This disparity is referred to as stereoacuity (a form of hyperacuity), and it can be as small as 3 arc seconds
• The visual cortex must contain a normal complement of binocular neurons for a highly developed stereoacuity
• Disorders of binocular vision, such as anisometropia and strabismus, may alter an infant’s visual experience and retard the development of binocular cortical neurons, resulting in ta reduction in stereoacuity
• Measurement of stereoacuity provides important information regarding a patient’s visual development

Question 31

What is the application of Monovision?

Answer 31

• Monovision – a common clinical practice of correcting:
  
  • One eye of a presbyope for distance
  • The other eye for near
  • Thereby enabling the patient to alternate between the two eyes to see both distant and near objects clearly
    
    • Contact lenses, refractive laser procedures
• Because the unused eye is largely suppressed, diplopia does not typically occur
• Monovision may interfere with stereopsis as patients’ binocularity is reduced at all distances
  
  • However, they retain depth perception due to monocular cues

Question 32

Will monovision correction cause a reduction in stereopsis that interferes with driving safety?

Answer 32

• Depends – not easy to answer -> some perform well, some don’t
• Monovision patients should be cautioned of a reduction in depth perception that could potentially interfere with driving
• Distance correction of the eye normally used for near should be considered