Lecture 2 - Individual Differences in Colour Vision (Colour Vision 1)

Question 1

Why does colour vision exist? Name one of the uses of colour and where it appears. (3)

Answer 1

Colour is used in communication.

Colour can indicate ripeness/quality of food, someone’s health (i.e., pale, feverish, etc.), symbolism in safety, and in aesthetics (clothes, appearance, decoration, etc.)

Question 2

Which two colours are often used symbolically as signals?

Answer 2

Red and Green.

Question 3

Name another use of colour towards survival.

Answer 3

Camouflage uses colours of the environment to help blend in and hide from predators/prey.

Question 4

What else can colour provide us in terms of environmental information? Give an example of what happens when we do not have access to colour.

Answer 4

Colour allows us to segment and differentiate between objects in space.

For example, viewing a grey-scale image limits the information we can process in a scene. We may not identify which flowers are in a field if we had no access to their colours.

Question 5

What is the difference between humans and dogs - and most other species - in their colour vision?

Answer 5

Humans and primates are trichromatic. This means we have three cones which process colour vision. Dogs and most other species are dichromatic, meaning they only have two types of cones which process colour.

Question 6

Fish have tetrachromacy, and most insects have pentachromacy. Explain what this means and provide an example.

Answer 6

Fish have tetra-chromatic vision, which means they have four cones. Pentachromacy means five cones. The peacock mantis shrimp has 12 types of cones - allowing them to perceive more colours than we could ever hope to imagine.

Question 7

TRUE or FALSE. We all see the same colours. Our eyes, brains, and the functions/mechanisms which allow us to see colour do not matter - all colours are the same.

Answer 7

FALSE. Colour is a construct. Colour vision depends heavily on the eyes and brain of the beholder. It also depends on the machinery and functions within the eye which code for colours.

We are constantly constructing a scene based on what we can and cannot process. Thus, individual differences can often arise in our perception of colour.

Question 8

What is central to colour vision, and what happens to colours when this changes?

Answer 8

Illumination is central to colour vision. Lower illumination limits the colours we can perceive.

Question 9

How do we perceive colour?

Answer 9

We perceive colour from the reflective spectrum. This is the number of wavelengths, or frequencies of light, reflected into our eyes.

Question 10

Which colour is most reflected in most individuals? How do we see this colour?

Answer 10

Red is reflected the most.

Reflected wavelengths allow us to perceive a colour as ‘red’, but the colour red doesn’t exist as a physical property. ‘Red’ is just different frequencies of light being reflected into our eyes.

Question 11

What are the three types of cones? How do they differ?

Answer 11

The three cones are Blue (S), Green (M), Red (L). These cones are more sensitive to different spectral frequencies. Sensitivity depends on size.

Question 12

Which two frequencies and cones overlap, and which two have a larger gap between them?

Answer 12

Red and green frequencies - and therefore red (L) and green (M) cones - overlap much more than green and blue frequencies or M and S cones.

Question 13

Where are blue (S) cones manufactured or produced?

Answer 13

The gene which produces S, or blue cones, sits on chromosome 7.

Question 14

Where are the genes which code for green (M) and red (L) cones located? What is their special quality?

Answer 14

M and L cones are made on the X chromosome at position q128. They are X-linked on this sex chromosome.

Question 15

What is a defining feature of the fovea, an indentation on the retina where visual acuity is the highest?

Answer 15

The fovea does not contain any S or blue cones. As such, it is considered ‘blue-blind’.

Question 16

Which spectral frequencies is the eye most sensitive to, and why is this the case?

Answer 16

Most of the spectral sensitivity in the eye relates to red and green cones. We often have twice as many red cones as green cones.

Question 17

TRUE or FALSE. We have no variability in our spectral sensitivity to red or green. Meaning, we all see red or green in the same way.

Answer 17

FALSE. Our spectral sensitivity to red and green frequencies may vary significantly.

Question 18

What are the two main causes of variability in red and green spectral sensitivity?

Answer 18

1) The spectral sensitivity of the green (M) and red (L) cones themselves.
2) The number of M and L cones an individual has.

Question 19

Explain the principle of univariance in relation to colour vision. (5)

Answer 19

The principle of univariance suggests that any single cone in the visual system is colourblind by itself. In other words, any combination of wavelengths and intensity can produce the same response in every cone.

Each cone, therefore, needs to compare inputs between spectral sensitivities. Ganglion cells exist to compare these inputs. This is to distinguish between spectral frequencies and wavelengths, resulting in perceiving different colours.

Question 20

Explain the blue-yellow channel of colour vision. This is otherwise known as S vs. (L + M).

Answer 20

The S cone becomes excitatory, which fires a signal. This signal is compared to inhibitory L and M cones by bi-stratified ganglion cells.

Question 21

YES or NO. Is the S vs. (L + M) or blue-yellow pathway recent? Does it code for spatial information?

