depth perception Flashcards
Stereopsis
Our eyes have two different views of the world, the use of these two views to get depth is termed stereopsis
Retinal disparity
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
Horopter
Locus of points in space that have the disparity as fixation
Polarity of disparity
Polarity of disparity governs whether seen as in front or behind horopter
Amount of disparity
Amount of disparity governs the amount of depth seen
Random dot stereograms - Beta Julez (1961)
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
Visual search task
Target defined by depth pops out
Autostereograms - Magic eye
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
Neural basis of disparity sensitivity
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
Disparity sensitive cells in V1
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
Stereoblindness
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
Motion parallax
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 :)
Rogers & Graham (1979)
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
Many pictorial cues to depth
Interposition
Height
Size
Perspective
Shadows
Size in depth
Any image will make a bigger visual projection the closer it is
Quick and automatic
Texture gradients
We can see a pattern changing depth and by understanding the pattern and how it changes through perspective, we can see depth
Shape from shading - Ramachandran (1998)
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
Casting shadows
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
Distortions due to misplaced depth
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
Ames Room
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
