Problem 3 - DONE

Question 1

three steps of colour perception

Answer 1

1. detection
2. discrimination
3. appearance

Question 2

step 1 - colour detection

Answer 2

• wavelengths must be detected

- cone-photoreceptors

Question 3

step 2 - colour discrimination

Answer 3

• we must be able to tell the difference between one wavelength (or mixture of wavelengths) and another
• principle univariance
• trichromatic theory of colour vision
• metamerism

Question 4

step 3 - colour appearance

Answer 4

• opponent colours

Question 5

step 1

cone-photoreceptors

Answer 5

• S-cones: short-wavelength pigment, with maximum absorption at 419-nm
• -> ‘blue cone’
• M-cones: middle-wavelength pigment, with maximum absorption at 531-nm
• -> ‘green cone’
• L-cones: long-wavelength pigment, with maximum absorption at 558-nm
• -> ‘red cone’

Question 6

step 2

principle of univariance

Answer 6

=

• one photoreceptor type cannot make colour discriminations based on wavelength
• -> receptor does only know the total amount it has absorbed
• once a photon of light is absorbed –> identity of the lights wavelength is lost
• anything a photon can do: hyper polarise more or less

Question 7

step 2

trichromatic theory of colour vision

Answer 7

= trichromacy = Young-Helmholtz theory

• theory that the colour of any light is defined in our visual system by the relationships of three numbers
• -> the outputs of three receptor types (three cones)
• colour vision depends on three receptor mechanisms, each with different spectral sensitivities
• based on colour-matching experiments

Question 8

trichromatic theory of colour vision

colour-matching experiments

Answer 8

• we can come up with any colour we perceive by matching three light sources
• background: metamerism

Question 9

trichromatic theory of colour vision

metamerism

Answer 9

• situation in which two physically different stimuli are perceptually identical
• metamers = any pair of stimuli that are perceived as identical in spite of physical differences
• -> reason (metamers look alike): both result in the same pattern of response in the three cone receptors
• background of colour matching experiments

Question 10

step 2

mixture of lights

Answer 10

additive colour mixture = if light A and light B are both reflected from a surface to the eye, the effects of those two lights add together –> adding up the wavelengths of each light in the mixture
=> all the light that is reflected from the surface by each light when alone –> also reflected when lights are superimposed
=> more wavelengths are be reflected –> each light adds wavelengths to the mixture

Question 11

step 2

mixture of pigments

Answer 11

subtractive colour mixture = if pigments A and B mix, some light will be subtracted by A and some by B –> remainder contributes to perception if colour
=> when mixed, both paints still absorb same wavelengths they absorbed when alone –> only wavelengths reflected are those that are reflected by both paints in common
=> fewer wavelengths are be reflected –> each paint subtracts wavelengths from mixture

Question 12

step 3

opponent colour theory

Answer 12

• theory that perception of colour is based in the output of three mechanisms, each of them resulting from an opponent between two colours: red-green, blue-yellow, black-white
• colour vision is caused by opposing responses generated by blue and yellow and by red and green
• based on results of phenomenological observations

Question 13

opponent colour theory

phenomenological observations

Answer 13

• perceptual pairing (B+ Y-; R+ G-)
• opponent neurones = respond with an excitatory response to light from one part of the spectrum and with an inhibitory response to light from another part
• -> single-opponent neurone = M+ L- receptive field; found in the retina, lateral geniculate nucleus and visual cortex; subtracts one type of cone input from another
• -> double-opponent neurone = side-by-side receptor field; found in the visual cortex; one region is excited by one cone type and inhibited by the opponent cones (R+ G-)

Question 14

colour deficiency

Answer 14

= partial loss of colour perception

• monochromat
• dichromate
• trichromat
• -> anomalous trichromat

Question 15

colour deficiency

monochromatism

Answer 15

• usually hereditary
• colour blindness
• poor visual acuity
• cone monochromat = individual with only one cone type –> characteristics of rod vision in both dim and bright lights
• rod monochromat = individual with no cones of any type; in addition are badly visually impaired in bright light

Question 16

colour deficiency

dichromatism

Answer 16

• experience some colours (lesser range)
• sex-linked
  types:
• protanopia
• deuteranopia
• tritanopia

Question 17

colour deficiency

trichromacy

Answer 17

• individual with three visual pigments –> third pigment increases number of colours that can be seen across visual spectrum

anomalous trichromat

• mixes wavelengths in different proportions
• not good at discriminating between wavelengths that are close together

Question 18

adaptation and afterimages

Answer 18

• colour induction = generation of illusory sensations of colour without direct stimulation of the corresponding retina
• -> negative afterimage = afterimage whose polarity is the opposite of the original stimulus

Question 19

colour constancy

Answer 19

= tendency that colours of objects/surface appear relatively unchanged (even if changed lighting conditions)

• why it occurs?
  1. chromatic adaption
• colour perception is changed by exposure to chromatic colour
• partial colour constancy = perception of object is shifted after adaptation
  2. effects of surroundings
  3. memory of colour

Question 20

light constancy

Answer 20

= achromatic colours are perceived as remaining the same even under changing illumination

• why it occurs?
  1. ratio principle
• lightness is determined by ratio of reflectance of object vs. reflectance of surrounding objects
• -> ratio remains the same = perceived lightness remains the same
  2. perception under uneven illumination
• in three-dimensional scenes: illumination is uneven because of shadows

Question 21

types of dichromatism

Answer 21

protanopia = colour deficiency due to absence of L-cones
- perceives short-wavelengths (blue)
- neutral point: 492nm
deuteranopia = colour deficiency due to absence of M-cones
- perceives blue at short-wavelengths; yellow at long-wavelengths
- neutral point: 498nm
tritanopia = colour deficiency due to absence of S-cones
- perceives blue at short-wavelengths; red at long-wavelengths
- neutral point: 570nm
–> neutral point = wavelength at which person perceives grey/no colours

Question 22

three dimensions of colour

Answer 22

1. hue/chromatic colours (blue, green, red)
   –> dominant wavelengths
   {1a. achromatic wavelengths (white, black, grey)}
2. brightness (value, intensity)
   –> amplitude of wavelength
3. saturation (chroma)
   –> spread of wavelength

Question 23

population coding

overview lecture

Answer 23

= analysis of a stimulus by multiple types of receptors/neurones

• important for colour vision based on different cone types + colour vision based on opponent cells
• solution for univariance
  1. perception of colour changes when relative pattern of activity of three receptor types changes
  2. perception of colour does not change if relative pattern of activity is the same
  3. response of a single receptor is ambiguous

Question 24

reflectance curves

Answer 24

• shows plot of percentage of light reflected vs. wavelength
• -> selective refraction = wavelengths of chromatic colours/hues are more reflected than others
• -> colours in environment are created by selective reflection of some wavelengths from objects

Question 25

transmission curves

Answer 25

• shows plot of percentage of light transmitted vs. wavelength
• -> selective transmission = only some wavelengths pass through the object/substance

Question 26

understanding #the dress

Answer 26

• light reflection from an object depends on
• -> reflectance properties of object (= surface): constant
• -> illumination spectrum (= light conditions): variable (temporal + spectral)
• colours in picture vary along blue-yellow axis
• -> correct for cool illumination: white/gold
• -> correct for warm illumination: blue/black