1. Illusions Are A Good Thing

2. Thesis Illusions are good because: – They tell us about perceptual strategies used by the brain – These perceptual strategies work most of the time; without them, our perception would be much worse

3. About Illusions Illusions occur when our perceptions are not consistent with the environment Illusions are rare Illusions are difficult to override

4. Example 1: Phantom WordsPhantom Words Problems for speech perception – Segmenting the speech signal – Variability in the speech signal What does the phantom words illusion reveal? – Top-down processing

5. Example 2: The Moon Illusion Problems for depth perception: – The retinal image is two dimensional – Size of the retinal image depends on both distance of the object and size of the object

6. What Does the Moon Illusion Reveal about the Brain? Contextual cues can affect perception of distance Unconscious inference: the brain makes perceptual calculations, such as apparent distance, quickly and without conscious awareness

7. Example 3: Negative Color Afterimage Problems for color perception: – Why do there seem to be “primary” colors? – Why can you say “bluish green” but not “reddish green?”

8. What do Negative Color Afterimages Reveal? Importance of neural adaptation Opponent pairs of colors: – Red-green – Blue-yellow – White-black

9. What if We Didn’t Have Illusions? How would we understand speech? How would we perceive distance and size? How would we discriminate between different colors?