Colour Vision

Question 1

What does the perception of colour involve?

Lecture 2

Answer 1

The perception of color involves the interaction between the object’s reflective properties, illumination, and the visual system’s processing in the brain.

Question 2

How does the brain process colour information?

Lecture 2

Answer 2

• Colour information is processed in the occipital lobe, which contains feature cells sensitive to colour and shape.
• The temporal lobe is responsible for object recognition
• The frontal cortex evaluates the significance of the perceived object (whether you like it or not).
• This information is then passed on to the dorsal stream for further processing (where it is/what you can do with it)

Question 3

What are the types of cones in the eye?

Lecture 2

Answer 3

• The eye contains three types of cones: S (blue), M (green), and L (red).
• These cones are sensitive to different wavelengths of light and are responsible for colour perception.

Question 4

Where are the different types of cone produced?

Lecture 2

Answer 4

• S cones are produced on chromosome 7 (q31)
• M and L cones are produced by two genes on the X chromosome (q28)

Question 5

Are there individual differences in number of cones in the fovea and if so what are they?

Lecture 2

Answer 5

• Yes there are individual differences
• In the average person there are twice as many L cones than M cones
• Also only 5% of all cones are S cones

Question 6

The Principle of Univariance

Lecture 2

Answer 6

The principle of Univariance states that a cone cannot distinguish between the wavelength and intensity of light that triggers its action potentials/ response.

Question 7

What are the two pathways of colour? (ganglion cell types)

Lecture 2

Answer 7

• Bistratified cells code for Blue/Yellow - S vs (L+M)
• Midget cells code for both Red/Green colour vision and spatial detail - L vs (L+M)

Question 8

How are the two color pathways organised in the brain?

Lecture 2

Answer 8

The Blue-Yellow and Red-Green pathways are segregated from the retina to the cortex

Question 9

What are the implications of the Red-Green Colour Processing System?

Lecture 2

Answer 9

• L vs (L+M) in the receptive field which is divided into a centre and a surround: In the Red/Green pathway a poppy can be seen in a green field, however when stripped of colour, you can still see the poppy (because of the lines around it) you just have to work a bit harder to see it.

Question 10

What are the implications of the Blue-Yellow Colour Processing System?

Lecture 2

Answer 10

• The Blue/Yellow pathway does not have a center-surround organisation in its receptive field.
• This lack of spatial detail in the Blue/Yellow pathway, known as the Boynton effect, results in limited information about object boundaries.
• The Blue/Yellow subsystem is also more vulnerable to toxic substances such as alcohol, potentially affecting color perception.

Question 11

What percentage of men are affected by colour deficiency?

Lecture 2

Answer 11

8%

Question 12

What are the different types of colour vision deficiencies, and how prevalent are they?

Lecture 2

Answer 12

Dichromat (missing one type of cone):
* Deuteranopia (1%) = missing M cones
* Protanopia (1%) = missing L cones

Anomalous trichroma (have all 3 types but one has a different spectral sensitivity):
* Deuteranomaly (5%) = M cones are shifted
* Protanomaly (1%) = L cones are shifted

Question 13

What vision did humans possess about 30 million years ago, and what is significant about it?

Lecture 2

Answer 13

About 30 million years ago, humans had vision similar to dogs, possessing S and L cones.
* S vs L: Blue/Yellow perception.
* L vs L: No Red/Green colour vision. There is still spatial detail.

Question 14

What evolutionary event occurred about 30 million years ago, leading to trichromatic vision in humans?

Lecture 2

Answer 14

A gene duplicated on the X chromosome. One of the duplicated genes then mutated, allowing humans to perceive green.
This advantageous mutation led to the emergence of M cones, contributing to trichromatic vision.

Question 15

Why is colour blindness more prevalent in men?

Lecture 2

Answer 15

• Men only have one X chromosome.
• Inter-genic recombination occurs: The result: One chromosome has two M cone genes and one with one missing.
  The chromosome lost its green gene - Dichromats.
• Intra-genic recombination occurs: Recombination is within each gene, producing a hybrid gene which is half green/half red. It is now producing a pigment which has shifted in its spectral sensitivity (picks up slightly different colours) - Anomalous Trichromacy

Question 16

How does tetrachromacy arise, and what is the role of female carriers in its inheritance?

Lecture 2

Answer 16

• Tetrachromacy can arise from gene variants passed down by female carriers.
• Female carriers may possess one normal X chromosome and one with a hybrid gene, contributing to the development of tetrachromatic vision in a small percentage of the population.

Question 17

What factors contribute to the variability in retinal cone mechanisms?

Lecture 2

Answer 17

Individual differences in retinal cone mechanisms result from hybrid genes, hybrid pigments, polymorphic pigments, and occasional mutations.

Question 18

What methods are used to measure individual differences in colour vision?

Lecture 2

Answer 18

• Ishihara plates
• Color matching experiments
  E.g. The Rayleigh match, as proposed by L. Rayleigh in 1881, which revealed significant variability in colour perception among both colour-blind and normal populations.

Question 19

What distinguishes isomers from metamers in colour perception?

Lecture 2

Answer 19

• Isomers are colours that are identical because they correspond to identical objects.
• Metamers are colours that appear the same despite being physically different