depth perception

Question 1

Stereopsis

Answer 1

Our eyes have two different views of the world, the use of these two views to get depth is termed stereopsis

Question 2

Retinal disparity

Answer 2

Any object not on the horopter has a retinal disparity
If we can calculate retinal disparity we know how far the object is away from our horopter
Means things aren’t exactly the same in both eyes

Question 3

Horopter

Answer 3

Locus of points in space that have the disparity as fixation

Question 4

Polarity of disparity

Answer 4

Polarity of disparity governs whether seen as in front or behind horopter

Question 5

Amount of disparity

Answer 5

Amount of disparity governs the amount of depth seen

Question 6

Random dot stereograms - Beta Julez (1961)

Answer 6

Pattern of random dots is copied with some of the dots shifted
Each pattern presented to a different eye
Each eye alone sees nothing but random dots
Together, the eyes extract the retina disparity and see the form
Takes a moment to do the computation

Question 7

Visual search task

Answer 7

Target defined by depth pops out

Question 8

Autostereograms - Magic eye

Answer 8

Usually, both eyes look at the same stripe, but getting our eyes to unfocus and do parallel viewing means we can see hidden images found in autostereograms

Question 9

Neural basis of disparity sensitivity

Answer 9

Cells cannot be driven by both eyes at the level of the retina
Left and right eyes drive different layers of the LGN (cells are monocular in the LGN)
First place info from both eyes come together is at V1

Question 10

Disparity sensitive cells in V1

Answer 10

Some only fire with little retinal disparity, some fire with lots
Disparity sensitive tuning occurs firstly in V1 and then in many later visual areas

Question 11

Stereoblindness

Answer 11

Many people have problems with stereovision
If there are problems in early life then the brain doesn’t learn to put the information from the two eyes together and the cue is lost

Question 12

Motion parallax

Answer 12

Movement through the world or of our eyes causes the image on our retina to move
Things closer to us move faster
If we have dots or a 2D screen and move them at different speeds they can look like they have depth
This is why kitties bob before pouncing at prey :)

Question 13

Rogers & Graham (1979)

Answer 13

Yoked (linked) the movements of dots on a 3D surface to the movements of the head
Subject saw static 3D surface when looking at moving 2D surface

Question 14

Many pictorial cues to depth

Answer 14

Interposition
Height
Size
Perspective
Shadows

Question 15

Size in depth

Answer 15

Any image will make a bigger visual projection the closer it is
Quick and automatic

Question 16

Texture gradients

Answer 16

We can see a pattern changing depth and by understanding the pattern and how it changes through perspective, we can see depth

Question 17

Shape from shading - Ramachandran (1998)

Answer 17

Tend to see middle element as sticking out
Brain makes assumption that light comes from the top
Turn the image upside down and you can see the elements swap

Question 18

Casting shadows

Answer 18

Shadow of an object gives us cues to depth
Can change perspective of where ball is moving to just by adjusting the shadow
Figure A shows ball rolling across floor
Figure B shows ball rising in air
Only thing that has changed is the shadow’s position

Question 19

Distortions due to misplaced depth

Answer 19

Our understanding of depth is used to then decide on how large or small things are and their spatial relationships
Can see this through dragon illusion

Question 20

Ames Room

Answer 20

People appear distorted in the room
The small person is actually further away, the room isn’t square
The visual cues (clock, squares on floor) make it look like people are same distance