Perceiving Depth

• Cue Approach to Depth Perception

  • this approach focuses on information in the retinal image that is correlated with depth in the scene: occlusion
  • we learn the connection between the cue and depth
  • the association becomes automatic through repeat exposure
• Oculomotor Cues

  oculomotor cues are based on sensing the position of the eyes and muscle tension

  • convergence: inward movement of the eyes when we focus on nearby objects
  • accommodation: change in the shape of the lens when we focus on objects at different distances
• Monocular Cues

  monocular cues from from one eye

  • partial cues: sources of depth information that come from 2-D images, such as pictures
  • occlusion: when one object partially covers another
  • relative height: objects below the horizon that are higher in the field of vision are more distant, objects above the horizon and are lower in the visual field are more distant
• Monocular Cues

  • relative size: when objects are equal size, the closer one will take up more of your visual field
  • perspective convergence: parallel lines appear to come together in the distance
  • familiar size: distance information based on our knowledge of object size
• Monocular Cues

  • atmospheric perspective: distance objects are fuzzy and have blue tint
  • texture gradient: equally spaced elements are more closely packed as distance increases
  • shadows: indicate where objects are located, enhance 3-D objects
• Motion-Produced Cues

  motion parallax: close objects in direction of movement glide rapidly past but objects in the distance appear to move slowly

  deletion and accretion: objects are covered or uncovered as we move relative to them

  • covering an object is deletion
  • uncovering an object is accretion
• Binocular Depth Information

  • stereoscopic depth perception
  • differences between 2D and 3D movies
  • binocular disparity: difference in images from two eyes
  • stereopsis: the impression of depth that results from information provided by binocular disparity
  • strabismus: eyes do not point in the same direction (ie lazy eyes)
• The Physiology of Binocular Depth Perception

  binocular depth cells/disparity selective cells: neurons that respond best to binocular disparity

  • cells respond best to a specific degree of absolute disparity between images on the right and left retinas

  disparity tuning curve
• The Physiology of Binocular Depth Perception

  experiment by Blake and Hirsch

  • cats were reared by alternating vision between two eyes

  results show

  • reared cats had fewer binocular neurons
  • were unable to use binocular disparity to perceive depth
• Perceiving Size

  distance and size perception are interrelated

  experiment by Holway and Boring

  • observer was at the intersection of two hallways
  • a luminous test circle was in the right hallway placed from 10 to 120 feet away
  • a luminous comparison circle was in the left hallway at 10 feet away
• Perceiving Size

  experiment by Holway and Boring

  • on each trial the observer was to adjust the diameter of the test circle to match the comparison
  • test stimuli all had same visual angle (angle of object relative to the observer's eye): visual angle depends on both the size of the object and the distance from the observer
• Perceiving Size

  Part 1 of the experiment provided observers with depth cues

  • judgements of size were based on physical size

  Part 2 of the experiment provided no depth information

  • judgements of size were based on size of the retinal images
• Size Estimation

  • based on actual sizes of objects when there is good depth information
  • when depth information is eliminated, visual angle strongly influences size estimation
• Size Constancy

  • perception of an object's size remains relatively constant
  • this effect remains even if the size of the retinal image changes
  • size-distance scaling equation: S = R x D, the cahnges in distance and retinal size balance each other
• The Price of Constancy

  inappropriate interpretations of physical reality

  example assumptions and related illusions:

  • converging lines are corners: Müller-Lyer illusion
  • linear perspective cues: Ponzo illusion
  • rooms are rectangular: Ames room illusion
• Müller-Lyer Illusion

  • S = R x D
  • distance is perceived as changing
  • since the retinal images (R) are the same, the lines must be different sizes (S)
• Ponzo Illusion

  • horizontal rectangular objects are placed over railroad tracks in a picture
  • the far rectangle appears larger than the closer rectangle but both are the same size
  • one possible explanation is misapplied size-constancy scaling
• The Ames Room

  • two people of equal size appear very different in size in this room
  • the room is constructed so that: a) the shape looks like a normal room when viewed with one eye, b) the actual shape has the left corner twice as far as the right corner
• The Ames Room

  one possible explanation: size-distance scaling

  • observer thinks the room is normal
  • women would be at same distance
  • woman on the left has smaller visual angle (R)
  • due to the perceived distance (D) being the same, her perceived size (S) is smaller
• The Ames Room

  second possible explanation: relative size

  • perception of size depends on size relative to other objects
  • one woman fills the distance between the top and bottom of the room
  • the other woman only fills part of the distance
  • thus, the woman on the right appears taller