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Perception and VR MONT 104S, Fall 2008 Lecture 12 Illusions

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Illusions. 2. Visual Illusions. Visual illusions are cases where the ... The existence of visual illusions shows us that what we see is not always a true ... – PowerPoint PPT presentation

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Title: Perception and VR MONT 104S, Fall 2008 Lecture 12 Illusions


1
Perception and VR MONT 104S, Fall 2008Lecture
12Illusions
2
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

3
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
4
Who do you see?
5
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.
6
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.

7
Orientation Illusions
The Zollner 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.
  • It is uncertain exactly what causes these
    illusions.

8
Perspective Distortions
Perspective distortions are caused by an
adjustment of size according to our estimation of
depth.
9
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. Perspecti
ve cues can give us a sense of perceived
distance, that then causes the brain to interpret
the size of images based on this distance.
10
Relative Size
Our estimate of size is dependent on the context
of the surrounding features. This relates to one
possible explanation of the moon illusion.
The Ebbinghaus illusion
11
Paradoxes
We tend to see surfaces that are touching as
being at the same depth.
The Impossible Triangle
Also recall the impossible trident and M.C.
Escher paintings from the last lecture.
12
Fictions
The brain fills in gaps according to the context
around them.
The Kanisza Triangle
The illusory contours forming the triangle do not
exist.
13
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 you agree, how can we determine
what is "real"? If you do not agree, why not?
How does this affect your sense of yourself?
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