Theory L6 - Size Constancy & The Brain

Question 1

What are depth cues?

Answer 1

Monocular cues:
Cues that provide depth information when viewing a stationary scene with one eye. E.g. pictorial.

Ocular cues:
Cues that provide depth information from the state of the eye - eg. accomodation, vergence.

Binocular cues (will not be covered):
Cues that provide depth information when viewing a scene with both eyes - due to each eye having a slightly diff perspective, the brain computes the slight difference between the two eyes to judge how far an object is. E.g. stereopsis.

Question 2

How are size and depth related?

Answer 2

Size influences depth perception
and
depth perception influences size

Question 3

What is convergence and divergence

Answer 3

Convergence - when eyes move inwards to look at something - closer

divergence - when eyes move outwards. - far.

This is a strong depth cue to the brain.

these are a type of OCULAR CUE

Question 4

What is Accommodation?

Answer 4

When the lens in the eyes changes curvature, to allow for crisp, clean images on the back of the eye.

Lens must curve to focus the image according to how far it is.

These are a type of OCULAR CUE

when looking @ images, lends doesnt change so tells brain its 2D

Question 5

Name some pictoral depth cues

Answer 5

• Occlusion
• Relative Size
• Texture Gradients
• Linear Perspective
• Aerial Perspective
• Image Blur
• Retinal Image Size

Question 6

What is occlusion?

Answer 6

When one object in an image occludes a part of another object in the image.

The one that is occluding the other is perceived to be IN FRONT of the other object.

Question 7

What is relative size?

Answer 7

This is a pictoral cue that allows depth perception.

Objects in an image are perceived as closer when they are bigger.
Objects that are smaller are perceived as further away.

Question 8

What are texture gradients?

Answer 8

Bigger and coarser texture is perceived to be closer than finer textures in an image

Question 9

What is linear perspective?

Answer 9

Parallel lines in an image becoming closer/converging gives the illusion of distance.

Question 10

What is aerial perspective?

Answer 10

Phenomenon that when we look into the distance it is more misty and hazy.

Thus in images, haziness = distance.

Question 11

What is image blur?

Answer 11

When an image is focussed (crisp part in image) on something, the lens accommodates for that specific distance.

Question 12

What does retinal image size tell us?

Answer 12

Ocular + Binocular cues tell us that an image is 2D, however, monocular cues give depth information.

yet, how big someting is on our retina gives illusion of depth

Question 13

In accomodation, what happens for an object that is closer?

Answer 13

Lens curves more to focus on closer objects.

Question 14

What is proprioception

Answer 14

• Prioprioceptors located in joints and muscles send signals to the brain and enable us to keep track of movement.
• Corollary discharge (or efference copy) is a copy of the motor command issued from the motor centres of the brain to other parts of the brain (Cerebellum).
• Perceptual constancy cannot work w/o efference copies - the influence of con/divergence and accommodation wouldn’t work.
• Vision constancy is also achieved by efference copies - to allow us to know our eyes are moving, to perceive the world as stationary.

Question 15

How do efference copies allow for depth perception?

Answer 15

Information about curvature of lens, or movement of eyes is sent to the perceptual centres of the brain.

This allows the brain to RE-SCALE the size of objects - allowing for perception!

Question 16

What are leads to size constancy and perception?

Answer 16

size on retina + conceptual knowledge + depth information = size perception

this mechanism is fundamental to being abel to interact in the world.

Question 17

What is the alice in wonderland syndrome?

Answer 17

A syndrome where there is a breakdown of size constancy

Question 18

What is the moon illusion?

Answer 18

The moon moves during the night - changes size - BREA DOWN IN SIZE CONSTANCY!

BUT we don’t have a concept of how big the moon is, so we cannot RE-SCALE - if we had a concept, the moon would not change size, because practically, we know that the moon doesnt -actually- change size.

Humans aren’t evolved to compute inter-planetary distance, however, are really good at seeing distance on the horizon.

Question 19

What is the ponzo illusion?

Answer 19

Demonstrates the brain rescaling size, when there is linear perspective.

Linear perspective tricks the brain into thinking that there is distance.

On railway tracks, our brain will rescale the linear lines further away (in the distance) and not rescale the ones in the foreground, because we register it as close..

the consequence of depth perception on size perception

Question 20

What have fMRI shown us about the pONZO illusion

Answer 20

• Measured brain activation while people looked at objects of same size in different perspectives/contexts.
• Same physical size (same retinal image) of the circle but had different apparently sizes.
• APPARENTLY larger circle will activate ANTERIOR and the apparently smaller one will activate POSTERIOR.

therefore

Retinotopic activity in v1 is modulated by PERCEIVED SIZE, even when retinal image of both is the same.

indicates v1 has important role to play in size constancy when APPARENTLY distance changes.

Question 21

Retinotopy?

Answer 21

Fovea = the centre of the visual field - Represented in the back of the visual cortex - posterior

Right visual field = in left HS

Left visual field - in right HS

upper visual field = dorsal V1

lower visual field = ventral V1

Periphery - anterior

SO if looking at small object - will only activate where it is - and POSTERIOR

BIG object = whole visual system - and ANTERIOR

Question 22

What is the Ames room?

Answer 22

Tricks brain into thinking that there is no distance difference/change, due to preconceived knowledge of a house/room

not a box shape, its trapezoidal.

So what we see instead, is a person changing size.

breakdown in size constancy

Question 23

Limitations of size constancy?

Answer 23

• When looking down from an airplane: Houses, cars, and people look like toys.
• In patients with Alice in Wonderland Syndrome. These patients lose size constancy with their migraine spells. Lewis Carroll, the author of Alice in Wonderland, is thought to have suffered from this syndrome.

Question 24

What is Emmert’s law?

Answer 24

The perceived size of an afterimage (in ideal conditions, when distance + depths cues are avail) is directly proportional to its apparent distance according to Emmert’s law.

size = distance x tan theta

therefore, when you increase the viewing distance, the size of afterimage increases proportionally.
double - double size.

VERY ROBUST PHENOMENON

Question 25

Chicken & Egg problem with the ponzo illusion?

Answer 25

Possibly a feedback look, where the brain is told what we think is in the foreground and distance.

this could prime areas of the brain to be more likely to activate, and others to be suppressed.

Question 26

What has an fMRI study on Emmert’s Law shown us?

Answer 26

When light at a fixed distance bleaches the retina, and subjects project an afterimage at varying distance, the farther away they project their afterimage, the bigger it appears.

Yet, retinal image stays the same!!!!!!!!

fMRI:

• did Retinotopic mapping - bigger rings activated anterior V1, Smaller activated posterior V1.
• when stimulus shown - posterior parts activated
• when after image started - anterior parts activated.

RETINOTPIC ACTIVITY IN V1 IS MODULATED BY PERCEIVED SIZE of an after image!!!

indicates that v1 has an important role in size constancy when the REAL distance changes.

Question 27

Overall, in this lecture, what have fMRIs shown us?

Answer 27

Suggests that the V1 has a role in size constancy, both when REAL and APPARENT distance changes.

Retinotopic activity in v1 is modulated by PERCEIVED SIZE, even when retinal image of both is the same