Transcript Lecture - ClassTools.Info

The $64,000 Vision Question
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 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
X
O
Sometimes these guesses produce
Errors 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 move
Other 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
O
O
X
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 produce
Errors 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 move
How 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
NE
E N E
Scary science revealed: Perception
You want to see convex
But it the lines don’t match
E
NE
Scary science revealed: Perception
So you see this instead 
E
NE
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 squares
Compensation for shadow from cylinder