reading assignment read chapter by pinker for wed. of next week

Reading Assignment

• Read chapter by Pinker for Wed. of next week.

Shape from Shading

Investigating Perception

• What is Science?

Investigating Perception

• What is Science?

• Science is a method for understanding how nature works

Science

• What isn’t Science?

• Headlines on www.reuters.com Science Page:

– Juan Strengthens Into Hurricane, Misses Bermuda

Science




– Japan's Mt Fuji Shows Subsidence, No Eruption Danger

Science





– Official Report: French Heat-Wave Death Toll 15,000

Science





– Official Report: French Heat-Wave Death Toll 15,000

– Man Questioned in Australia Over Mutilated Whale Carcass

Science

• Science is like a set of procedures:

Identify a question

Science


Identify a question

Make a Hypothesis

Science


Identify a question

Make a Hypothesis

Make a Prediction

Science


Identify a question

Make a Hypothesis

Make a Prediction

Test It

Science


Identify a question

Make a Hypothesis

Make a Prediction

Test ItPrediction holds

Prediction Fails

Example: The horizontal-vertical illusion


• Question:

• Hypothesis:

• Prediction:

• Test:


• Question: Why does the vertical line seem longer?

• Hypothesis:

• Prediction:

• Test:



• Hypothesis: Wundt -> vertical and horizontal eye movements scan the image differently

• Prediction:

• Test:




• Prediction: Stabilized image should eliminate the illusion

• Test:




• Prediction: Stabilized image should eliminate the illusion

• Test: Illusion is still present in afterimage!

Muller - Lyer Illusion

Muller-Lyer Illusion

• What theory does Gregory take up?

Muller-Lyer Illusion

• What theory does Gregory take up?– Thiery / Woodworth : Muller-Lyer figure

“suggests perspective”

– implicit is that the brain shrinks things that are perceived as near and expands things that are perceived as far away

Size Constancy

• Brain shrinks things that are near and expands things that are far

The Ponzo Illusion:

Size Constancy

• The lines of perspective in the Ponzo Illusion make the top line appear farther away

• Why isn’t it this simple for the Muller-Lyer Illusion ?

Size Constancy

• The lines of perspective in the Ponzo Illusion make the top line appear farther away

• Why isn’t it this simple for the Muller-Lyer Illusion ?

– Because the Muller-Lyer figure looks flat !

Paradox of Pictures

• What does Gregory find so paradoxical about pictures?

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Paradox of Pictures

• What does Gregory find so paradoxical about pictures?

• What does he do to remove the paradox so that the Muller-Lyer figure no longer looks flat?

Paradox of Pictures

Removing the Background

• With background removed, Muller-Lyer figure actually looks 3D

• What prediction can you make about how the perceived size and the perceived distance of the center line are related?

• How did Gregory test that?

Removing the Background

• Perceived size and perceived distance are tightly related

Size Constancy in Motion

• What does Gregory wonder about size constancy and motion?

• How does he test that?


• Gregory finds that motion does invoke size constancy in certain cases:


• Gregory finds that motion does invoke size constancy in certain cases:– only when observer is accelerating

General Theory of Illusions

• What’s the more general theory about why illusions occur ?


• What’s the more general theory about why illusions occur ?

• That illusions occur because of perceptual mechanisms evolved to handle vision under normal circumstances


• What is Gregory’s theory about how the brain accomplishes this?


• 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”

Modern “Two-System” Model of Vision

• What Gregory called “Control System” vs. “Lookup System” we often now call “Bottom-Up” vs. “Top-Down” or “Stimulus-Driven” vs. “Goal Driven”

Two-System model of Vision

• Bottom-up System– incoming stimulus is “processed” by

successively higher stages until its identity is obtained

– Percept is controlled directly by the retinal image

Pattern on Retina

brightness and line orientation

Surfaces and background identity

Surfaces and background


• Top-Down System– incoming stimulus triggers a comparison between a

top-down “hypothesis” and the actual image

Pattern on Retina


COMPAREpossible identities

Brain “tests” differentrepresentations until it finds a match

identity

Surfaces and background


• Top-Down and Bottom-Up systems work together

Pattern on Retina


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


Surfaces and background identity

Blobsbig blobs and little blobs

nope just blobs modern

art

nope just blobs


• Top-Down System

• What It’s Telling You:

Blobs Big blobs and little blobs

COMPAREcat? bird? A beer?

none of the above = modern art


• Top-Down System


COMPAREcat? bird? A beer?

nope just blobs


“it’s a dog”


• Top-Down System


COMPAREDog?

nope just blobs


“it’s a dog”


• Top-Down System

BlobsBig blobs and little blobs

COMPAREDog?

blobs that look a bit like a dog

DOG !

“it’s a dog”

Testing the Two-Systems Model

• Have we talked about any phenomena that cannot be explained (easily) by a strictly bottom-up model of vision?

Testing the Two-Systems Model

• Have we talked about any phenomena that cannot be explained (easily) by a strictly bottom-up model of vision?

• Once a stimulus enters an auditory or visual pathway, it should be unaffected by subsequent stimuli (according to the bottom-up model).

Single Target - Identify shape

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are needed to see this picture.

Single Shape/ Lingering Dots

• Lingering dots do not interfere with percept of shape



• Let’s overload the system and see what happens.



• Let’s overload the system and see what happens.

• What would a strictly bottom-up model predict?

Many targets - Identify shape inside dots

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Object Substitution Masking

• 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 Tuesday!

More about 3D Vision

reading assignment read chapter by pinker for wed. of next week

Documents

vertical line

wundt vertical

science page

horizontal eye movements

mullerlyer figure

set of procedures

image differentlyprediction

stabilized image