4. Depth perception

Question 1

Name the spatial and temporal monocular cues

Answer 1

Spatial
1. linear perspective
2. Overlay of contours
3. Highlights and shadows
4. Retinal image size
5. Aerial perspective
6. Size of known objects
Temporal
7. Motion parallax

Question 2

How does monocular cues affect stereoacuity?

Answer 2

Monocular cues enhance stereoacuity. Thus hides abnormalities resulting in false negatives.

Question 3

What are the 2 oculomotor cues?

Answer 3

1. Accommodation
2. Convergence

Question 4

How does accommodation occur?

Answer 4

1. Ciliary muscles contract
2. Zonules relax
3. lens becomes rounder and thicker
4. Refractive power increases for near viewing

Question 5

What does stereoacuity depend on?

Answer 5

1. Whether target is moving or not
2. Whether eyes are moving or not

Question 6

Which test is used to determine stereo thresholds?

Answer 6

3 needle test

Question 7

Describe the 3 needle test

Answer 7

1. All monocular cues must be removed
2. 3 pins viewed under controlled conditions
3. 2 outer needles are fixed, the central one can be moved back and forth to create change in disparity (depth)
4. Px reports whether the needle is in front or behind the other two.

Question 8

Describe the visual pathway to process depth

Answer 8

V1 = contains monocular and binocular neurons
beyond V1 the receptive fields are mostly binocular

V2 = gateway to further depth processing

Further stereoscopic processing occurs in V4 and MT.

Question 9

What are the two categories of stereopsis?

Answer 9

1. Fine stereopsis
2. Course stereopsis

Question 10

Describe Fine stereopsis

Answer 10

1. High spatial frequencies
2. Retinal disparity = 30minArc
3. Stationary/ Slow targets
4. Mainly foveal vision
5. Parvocellular pathway
6. High quality stereopsis

Question 11

Describe course stereopsis

Answer 11

1. Lower spatial frequencies
2. Retinal disparity = 30 - 600 minArc
3. Moving targets
4. Foveal and peripheral vision
5. Magnocellular pathway

Question 12

What are the two types of binocular summation?

Answer 12

1. Probability summation
2. Physiological summation

Question 13

What is probability summation?

Answer 13

One eye catches an image the other eye misses. Increases detection rate

Question 14

What is physiological summation?

Answer 14

The addition of signals from left and right eyes at binocular sites in brain.

Question 15

What are free fusion stereograms?

Answer 15

Two pictures of an object are the same but slightly shifted to one side. This creates disparity, objects seen in stereoscopic depth

Question 16

What are two examples of free fusion stereograms?

Answer 16

1. Random dot stereogram - in one image of one eye some of the dots are slightly shifted to create disparity
2. Auto stereograms (Magic eye)
   The eyes must convere or diverge one repetitive width. RE and LE see slightly disparate images.

Question 17

What are the 3 conditions to be able to have stereopsis?

Answer 17

1. Good VA
2. Vergence
3. Accommodation

Question 18

At what ages does stereopsis develop?

Answer 18

Gross stereopsis = 4 months
Mature stereopsis = 6 months
Full stereopsis = 3 years

Question 19

What are the requirements of stereopsis tests?

Answer 19

1. All monocular cues removed
2. Stereopsis can be simulated by 2D viewing in two ways:
   a. Binocular presentation = present similar images binocularly to allow fusion
   b. Dichoptic presentation = present images with a slight relative shift to each eye

Question 20

What are the limitations of stereopsis tests?

Answer 20

1. Ceiling effect- Can’t check who has high levels of stereoscopic acuity due to maximum measurable disparity
2. May not achieve actual stereoacuity level due to attention issues
3. Some tests have monocular cues
4. Children may not be happen to wear glasses
5. Need to ensure test is set up properly and Px doesnt move head
6. Different stereotests are not comparable to each other.