Lecture 6 - Depth/ size perception

Question 1

What are depth cues?

Answer 1

• we perceive depth with the help of cues
• there are body related
  -> oculomotor cues (state of the eyes)
  -> we feel that an object is close to us or far away
• there is also stimulus related cues
  -> monocular cues (work with one eye): pictorial and motion based cues
  -> binocular cue (depends on both eyes): retinal disparity - the most powerful of all depth cues

Question 2

Monocular cues - pictorial cues?

Answer 2

• Occlusion:
  -> occluded (partially hidden) objects are further away than occluding objects
• Relative height:
  -> objects higher in the visual field are further away than objects lower in the visual field
• Relative size:
  -> when 2 objects are equal in size, the one that is further away will take up less of the visual field
• Familiar size:
  -> knowledge about physical size of objects
  -> Epstein experiment: 1 pound coin is judged to be further away because it is known to be larger than the 5p coin
• Perspective convergence:
  -> converging parts of objects are further away
• Atmospheric perspective:
  -> objects further away are less sharp and have a blue tint
  -> particles in the atmosphere scatter light (preferentially short wavelengths which we see as blue) which softens and colours further away objects
• Texture gradient:
  -> denser textures indicate further away objects
• Shadows
  -> can make objects seem closer or further away from us

Question 3

Monocular cues - motion based cues?

Answer 3

• Motion parallax:
  -> nearby objects glide away quickly
  -> far away objects glide along slowly
• Deletion & accretion:
  -> an object being covered and uncovered is further away (occlusion in motion)

Question 4

Binocular cues?

Answer 4

• binocular vision = stereoscopic vision
• the binocular cue = retinal disparity

Question 5

Retinal disparity?

Answer 5

• the retinal images in the left and right eye are slightly shifted (because our eyes are a few cm apart)
• the point of fixation is crucial for binocular depth perception

Question 6

Corresponding retinal points?

Answer 6

• the 2 foveae are corresponding points
• all objects with the same distance as the fixation point will be represented on corresponding retinal points (same relative position in both eyes)
• if eyes are overlaid corresponding points overlap exactly
  can look at the Frida e.g.

Question 7

What is the horopter?

Answer 7

imaginary plane through the fixation point connecting all objects for which retinal images fall on corresponding retinal points (connects all objects that are equidistant from observer)

Question 8

Non corresponding retinal points?

Answer 8

look at carol/frida e.g.
- if eyes are overlaid non-corresponding points do not overlap
- this is the same for objects that are further away than the fixation point

Question 9

Stereopsis?

Answer 9

• this is the ability to use retinal disparity as a cue to perceive depth
• each distance from the fixation produces a different amount of retinal disparity
• the larger the disparity, the further away the object is from fixation

Question 10

Neuronal evidence?

Answer 10

• Disparity-selective neurons in V1 and along both ventral and dorsal streams
• Retinal disparity can transform 2D images into 3D perception
• But stereoscopic photographs also contain pictorial cues which create depth perception

Question 11

Size perception?

Answer 11

• our perception of size can be affected by our perception of depth
• the size of retinal object representations are measured in degree of visual angle: the angle of an object relative to the observers eye, the size of the retinal object representation
• it depends on physical object size and physical distance from the observer
• the same object has a larger visual angle when it is closer to the observer and a smaller visual angle when it is further away
• 2 differently sized objects can have the same visual angle when the larger one is further away

Question 12

Size constancy?

Answer 12

perception of an objects size is relatively constant even when viewed from different distances (producing different visual angles)

Question 13

How do you calculate size constancy?

Answer 13

• size distance scaling equation
• S = K(R x D)
• S = perceived size R = size of the retinal image D = perceived distance