Week 5 : Depth Perception

Question 1

The problems for the visual system…

Answer 1

• interpreting a 3D world on a 2D retina
• we have an organized representation of visual space that is maintained in the visual cortex
• but… the representation is 2D which our world is 3D
• regardless of the distance an object is from the eye, light is imaged by the same receptors on the flat retina… so there is no depth perception at the retina

Question 2

Monocular depth cues

Answer 2

• cues that can be perceives with one status eye
• 2 categories… pictorial cues & motion cues

Question 3

Pictorial cues (monocular)

Answer 3

• cues from which we can judge depth from static non-moving pictures
• top-down understanding from experience

Question 4

Occlusion - pictorial cue

Answer 4

• occurs when one object partially hides/occludes the view of a second object
• we infer the hidden object is farther away from us than the object that obstructs it

Question 5

Relative height - pictorial cue

Answer 5

• objects closer to the horizon are seen are farther away (horizon = earth meets sky)
• objects at the lower bound of an image are closed to us than the horizon & objects above horizon are closer (e.g. cloud, tree branch)
• used a lot in art for depth

Question 6

Familiar size - pictorial cue

Answer 6

• when we judge distance based on our existing knowledge of the sizes of objects
• if we know something is smaller than another object but it is taking up more space on the retina, we assume it is closer to us
• this allows us to predict the relative distance each object is located from the eye
• this is why you arent likely to be fooled by a pic of someone leaning on the tower of Pisa

Question 7

Linear perspectives - pictorial cues

Answer 7

• percept that parallel lines appear to get closer as they get farther away
• relies on experience & we know parallel lines dont meet to must be cause we are far away
• perceptually parallel lines meet at the horizon (relative height)
• 1st cue to appear in kids art

Question 8

Texture gradients - pictorial cue

Answer 8

• many environments are comprised of an arrangement of objects that we know are approx. same size & same distance from each other
• so, an area of the image where objects appear larger & less tightly packed it closer to us than smaller tightly packed objects
• texture finer/smoother when far away

Question 9

Atmospheric perspective - pictorial cue

Answer 9

• objects in distance looks blurred and bluish
• our atmosphere scatters more blue light than other wavelengths, so more distant objects should appear blueish
• because far away objects appear fuzzy cuz not all the light is travelling in a straight line to us

Question 10

Shadows - pictorial cue

Answer 10

• shadows provide a depth cue cuz the object is in front of the shadow and the angle of the shadow gives cues
• objects in the shadows must be farther from the light
• depend on our life experiences w/ light sources like the sun

Question 11

Motion cues (monocular)

Answer 11

• arise from the motion of an object across the retina, or our own body movements to judge depth & distance

Question 12

Deletion & accretion - motion cue

Answer 12

• simplist motion cue
• essentially dynamic occlusion cues generated by a moving object
• an object moves across the visual field, if it disappears behind another object (deletion) it must be rather than the occluding object
• e.g. walking behind bookshelves

Question 13

Motion parallax - motion cue

Answer 13

• different speeds at which objects at different depths sweep across the retina
• objects closer to us, sweep across a larger portion of the retina when we move the same distance as objects farther away
• looking out car window, far objects move slow & close objects fly by
• eye can be stationary & still observe it
• objects moving sideways through the visual field

Question 14

Optic flow - motion cue

Answer 14

• motion of objects as we move forward/backward through environment
• point of focus = focus of expansion
• far away objects appear to move more slowly than close objects that rush toward us quicker (periphery so fast)
• helps navigate movement -> flow & movement cycle (e.g. gymnasts)
• crucial to balance & staying upright esp in early development

Question 15

Oculomotor cues

Answer 15

• involve the movement of the eyeballs themselves
• accommodation & vergence

Question 16

Accommodation - oculomotor cue

Answer 16

• to focus an image produced by incoming lights at different distances, the lens changes shape
• process can be felt & provide cues to depth, but its minimal use cause of the imprecision observed across different lighting conditions

Question 17

Vergence - oculomotor cue

Answer 17

• vergence occurs when the eyes rotate inward to see a near object & outward (diverges) when looking at a more distant object
• automatic process but brain senses movements & this feedback provides info about distance
• can provide info to about 2m in distance
• challenge for developing VR

Question 18

Binocular vision & disparity

Answer 18

• a large portion of what is visible to humans exists in binocular vision
• visible to both eyes
• representations on each retina aren’t identical
• our brain stitches these images together to extract depth cues & perceive a continuous single scene (stereopsis)
• this is binocular disparity
• how 3D glasses work

Question 19

Corresponding retinal points

Answer 19

• the 2 retinae can be thought of as having corresponding points/locations that would overlap perfectly if the 2 eyes were superimposed
• non corresponding points would not overlap perfectly = double image (think of finger and lamp)

Question 20

Horopter

Answer 20

• corresponding points fall on the horopter
• horopter is an imaginary arc through the visual field that contains the fixation object & all other objects located the same distance from viewer as fixation object

Question 21

Horopter & double vision

Answer 21

• non-corresponding points not in the horopter give rise to seeing doubles (diplopia)

Question 22

Horopter & visual disparity

Answer 22

• horopter related to point of fixation so it changes
• location of horopter also influences the type of disparity generated by other objects in the visual field
• crossed vs uncrossed disparity

Question 23

Crossed disparity

Answer 23

• direction of disparity for objects that are closed to the viewer than the horopter (cross eyes)
• image in the left eye is to the right of the image of the object in the right eye

Question 24

uncrossed disparity

Answer 24

• farther than the horporter (uncrossed eyes)
• image in the left eye is to the left of the image in the right eye

Question 25

Disparity tuning

Answer 25

- use magnitude of disparity to provide depth cues - in cortex, after 2 images have been combined, we have binocular cells that have similar tuning properties & discern object depth - many of these cells receive their inputs from corresponding points on the retina - however, there is a subset of binocular cells that receive their inputs from non-corresponding retinal areas & thus tuned to visual disparities (V4-V1) - we do not have binocular depth info till we are 4 months old