Name the spatial and temporal monocular cues
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
How does monocular cues affect stereoacuity?
Monocular cues enhance stereoacuity. Thus hides abnormalities resulting in false negatives.
What are the 2 oculomotor cues?
- Accommodation
- Convergence
How does accommodation occur?
- Ciliary muscles contract
- Zonules relax
- lens becomes rounder and thicker
- Refractive power increases for near viewing
What does stereoacuity depend on?
- Whether target is moving or not
- Whether eyes are moving or not
Which test is used to determine stereo thresholds?
3 needle test
Describe the 3 needle test
- All monocular cues must be removed
- 3 pins viewed under controlled conditions
- 2 outer needles are fixed, the central one can be moved back and forth to create change in disparity (depth)
- Px reports whether the needle is in front or behind the other two.
Describe the visual pathway to process depth
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.
What are the two categories of stereopsis?
- Fine stereopsis
- Course stereopsis
Describe Fine stereopsis
- High spatial frequencies
- Retinal disparity = 30minArc
- Stationary/ Slow targets
- Mainly foveal vision
- Parvocellular pathway
- High quality stereopsis
Describe course stereopsis
- Lower spatial frequencies
- Retinal disparity = 30 - 600 minArc
- Moving targets
- Foveal and peripheral vision
- Magnocellular pathway
What are the two types of binocular summation?
- Probability summation
- Physiological summation
What is probability summation?
One eye catches an image the other eye misses. Increases detection rate
What is physiological summation?
The addition of signals from left and right eyes at binocular sites in brain.
What are free fusion stereograms?
Two pictures of an object are the same but slightly shifted to one side. This creates disparity, objects seen in stereoscopic depth
What are two examples of free fusion stereograms?
- Random dot stereogram - in one image of one eye some of the dots are slightly shifted to create disparity
- Auto stereograms (Magic eye)
The eyes must convere or diverge one repetitive width. RE and LE see slightly disparate images.
What are the 3 conditions to be able to have stereopsis?
- Good VA
- Vergence
- Accommodation
At what ages does stereopsis develop?
Gross stereopsis = 4 months
Mature stereopsis = 6 months
Full stereopsis = 3 years
What are the requirements of stereopsis tests?
- All monocular cues removed
- 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
What are the limitations of stereopsis tests?
- Ceiling effect- Can’t check who has high levels of stereoscopic acuity due to maximum measurable disparity
- May not achieve actual stereoacuity level due to attention issues
- Some tests have monocular cues
- Children may not be happen to wear glasses
- Need to ensure test is set up properly and Px doesnt move head
- Different stereotests are not comparable to each other.
