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

COMPAREpossible identities

Brain “tests” differentrepresentations 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

COMPAREpossible identities

identity

BOTTOM-UP TOP-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

Blobsbig 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

COMPAREcat? bird? A beer?

none of the above = modern art

Two models of Vision

• Top-Down System

Blobs Big blobs and little blobs

COMPAREcat? 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

COMPAREDog?

nope just blobs

none of the above = modern art

“it’s a dog”

Two models of Vision

• Top-Down System

BlobsBig blobs and little blobs

COMPAREDog?

blobs that look a bit like a dog

DOG !

“it’s a dog”

Shape Identification Requires Interaction between Top-Down and

Bottom-Up Processes

QuickTime™ and a decompressorare needed to see this picture.

Single Target - Identify shape

Single Target - Identify shape

QuickTime™ and a decompressorare needed to see this picture.

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

QuickTime™ and a decompressorare needed to see this picture.

Many targets - Identify shape inside dots

QuickTime™ and a decompressorare needed to see this picture.

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