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

2. 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. 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. 4 Who do you see?

5. 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. 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. 7 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. It is uncertain exactly what causes these illusions.

8. 8 Perspective Distortions Perspective distortions are caused by an adjustment of size according to our estimation of depth. The Muller-Lyer illusion The Ponzo illusion Which line appears longer?

9. 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. 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.

10. 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. 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. 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. 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?