Mechanisms of colour vision

Question 1

Describe trichromatic theory, colour matching and metamers

Answer 1

• by Thomas Young, early 19th century
• 3 types of ‘particles’ in the retina for the sensations of red, green and blue.
• other hues are mixtures of these primaries
• validated by colour matching
  -metamers; colour matching

Question 2

List the unique hues and explain opponent colours

Answer 2

• Hering 1878: postulated from introspection, two pairs of opponent colours: blue-yellow, green-red (4UH)
• OC are mutually exclusive: no hue can be both bluish and yellowish at the same time or both greenish and reddish
• gives two axes in colour space
  -colour after-effect

Question 3

Trichromacy to opponent coding: an attempted synthesis

Answer 3

• in humans opponent coding seems to be based on two comparisons:
  
  • blue v yellow
  • green v red
• How do we get from Young-Helmholz trichromacy to Hering’s opponent colours (4UH in 2 opponent pairs)?

Question 4

Opponents processing of cone signals

Answer 4

Hypothetical scheme for producing two chromatic and one luminance (black/white) channels from combining cone signals

Question 5

Is there neurophysiological evidence for opponent processing?

Answer 5

• Yes and what would count as evidence?
• If colour opponent neurons, expect inputs of opposite sign from cones of different spectral classes
• earliest attempts: De Valois, Abramov, & Jacobs (1966)

Question 6

describe Colour sensation

Answer 6

• Light reflected from an object surface will differentially affect the 3 sets of cones according to the SSR
• Oversimplified: light does not normally appear coloured, but there is nevertheless variation in its spectral quality
• This means that under different illuminants light reflected from the object will be ‘distorted’ from the SSR
• Therefore the relative cone responses will change in different lighting conditions

Question 7

Discuss the retinal circuitry that may give rise to colour opponent receptive fields

Answer 7

-colour opponent neurons in retina and LGN
-red/green cells
L+ M- or M+ L-
-blue/yellow cells
-weak and strong excitation => net excitation
-L/M cones feed into inhibitory input into short bistratified ganglion cells
-input from blue cones/ short wave cones is fed via an excitatory pathway to same small bi-st GC

Question 8

Spectral radiance of two different light sources

Answer 8

Power (au) vs wavelength for a daylight simulation and indoor lighting with tungsten light bulbs

Question 9

what is the basic centre surround arrangement

Answer 9

inhibition from cones in surround
excitation from cones in centre

Question 10

what does the actual reflected light depend on/

Answer 10

quality of the illuminating light

Question 11

what does spectrum of light reaching the eye depend on?

Answer 11

SSR and spectrum of illuminant

Question 12

Explain the concept of colour constancy

Answer 12

• For a given surface, wavelength composition of light reaching the eye will change if wavelength composition of illuminant changes
• If the real-world property of colour is SSR then the job of the visual system may be to estimate real SSR
• But this can only be done if the spectrum of the illuminant is known c = I s
• Wavelength composition of light reaching the eye is coded by relative responses of the 3 cone classes
• For a given object, wavelength composition of light reaching the eye (and therefore relative cone responses) will change if wavelength composition of illuminant changes
• However, perception of object colour appearance does not normally change
• Therefore colour appearance cannot be the same as wavelength composition
• The converse can also be true; wavelength composition of light reaching the eye can remain the same in situations where colour appearance does change

Question 13

describe equation c = i s

Answer 13

• c = spectrum of light reaching the eye
• i = the illuminant
• s = SSR

Question 14

COLOUR APPEARANCE DEPENDS ON CONTEXT: SIMULTANEOUS COLOUR CONTRAST. True or false?

Answer 14

TRUE

Question 15

how are Colour appearance and wavelength composition dissociable

Answer 15

• Colour constancy shows that wavelength can change while colour appearance remains constant
• Simultaneous colour contrast shows that colour appearance can change even though wavelength remains constant
• Opponent processing along retinogeniculate pathway seems suited to signal wavelength composition of light
• Perhaps further cortical processing is required to account for perceived colour e.g. colour constancy