Task 3

Question 1

Basics of color perception

Answer 1

• color as result of interaction of a physical stimulus with a particular nervous system
• three steps:
  1: Detection -> detection of wavelengths
  2: Discrimination -> tell difference between one wavelength and another
  3: Appearance -> assign perceived colors to lights and surfaces in the world

Question 2

Step 1: Detection

Answer 2

• three types of cone photoreceptors: differ in photopigment they carry -> differ in sensitivity to light of different wavelengths
  
  • S-Cones: 420 nm
  • M-Cones: 535 nm
  • L-Cones: 565 nm
• their spectral sensitivities overlap

Question 3

Step 2: Color Discrimination

• principle of univariance
• cone-opponent cell

Answer 3

• principle of univariance: an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor -> one photoreceptor cannot make color discriminations based on wavelength
• cone-opponent cell: cell type that subtracts one type of cone input from another

Question 4

Step 3: Color Appearance

- color space

Answer 4

• color space: three-dimensional space, established because color perception is based on the outputs of three cone types, that describes a set of all colors

Question 5

Color Blindness:

- determining factors

Answer 5

• type of cone affected
• type of defect (photopigment for cone missing or anomalous)
• mostly: M- and L-cone defects

Question 6

Color Blindness:

- Types

Answer 6

• Protanope: absence of L-cones
• Deutanope: absence of M-cones
• Tritanope: absence of S-cones
• Cone monochromat: only one cone type; truly color-blind
• Rod monochromat: no cones; truly color-blind; badly visually impaired in bright light
• Cerebral achromatopsia: total color blindness

Question 7

Color Blindness:

- Anemia

Answer 7

inability to name objects in spite of the ability to see and recognize them
- typically due to brain damage

Question 8

Color Blindness:

- Agnosia

Answer 8

failure to recognize objects in spite of the ability to see them
- typically due to brain damage

Question 9

Adaptation and Afterimages:

• negative afterimages
• adapting stimulus
• neutral point

Answer 9

• negative afterimages: afterimages whose polarity is the opposite of the original stimulus; light stimuli produce dark negative afterimages; colors are complementary
• adapting stimulus: stimulus whose removal produces a change in visual perception or sensitivity
• neutral point: point at which an opponent color mechanism is generating no signal

Question 10

Color Constancy

Answer 10

= tendency of a surface to appear the same color under a fairly wide range of illuminants

Question 11

Light Constancy

Answer 11

= fact that we see whites, grays, and blacks as staying about the same shape under different illuminations
- information of shadows: shadow’s meaningful shape must be taken into account

Question 12

Selective Reflection

Answer 12

= process in which some wavelengths are reflected more than others -> leads to occurrence of chromatic colors (blue, green, and red)
- achromatic colors = white, gray, and black

Question 13

Selective Transmission

Answer 13

= only some wavelengths pass through the object or substance -> color of things that are transparent (liquids, plastics, and glass)

Question 14

Color Mixing

- Paints -> subtractive color mixture

Answer 14

• both paints still absorb same wavelengths they absorbed when alone -> only wavelengths selected are those selected by both paints common –> subtractive color mixture

Question 15

Color Mixing

- Lights -> additive color mixture

Answer 15

• all of the light that is reflected from the surface by each light when alone is also reflected when the lights are superimposed

Question 16

Perceptual Dimensions of Colors

Answer 16

• spectral colors: colors in spectrum
• nonspectral colors: colors that do not appear in the spectrum because they are mixtures of other colors (e.g. magenta -> red and blue)

Question 17

Trichromatic Theory of Color Vision

- in general

Answer 17

= theory that the color of any light is defined in our visual system by the relationships of three numbers - outputs of three cones
- Young-Helmholtz theory

Question 18

Trichromatic Theory of Color Vision

- Color-Matching Evidence

Answer 18

• color matching
• light of a particular wavelength stimulates each receptor mechanism to different degrees -> pattern of activity in three mechanisms result in the perception of a color

Question 19

Trichromatic Theory of Color Vision

• Physiological Evidence
  
  • metamerism

Answer 19

• cone pigments (S-, M-, and L-cones)

- metamerism: two physically different stimuli are perceptually identical

Question 20

Trichromatic Theory of Color Vision

- three receptor mechanisms

Answer 20

• monochromat:
  
  • principle of univariance
• dichromat:
  
  • color perception depends on the pattern of activity in three receptor mechanisms

Question 21

Opponent-Process Theory of Color Vision

- in general

Answer 21

= color vision is caused by opposing physiological responses generated y blue and yellow, red and green, and black and white

• evidence:
  
  • color circle -> all colors fall in four different categories

Question 22

Opponent-Process Theory of Color Vision

- psychophysical measurements

Answer 22

• hue cancellation

- discovery of opponent neurons

Question 23

Opponent-Process Theory of Color Vision

- physiological evidence

Answer 23

• opponent neurons
• BUT:
  • both theories correct –> each describes neural processes that take place in different parts of the visual system

Question 24

Article: 
#thedress

Answer 24

• color constancy –> perception of the dress is based on individual differences
• people either correct for “cool illumination” (white and gold) or “warm illumination” (blue and black)

Question 25

Population coding

• specificity coding
• distributed coding

Answer 25

• specificity coding: firing of one neuron to one specific stimulus
• distributed coding: large number of stimuli that can be represented by a large number of neurons