Illusions Are A Good Thing

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 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?