Gregory’s Theory of Illusions What’s the “Big Picture” theory, amid the more specific hypothesis? (hint: look in the first paragraph) That illusions occur because of perceptual mechanisms evolved to handle vision under normal circumstances

Remember Last day to take test. No class on Friday.

Gregory’s Theory of Perception What is Gregory’s theory about how the brain accomplishes this?

Gregory’s Theory of Perception What is Gregory’s theory about how the brain accomplishes this? “it is as though the brain entertains alternative hypotheses of what object the eye’s image may be representing”

Two models of Vision “Control System” vs. “Lookup System” Now often called: “Bottom-Up” vs. “Top- Down”

Two models of Vision Bottom-up System –incoming stimulus is “processed” by successively higher stages until its identity is obtained –Percept is controlled ultimately by the retinal image Pattern on Retina brightness and line orientation Surfaces and background identity

Surfaces and background Two models of Vision Top-Down System –incoming stimulus triggers a comparison between a top-down “hypothesis” and the actual image Pattern on Retina brightness and line orientation COMPARE possible identities Brain “tests” different representations until it finds a match identity

Surfaces and background Two models of Vision Top-Down and Bottom-Up systems work together Pattern on Retina brightness and line orientation COMPARE possible identities identity BOTTOM-UPTOP-DOWN

Top-Down System at Work Your previous experience with this image helps you identify the object much faster

Two models of Vision Bottom-Up System: What it’s telling you: Pattern on Retina brightness and line orientation Surfaces and background identity Blobs big blobs and little blobs nope just blobs modern art

nope just blobs Two models of Vision Top-Down System What It’s Telling You: Blobs Big blobs and little blobs COMPARE cat? bird? A beer? none of the above = modern art

Two models of Vision Top-Down System Blobs Big blobs and little blobs COMPARE cat? bird? A beer? nope just blobs none of the above = modern art “it’s a dog”

Two models of Vision Top-Down System Blobs Big blobs and little blobs COMPARE Dog? nope just blobs none of the above = modern art “it’s a dog”

Two models of Vision Top-Down System Blobs Big blobs and little blobs COMPARE Dog? blobs that look a bit like a dog DOG ! “it’s a dog”

Shape Identification Requires Interaction between Top-Down and Bottom-Up Processes

Single Target - Identify shape

Single Shape/Brief Dots Shape is identified by top-down system before information in the bottom-up system gets changed

Single Shape/Brief Dots Shape is identified by top-down system before information in the bottom-up system gets changed Let’s push the system: overload top-down system AND change the signal in the bottom-up system

Single Shape/Brief Dots Shape is identified by top-down system before information in the bottom-up system gets changed Let’s push the system: overload top-down system AND change the signal in the bottom-up system What do you predict?

Many targets - Identify shape inside dots

Object Substitution Bottom-up info gets changed before top- down process completes all that’s left in the bottom-up signal is the four dots

Next Time: Read Pinker for next Friday! No class this Friday! More about 3D Vision