L8 - Depth Perception Flashcards
Why are our eyes both facing the same direction?
- Most animals have two eyes - means you cannot sneak up on an emu behind as their eyes are on the side
- Front facing eyes have almost identical views of the world: thumb and closed eye and the movement or the pointing
- Both eyes have slightly different images
- Therefore different objects with different depths for both eyes, must be more important than seeing everywhere
What is Stereopsis?
- Our eyes have two diff views of the world to get depth
- Any object not on the horopter (locus of points in space that have the same disparity as fixation) has a retinal disparity (the distance between two objects far and close)
- Object both eyes are looking at have same RD, something further will have retinal dis, called cross-RD, things that are closer will have RD and indicated as sign
- Once calculated, we know how far the object is away from our horopter
- Objects at diff distances will create diff amounts of disparity = If you can cause disparity = depth
What are ways of getting different images into each eye?
- Red-green anaglyphs
- Polaroid over each eye - lights oscillates in different directions with filters = old cinema glasses
- Mirrors
- Free fuse: get one eye to look on the other side
- Autostereograms
What does disparity govern?
- The polarity of disparity governs whether seen as in front or behind horopter (screen)
- Amount of disparity governs the amount of depth seen
What are Random dot stereograms?
- Patterns of random dots is copied with some of the dots shifted
- Each pattern presented to diff eye
- Each eye alone sees nothing but random dots
- Together the eyes extract the retinal disparity and see the form
- Depth must precede form
What was the use of visual search?
- Target defined by depth pops out
- Info needed to pick up a target with fingers: stereovision important in action
How do these pictures look different if it is the same picture (Autostereograms)?
- Stripes
- Both eyes look at stripe, look at same image = looks flat
- Look at different stripes with different eye = depth
What is the 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 LGN - so still no binocular cells
- First place info from two eyes come together is area V1
- Many cells here are disparity sensitive
What is stereoblindness?
- Many people have problems with their stereovision (10%)
- If there are any problems in early life then the brain does not learn to put the info together/eyes do not work together and this cue to depth is lost
- Such problems sometimes don’t know that they have this problem - must be other cues to depth
- Have a lack of finesse for depth perception but still reasonably good perception
What is motion parallax?
- Movement through the world or our eyes causes the image on our retina to move
- Things closer to us move faster and things further away move slower so if we have dots and move them at different speeds they can look like they have depth
- Subjects see a static 3D surface when looking at a moving 2D surface because they moved their heads
What are pictorial cues to depth?
- Lack of stereo cues and lack of motion = we can still see depth using the following:
- Interposition
- Height
- Size
- Perspective
- Shadows
Describe size?
For a given object, it will cast a smaller and smaller retinal image as it gets further away
Describe texture gradients?
Can make it seem like pictures and shapes are angled to another side even though the spatial cues are changed purposely
Shade and shading?
- Some buttons stick out and some stick in because brain assumes that light comes from the top
- If you turned picture upside down = reverse
Cast shadows?
Shadow of object gives us cue of depth and can be very powerful
