Week 6 Lecture 6 - depth perception Flashcards
If we have so many different cues to depth what is the problem?
- Many cues are ambiguous – 2D image compatible with infinite 3D worlds
- With multiple cues available – how do we perceive a single unified world?
What is ambiguity in depth perception?
Many of the 3D cues (particularly the pictorial cues) are ambiguous
Give an example of ambiguity in perspective
- Imagine there is a 2D image of some railway tracks
This 2D retinal image could have been produced by:
- parallel lines in the world receding away from you
- converging lines that are a constant distance from you (or infinite other 3D stimuli)
Give an example of how ambiguity exists in shading
- imagine there’s a 2D image of a shaded circle on top of a square
This retinal image could have been created by:
- a convex object that is lit from above
- a concave object lit from below (or it could be a flat surface with a pattern of light & dark greys painted on it)
How can we overcome ambiguity in depth perception?
- By using prior knowledge, or prior assumptions, to interpret the image
- These assumptions are gained through our knowledge and experience of the physical properties of the world
- This is a type of top-down processing and supports the constructivist approach to vision
Can assumptions sometimes lead to errors?
yes
How is Ames Room an example of an erroneous assumption?
- When we view the Ames room we assume lines are parallel or at right-angles, however this is not the case
- The perspective assumption of parallel lines is invalid which causes an erroneous percept
What is multi-cue perception?
- Real-world scenes have multiple cues present
- Cues must be integrated to achieve a single unified percept
What problems can multi-cue perception help overcome?
problems of:
- Reliability
- Ambiguity
- Conflict
What are the 3 types of cue integration?
Compromise
Dominance
Interaction
What is compromise (cue integration)?
When two sources of depth information are conflicting, the brain will try to find a compromise between the two.
What study investigated compromise as a type of cue integration?
Young et al (1993):
- Participants viewed computer generated cylinder defined by 2 cues:
Texture
Motion
- Task: Apparently circular cylinder task
- Judgement: Does the cylinder look flattened, circular or stretched?
- Put 2 cues into conflict each define a different shape:
Texture – ‘flattened’
Motion – ‘stretched’ - ppts perceived shape is a compromise between the two cues
What if one cue is more reliable than the other?
- When texture made less regular (hence less reliable) perceived shape is biased towards motion cue
- When motion made less smooth (hence less reliable) perceived shape is biased towards texture cue
What conclusions can be taken from Young et al’s (1993) study on compromise?
- When 2 cues are conflicting the brain will try to average them
- Final percept of shape will be biased towards most reliable cue
What is dominance (cue integration)?
- When two cues define very different shapes or depths, the brain may choose to ignore one in preference for the other
- Large cue-conflicts tend to arise when one cue’s assumptions are invalid
- Usually the cue with valid assumptions will dominate
When an invalid cue dominates, what happens?
Give an example
- When an invalid cue dominates – this creates illusion
- Ames room illusion –> relative size in conflict with perspective (perspective wins)
