depth perception

Question 1

what is stereopsis?

Answer 1

• retinal disparity
• our eyes have two different views of the world - the use of these two views to get depth is termed “stereopsis”
• object we are looking at is not perfectly aligned in both eyes
• objects at different distances will create different amounts of disparity
• object that both eyes are looking at will have 0 retinal disparity
• but if something is closer than what you are looking at will have less RD

Question 2

what is the ‘horopter’?

Answer 2

• any object not on the horopter (the locus of points in space that have the same disparity as fixation) has a retinal disparity
• if we can calculate this retinal disparity we know how far the object is away from our ‘horopter’

Question 3

how can we see depth?

Answer 3

• if we can create images that have retinal disparity they should appear to have depth
• different retinal disparities = we will see depth

Question 4

what is cross-retinal disparity?

Answer 4

• but something that is further away will have RD, we call this cross-retinal disparity

Question 5

why are our eyes facing in the same direction?

Answer 5

• if animal has eyes on both side of head - they can see all the way around e.g., ostrich
• front facing eyes have almost identical views of the world

Question 6

how to we get different images into each eye?

Answer 6

• red-green anaglyphs (there is nothing special about red and green, can be different colours)
• polaroid glasses over each eye
• mirrors
• free fuse
• autostereograms (Magic Eye)

Question 7

polarity of disparity governs?

Answer 7

whether seen as in front of behind horopter (here defined by the blue lines)

Question 8

amount of dispairty governs?

Answer 8

the amount of depth seen

Question 9

what are random dot stereograms?

Answer 9

• pattern of random dots is copied with some of the dots shifted
• each pattern presented to different eye (see last slide)
• each eye alone sees nothing but random dots
• together the eyes extract the retinal disparity (due to the shifter dots) and see the form
• DEPTH MUST PRECEDE FORM

Question 10

what are autostereograms?

Answer 10

• Magic eye
• one eye looks at one stripe (e.g., vertical ones) and the other eye looks at a different stripe (e.g., horizontal ones)
• makes us see depth

Question 11

what is the cross-viewing method to get the magic eye picture?

Answer 11

put finger in front of you and look at image through your finger so eyes cross over at finger

Question 12

what is the correct way of doing the cross-viewing method to get the magic eye picture?

Answer 12

place book in front of you really close and slowly move it away from your face, then you will see the magic eye picture

Question 13

what is the neural basis of disparity sensitivity?

Answer 13

• clearly, 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 information from two eyes comes together is area V1

Question 14

what did Barlow et al., (1968) say about cells in V1?

Answer 14

they are disparity sensitive

Question 15

what is stereoblindness?

Answer 15

• many people (10%) have problems with their stereovision
• if there are any problems in early life (e.g. squint) then the brain doesn’t learn to put the information from the two eyes together and this cue to depth is lost
• such people sometimes don’t know that they have this ‘problem’ - must be other cues to depth

Question 16

what is motion parallax?

Answer 16

• movement through the world, or of our eyes causes the image on our retina to move
• things closer to us move faster
• so if we have dots or a 2D screen and move them at different speeds they can look like they have depth

Question 17

Rogers and Grahama (1979): motion parallax?

Answer 17

yolked the movement of dots to the movements of the head - subject ‘saw’ static 3-D surface when looking at moving 2-D surface

Question 18

what are some pictorial cues to depth?

Answer 18

• interposition
• height
• size
• perspective
• shadows
• texture gradients
• shape and shading

Question 19

pictorial cue to depth: size?

Answer 19

for a given object size it will cast a smaller and smaller retinal image as it gets further away

Question 20

pictorial cue to depth: shadows?

Answer 20

• the shadow of an object also gives us a cue to depth
• can be very powerful
• Kersten et al. (1997)

Question 21

what happens during distortions due to misplaced depth?

Answer 21

our understanding of depth is used to then decide on how large or small things are, and their spatial relationships

Question 22

what is Emmert’s Law?

Answer 22

• images of same retinal size will appear different sizes when located at different distances (linear relationship)
• hollow face illusion
• Ponzo illusion
• Ames room

Question 23

what is Ames room?

Answer 23

• people appear distorted in this room
• the ‘little’ person is actually farther away (the room is not square) and so has a smaller retinal image
• however, the cues trick you into thinking they are at the same distance