Chapter 6 - Depth Perception

Question 1

Describe the inverse problem?

Answer 1

multiple objects can be represented the same way on the retina because of depth info

Question 2

What two things do we need to know in order to perceive the depth an object is located in?

Answer 2

1. the size of the object

2. the distance we are from the object

Question 3

What are oculormotor depth cues?

Answer 3

depth cues based on feedback from oculormotor muscles that control the shape of the lens and the position of the eye
- the accomodation mechanism is only engaged in nearby objects (about 2m or farther)

Question 4

what are the two types of vergence?

Answer 4

convergence and divergence

Question 5

What is convergence?

Answer 5

when the eyes rotate inward in order to maintain the image of an object in the fovea (close objects = strong convergence)

Question 6

What is divergence?

Answer 6

when the eyes rotate outward to maintain the focus of the image as it moves away from you

Question 7

What is occlusion?

Answer 7

Non-metrical depth cue = does not provide info about depth magnitude

• monocular
• provides info on the relative ordering of depth
• is always correct = is most reliable
• gives no info on depth magnitude or size of objects occluding eachother

Question 8

Describe the pictorial depth cue of occlusion?

Answer 8

T-junctions

• powerful indicator of occlusion
• the continuous edge (that obeys good continuation) is typically seen as in front of the other object

Question 9

What is a metrical depth cue?

Answer 9

Provides information about depth magnitude

Question 10

What do relative height, relative size and familiar size provide information about?

Answer 10

How far an object is located from you

Question 11

What is a relative metrical depth cue?

Answer 11

A cue that could provide enough information to say that object A is twice as far as object B, without providing information towards the absolute distance between A and B
- relative size, relative height

Question 12

What is an absolute metrical depth cue?

Answer 12

A depth cue that provides quantifiable information about distance in the third dimension
- familiar size

Question 13

Describe relative height

Answer 13

• below the horizon (near objects are lower than the horizon)
• above the horizon (far object are above the horizon)
• linear perspective: parallel lines in a 3D world will converge in 2D space (lines that are more straight away from you in the real world converge onto the retina)
• nearby objects generate larger retinal images than distant ones – that’s why we use degrees of visual angle)

Question 14

Describe familiar size

Answer 14

• a size based depth cue
• knowing the retinal image size of a familiar object at a familiar distance lets us use its retinal image size to gauge depth

Question 15

Describe relative size

Answer 15

• A size based depth cue
• under the assumption that two or more objects are about the same size, the relative size of their retinal images can be used to judge their relative distance

Question 16

Describe texture gradient?

Answer 16

• items of the same size form smaller images when they are farther away
• an array of items that changes in size smoothly over the retinal image will appear to form a surface tilted in depth
• the more texture we see, the closer it is to us

Question 17

Name all the monocular pictorial depth cues.

Answer 17

1. occlusion
2. relative height
3. relative size
4. texture gradient
5. retinal image size
6. familiar size
7. aerial perspective
8. linear perspective
9. shading and shadows

Question 18

What is motion parallax?

Answer 18

The difference in the speed and direction with which objects appear to move in the retinal image as an observer moves within a scene
- near objects appear to move more and faster than further objects

Question 19

What is optic flow?

Answer 19

The relative motion of objects on the retinal image as an observer moves forward or backwards within a scene

Question 20

What are ocular depth cues?

Answer 20

Depth cues available by the position and accomodative state of the eye

Question 21

What are monocular depth cues?

Answer 21

Depth cues, pictorial or motion based, that is available through one eye alone

Question 22

What are binocular depth cues?

Answer 22

Depth cues that rely on the comparison of the retinal image in both eyes to infer depth
- stereopsis

Question 23

What is binocular summation?

Answer 23

The combination of signals from each eye in a way that makes performance on many tasks better with both eyes than either eye alone

Question 24

Describe cyclopean vision?

Answer 24

Both eyes see differently, but some objects fit at the same area in both eyes

Question 25

What is binocular disparity?

Answer 25

A depth cue based on differences in the relative positions of the retinal images of objects in the two eyes

Question 26

What is stereopsis?

Answer 26

The ability to use binocular disparity to obtain a vivid sense of depth arising from the difference in the retinal image of both eyes

Question 27

What is a horopter?

Answer 27

An imaginary surface defined by the locations in a scene from which objects would project retinal images at corresponding points (typically dictated by the object that you are fixating)

• things that fall on the horopter will fall on the corresponding points of the retina (zero disparity)
  
  • a point on both retinas that would coincide if they were superimposed
  • the fovea is a corresponding point
• things that fall behind or in front of the horopter will fall on non-corresponding points (disparity)
  
  • a point on both retinas that would not coincide if they were superimposed

Question 28

What is crossed disparity?

Answer 28

the value of the thoracic disparity is greater than the nasal disparity (aka plus disparity)

Question 29

What is uncrossed disparity?

Answer 29

the value of the thoracic disparity is smaller than the nasal disparity (aka minus disparity)

Question 30

Describe disparity in the cortex

Answer 30

• striate cortex combines information from both eyes
• still shows binocular preference
• prefers info from same point on retina
• there is a range of optimal disparity (disparity tuning) in the cortex
  
  • the majority of binocular neurons are tuned to zero disparity
• the range of possible disparities to encode is finite
  
  • objects very close or very far from the horopter will not be encoded as one, but two images, since no binocular neuron can encode the disparity

Question 31

Describe Panum’s Fusional Area

Answer 31

a) cyclopean perception
- objects within Panum’s Fusional Area are seen as a single image
- the image presented to both eyes can be fused into one
b) diploplian perception
- objects outside of Panum’s Fusional Area will be seen as two images
- diploplia = double vision

Question 32

What is the correspondence problem?

Answer 32

the problem of determining which features in the retinal image in one eye correspond to which features in the retinal image of the other eye

Question 33

What are the 2 hypotheses as to how the visual system solves the correspondence problem?

Answer 33

1. object recognition is performed on each eye individually prior to the scene being reconstructed in depth
   - each object is therefore labelled and correctly combined afterwards
2. the visual system matches parts of the retinal image on simple properties first (colour, size, orientation) before proceeding to object recognition
   - the scene is therefore reconstructed in depth prior to objects being labelled

Question 34

What is a stereogram?

Answer 34

Two slightly disparate images are presented to both eyes independently = result is perception of a 3D image

Question 35

How is correspondence accomplished?

Answer 35

• each feature in one eye should match with only one feature in the other eye
• most mammals use facilitation between neurons with similar preferred disparity and inhibition between neurons with different preferred disparity to solve the correspondence problem
  
  • in a population of neurons, this type of interaction between neurons, only one depth solution would be common to the process

Question 36

What is binocular rivalry?

Answer 36

a phenomenon experienced when different images are presented at the same retinal location in both eyes
- the two images compete for perceptual dominance