VF - Stereopsis - Week 3 Flashcards
Name the three sources of information about distance, and for each, give examples of cues (9 total).
Monocular cues
- Retinal image size
- Aerial/atmospheric perspective
- Distribution of light and shade
- Interposition/overlapping
- Geometric perspective
- Motion parallax
Intrinsic proprioceptive cues
- Accommodation
- Convergence
Binocular sources
-Stereopsis
Describe the interpretation of size. What provides a cue to distance if size is known?
Give an example using cats and fences.
When size is known, retinal image size provides a cue to distance.
The retinal image size in the context of geometric perspective provides information about relative distance and size.
Consider a fence going into the distance, getting smaller.
Cats of the same size can be placed under the fence at regular intervals.
Despite the cat’s size shrinking with distance, all cats are seen as the same size.
Describe the terror sub-terra example for interpretation of size.
Consider a long corridor fading into darkness in the distance.
A small figure placed at the front of the corridor will appear small.
The same figure placed at the end of the corridor will appear much larger even though retinal image size remains the same.
Define the following in terms of position relative to the horopter:
Crossed diplopia
Uncrossed diplopia
Too near - crossed diplopia
Too far - uncrossed diplopia
Define disparity, and what this means for objects that fall on the horopter, but are still at slightly different depths.
Explain in terms of corresponding points.
Images of objects at the same depth of a fixation point fall on corresponding points.
Images at different depths fall on disparate points.
If the disparity is not too large, they will appear single, but at different depth compared to the fixation point.
Describe an example of depth judgement experimentation using a sheet and rod. Describe how the experiment is carried out, and modifications to account for and eliminate different cues for depth (4 total).
A sheet and rod are placed in front of an observer, who can modify the position of the rod with the aim of aligning them in depth.
Size difference cue - using a sheet edge, rather than a second rod eliminates this cue.
Motion parallax - chin and head rest prevents this.
Light modelling cue - Illuminated background to eliminate the silhouette
Geometric perspective - using a screen with a viewing slot to eliminate the top and bottoms of the sheet/rod.
Consider the sheet and rod experiment for depth judgement.
Describe the method of adjustment and method of limits for this experiment, and define what the threshold would be.
Method of adjustment - set the rod to align with the sheet edge. Standard deviation is the threshold.
Method of limits - Move the rod until a depth difference is evident. Mean of settings is the threshold.
Consider the sheet and rod experiment for depth judgement.
Describe the method of constant stimuli for this experiment, and define what the threshold would be.
Rods are in set positions, and observer judges whether they are closer or further than the sheet.
Standard deviation is the threshhold.
Consider depth interval vs retinal disparity relationships. What is the relationship like, and how does it chance with differing distance?
Explain what this means in terms of depth perception.
Linear relationship that decreases with increasing distance.
This means capacity to discriminate depth decreases with distance.
Describe the geometric limit of stereopsis, and why depth perception changes with increasing distance in terms of disparities.
As viewing distance increases, the same depth translates to smaller and smaller disparities.
At one point, this reaches the viewing distance itself, which is the geometric limit.
This is ~1,300m
Describe how stereoscope image pairs work.
Two images are presented to each eye, differing slightly in contours. This gives the two fused images depth.
Are differing contours necessary for stereopsis?
No, random dot stereograms show they arent needed.
Consider random dot stereograms. How does the brain know which points are disparate and which are not?
These images, when used in stereoscopes, give a strong impression of depth, and show disparity alone can lead to depth perception, without monocular cues.
Compare quantitative and qualitative stereopsis. Describe what each is useful for.
Quantitative - improves with exposure duration, good for judging shape and orientation.
Qualitative - optimal with brief presentation. Good for detecting objects that appear suddenly.
Between qualitative and quantitative stereopsis, which helps in initiating vergence alignments of the eyes?
Qualitative.
