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 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 XX 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 X O X O

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 EN E 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 squares Compensation for shadow from cylinder