How do we take the 2-dimensional image that is projected onto the back of the eye, and figure out from

that what the 3-dimensional world that caused the image is like?

The $64,000 Vision Question

The Inverse Optics Problem

Answer: We don’t. We can’t.

So what do we do?

We guess, based on biases about what arrangements in the world probably lead to the image on the retina.

Example: Guessing the depth

Move your head from side to side

Object in the world move across your retina

Closer objects move more on your retina

Farther objects move less on your retina

** Amount of motion on retina of object is a depth cue

IF THE OBJECT IS STATIONARY

Example: Guessing the depth

X

O

O

X

O

X

O

X

Sometimes these guesses produceErrors in perception (illusions)

Example: Moon over my hammy

Inputs to motion system:

Retinal motion (function of actual distance and actual motion)

Distance calculated

Knowledge (biases) about how things moveOther stuff too

Retinal Motion = 0 (moon is 240,000 miles away)

WRONG Perceived distance = 1 mile

Illusion = Moon is following me

Example: Guessing the depth

What if you are moving and the object stays the same on your retina?

It is moving with you

OR

It is REALLY far away (rocky mountains)

Moon follows you on a train – because your brain can’t quite appreciate how far away it is – so your brain concludes it is moving with you

Example: Guessing the depth

X

X X

X???

Example: Guessing the depth

If an X was moving across your eye faster than O

AND you thought O was closer

THEN

You would think

“Object O must be moving with me some"

Example: Guessing the depth

X

O

O

X

OX

O

X

O

Example: Guessing the depth

Wait – how do you know how far away it is when all you

Have is the information on the retina?

Stereo cues

Differences in the two images on the two eyes

Monocular cues

Interposition, atmospheric cues, size

And

Shape from shading

Sometimes these guesses produceErrors in perception (illusions)

Example: Movin on

Inputs to motion system:

Retinal motion (function of actual distance and actual motion)

Distance calculated

Knowledge (biases) about how things moveHow much of visual field is moving

Retinal Motion = A lot

Perceived distance = 20 feet

Whole field is moving

Illusion = I AM MOVING

Bumps or Dents?

Shape from shading

Could be a bump lit from above or dimple (OR A CHAD) lit from below

Assume that things are lit from above

Why? Look up and see the sun – it shineth from above

This BIAS helps us in this world big time because the sun is up

Scary Science

Disneyland haunted faces seem to turn their head and follow you as you walk or ride by them

Most people assume that it is some sort of high-tech holographic motion-sensitive tracking program…

But of course, it’s not …

Let’s see what it looks like first…

Scary Science

Can anybody tell me why this occurs?

In order to figure it out – you need one more piece of info that I have not given you…

Scary Science

Is this face convex or concave

It looks convex, but I’m a dirty rotten scoundrel – it’s concave – see???

Now – how do these fit together to produce the effect?

Scary Science

Think of light from eyes and light from tip of nose

Light coming from tip of nose is further away – so a smaller motion signal on your retina

But you think it is closer because you think it is convex like normal faces (shape from shading and knowledge of faces)

How does your brain resolve this? By thinking that the nose is moving with you – IT’S FOLLOWING YOU

Scary science revealed: Reality

E ENE

E

N

Scary science revealed: Perception

E

E

N

You want to see convex

But it the lines don’t match

Scary science revealed: Perception

So you see this instead

E

E

N

Most Important Point: Scary Science

Perception is an ACTIVE construction

Usually does a good job of telling us what is really out there

But it can be fooled – ILLUSIONS

What illusions tell us about the mind

What biases the brain has in constructing perception

We don’t experience reality – we experience our CONSTRUCTED PERCEPTIONS

What illusions tell us about the mind

What biases the brain has in constructing perception

We don’t experience reality – we experience our CONSTRUCTED PERCEPTIONS

Perception is an illusion

Motion after-effect

Neurons that detect motion act in opposition

Some signal inward motion

Some signal outward motion

Normally when looking at stationary things

they balance – net signal = 0 so

no motion is perceived

Perception is an illusion

Motion after-effect

Watch a display spiralling inward

Inward neurons get tired

When you stop looking at the spiral

Normal balance is upset because

outward motion neurons overpower inward motion neurons

And you see exploding faces!

Perception is an illusion

Relative contrast of surrounding squaresCompensation for shadow from cylinder