Transcript Session 12

Perception, Illusion and VR
HNRS 299, Spring 2008
Lecture 12
Illusions
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Visual Illusions
Visual illusions are cases where the perception of an object or
drawing is not consistent with its physical reality.
Illusions can give us some insight into how the brain processes
image information and what assumptions the brain makes to
determine the 3D scene from a 2D image.
Gregory describes 4 categories of illusion:
• Ambiguities
• Distortions
• Paradoxes
• Fictions
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Ambiguous Figures
Ambiguous figures can be seen in one of two ways.
Often the perception will flip back and forth between the
two interpretations.
Example: The Necker Cube
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Who do you see?
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What is this?
With ambiguous figures, we see one interpretation or the other,
but not both at the same time.
This probably helps us see the world unambiguously.
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Distortions
In some illusions, what we see appears to be a distortion of the
actual image.
There is some controversy as to the cause of these illusions.
Explanations include:
• "Bottom up": The distortion arises because of the way the
brain processes the image information to determine basic
features such as edges, orientation, motion, etc.
• "Top down": The distortion occurs because the brain uses
knowledge about the world (e.g. faces are convex) to
interpret the image.
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Orientation Illusions
The Zollner Illusion
The Poggendorf Illusion
Bottom up explanation:
•Orientations that are close to one another tend to repel each other.
•This is thought to be caused by "lateral inhibition" by neurons
processing similar orientations.
•This makes acute angles appear wider than they are.
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• It is uncertain exactly what causes these illusions.
Perspective Distortions
Perspective distortions are caused by an adjustment of size
according to our estimation of depth.
The Ponzo illusion
The Muller-Lyer illusion
Which line appears longer?
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Size Constancy and Emmert's
Law
Our perception of size relates to the perceived distance to an
object.
Emmert's Law states that the perceived size of a constant retinal
image is proportional to its perceived distance.
Perspective cues can give us a sense of perceived distance, that
then causes the brain to interpret the size of images based on this
distance.
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Paradoxes
The Impossible Triangle
We tend to see surfaces that are touching as being at the same
depth.
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Fictions
The brain fills in gaps according to the context around them.
The Kanisza Triangle
The illusory contours forming the triangle do not exist.
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Discussion topic
The existence of visual illusions shows us that what we see is
not always a true representation of physical reality.
The brain misinterprets orientations, assumes configurations and
creates features that are not present in the image.
Gregory makes the provocative statement that:
"perceptions are not essentially different from hallucinations."
Do you agree with this statement?
If so, how can we determine what is "real"?
If not, why not?
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