Chapter 10: Perceiving Depth and Size

• Questions needing answers

  • How can we see far into the distance based on the flat image on the retina

    • 2D image - where does 3D come from

  • Why do we see depth better with 2 eyes than with one?

    • What judgements require 2 eyes vs one?

  • Why don’t people appear to shrink in size when they walk away?

    • What size is constancy?

• Cues to Depth Perception

  • Oculomotor Cues

    • cues related to eye positions and muscle tension

  • Monocular Cues

    • cues needing only one eye

  • Binocular Cues

    • cues needing two eyes

• Oculomotor Cues

  • Convergence

    • eyes turn in more for a near object than a far object

    • eyes have no convergence for very far objects

    • creates muscle tension

  • Accommodation

    • lens fattens for near objects

    • lens fattens for far objects

    • the ciliary muscles provide feedback on distance

    • also considered a monocular cue

• Monocular Cues

  • Pictorial cues - used in paintings and drawings, stationary cues

    • accommodation

    • occlusions

    • relative height

    • relative size

    • linear perspective

    • familiar size

    • atmospheric perspective

    • texture gradient

    • shadows

  • Motion Parallax

  • Deletion and Accretion

• Relative Height

  • objects higher in the plane are seen as further away

• Relative Size

  • near objects seem larger

  • distant objects seem smaller

• Size constancy - recognize the object as the same actual size

  • we retain our knowledge of relative size to produce size constancy

• Linear perspective

  • parallel lines seem to converge in the distance

  • called Perspective Convergence in the text

• Familiar Size

  • when using one eye, subjects judged using familiar size

  • when using two eyes, they could see the coins were the same distance

  • familiar size cue is useful when other cues are absent

• Atmospheric Perspective

  • also called Aerial Perspective

  • distant objects appear hazier often bluer than near objects

• Texture Gradient

  • large field of similar objects

  • objects get smaller in distance

• Shadows

  • shadows cast by objects can provide good cues to depth

• Binocular Cues

  • Binocular Disparity

  • images seen by the 2 eyes aren’t the same

  • disparity creates depth

  • corresponding points on retinas

• Retinal Disparity

  • also called binocular disparity

  • causes retinal disparity

  • viewpoint of the 2 eyes is slightly different

  • stereoscope

• Disparity

  • the disparity of the points on the retina for the far bar perception of the depth

Chapter 10a: Perceiving Color

• Functions of Color Vision

  • Perceptual Segregation

    • determine the boundaries of an object

    • necessary for finding food

    • illumination contours

  • Signaling

    • come colors have specific meanings attached

    • colors may signal danger, a mate, health, etc.

• Hue - what we think of as “color”

  • achromatic - no hue

  • chromatic - having hue

• Saturation - the purity of the color

  • hue saturation scale goes from less white added to more white added

• Brightness - intensity of color

  • goes from less gray added to more gray added

• Color Experience

  • we can perceive about 2,000,000 colors

  • use 4 names to describe them - red, yellow, green, blue

• Color and Wavelength

  • visible spectrum is from 360 - 760 nm

  • different wavelengths appear to look different colors to us

• Selective Reflection

  • white light coming in → green surface (leaf) → green light wavelength appearing to eye

• Surface Reflectance

  • light hits white paper → 10% absorbed by paper → 90% reflected

  • light hits black paper → 90% absorbed by paper → 10% reflected

• Sensory Code

  • Specificity Coding

    • Neurons are tuned to specific wavelengths

    • Problems

      • requires more receptors than are present in a location on the retina

      • too many different colors to have specificity

    • Cone Properties

      • respond to range of wavelengths

      • respond to stimulus intensity

  • Across Fiber Pattern Coding

    • Different qualities (color) are signaled by a pattern of neural activity

    • Three cone types respond best to 3 different wavelengths

      • S cones - 419 nm (Short - blue)

      • M cones - 531 nm (Medium - green)

      • L cones 558 nm (Long - red)

    • The combination of firing across receptors allows discrimination

• Combination Theory of Color

  1) Trichromatic Stage

  • every color perceived by the brain is a combination of blue, green, and red

  • these colors are created by the action of different proportions of red, green, and blue light, which can be mixed to produce any color in the visible spectrum

  2) Opponent Process Stage

  • explains how color is coded at the level of ganglion cells and lateral geniculate nucleus of the thalamus

  • color is coded in opposite pairs: black+white, yellow+blue, and green+red

• Color Metamers

  • metamers: two lights have different wavelength distributions but are perceptually identical

• Color Deficiency

  • Monochromat

    • rod monochromat - truly colorblind (black and white)

    • monochromat - one functioning cone (can’t distinguish colors)

  • Dichromat

    • Protanopia - L cone absent

    • Deuteranopia - M cone absent

    • Tritanopia - S cone absent

    • Anomalous Trichromat - deficient cone type (combo of any above cone absence)