Answer 21

NO. The S vs. (L + M) or blue-yellow pathway is ancient. It does not code for spatial information, it only codes for colour.

Question 22

What is the blue-yellow channel sensitive to? What can happen if the blue-yellow channel is damaged?

Answer 22

The blue-yellow channel is sensitive to toxic substances. This includes drugs, alcohol, and nicotine. An individual may develop a loss in the blue-yellow channel if these substances are abused.

Question 23

Explain the red-green channel of colour vision. This is known as L vs. (L + M). Is this channel connected to spatial information?

Answer 23

The L cones become excitatory. It then fires an action potential, which is compared by midget ganglion cells to the surrounding inhibitory L (Red) and M (Green) cones. This channel is connected to spatial information.

Question 24

Different to the blue-yellow colour system, which uses bi-stratified ganglion cells, what other functions do the midget ganglion cells of the red-green colour system have? (2)

Answer 24

Midget cells control spatial vision and processing the peripherals or edges of an object.

Question 25

YES or NO. Is the red-green colour channel/system ancient?

Answer 25

NO. The red-green colour channel/system is relatively recent.

Question 26

TRUE or FALSE. Both the red-green and blue-yellow colour channels interact.

Answer 26

FALSE. The two types of colour channels are independent throughout the cortex. They process colour vision throughout the retina separately.

Question 27

YES or NO. Is everyone’s colour vision the same?

Answer 27

NO. It is common to have individual differences in normal colour vision, such as colour vision deficiency.

Question 28

What percentage of men are affected by red-green colour deficiency?

Answer 28

8% of men are affected by a form of red-green colour deficiency.

Question 29

Name the two types of dichromats, what this means, and give their percentages.

Answer 29

Dichromat means only two types of cone instead of three. It is the complete loss of one type of cone. The two types of dichromat are deuteronopia and protanopia.

Deuteronopia affects 1% of men. It is where the green cones are absent.

Protanopia affects 1% of men. The red cones are absent.

Question 30

Explain anomalous trichromacy, the two types, and their percentages.

Answer 30

Anomalous trichromacy means all three types of cone are present, but the red or green cones are less sensitive than normal.

Deuteranomaly affects 5% of men. This is where the green cones are less sensitive.

Protanomaly affects 1% of men. This is where the red cones are less sensitive.

Question 31

Which condition occurs in women and results in 4 types of cones instead of 3?

Answer 31

Tetrachromacy can occur in women. This is where the gene codes for four types of cones instead of three. These women can see more colours than normal.

Question 32

Around 30 million years ago, all humans were dichromatic, with only S (blue) and L (red) cones present.

Why are we not dichromatic in modern times?

Answer 32

We are no longer dichromatic as a gene duplication on the X chromosome, and a mutation on position q128, led to the differentiation between L cones. This resulted the M cones (green).

Question 33

TRUE or FALSE. The genetic gap between L and M cones is small. There is only a small difference between their spectral sensitivity.

Answer 33

TRUE. L and M cones share 98% of their genetic mapping. They are very similar in terms of their spectral sensitivity.

Question 34

What is dichromacy - specifically deuteronopia - often related to?

Answer 34

Deuteronopia is related to the loss of a gene which codes for green cones. If this gene is not present, the individual will have no green cones, and will therefore be dichromatic with only L (red) and S (blue) cones.

Question 35

Explain what a hybrid gene is relating to anomalous trichromacy.

Answer 35

The gene array of an anomalous trichromat includes a hybrid gene. This is a gene halfway between red and green.

When this gene is expressed in the retina, it results in a photo-pigment receptor halfway between green and red. This reduces colour discrimination.

Question 36

If a son is found to be an anomalous trichromat, what is the mother likely to be?

Answer 36

Tetrachromat. The mothers of those with anomalous trichromacy/colour deficiencies have a higher chance to express four types of cones: S, M, L, and L’ Prime cones. The last is a hybrid cone.

The hybrid cone is linked to midget ganglion cells comparing frequency spectral signals. Around 12% of women are carriers of this hybrid gene and may be tetrachromates.

Question 37

YES or No. Due to the common variations in L and M cones, is it likely we all see the same colours?

Answer 37

NO. It is unlikely we see all the same colours. Variations, along with differences in sensitivity and number of cones, can relate to wild-type pigments, hybrid pigments, and polymorphic pigments as well.

Question 38

Name the two methods of measuring individual differences in colour vision.

Answer 38

Ishihara plate and Rayleigh match.

Question 39

Explain how Ishihara plates work.

Answer 39

Ishihara plates are a three-dimensional construct using random patterns of grey dots, changing with saturation, hues, and shades. All dots are the same brightness/luminance value to prevent colour shifts.

Three patterns are overlaid on top of each other. The first is grey. The second is defined by yellow and blue variation only. This is placed underneath a pattern, which is defined by a red-green variation only.

Those with red-green colour deficiency will only see the number on the yellow-blue variation.

Question 40

Explain how the Rayleigh match works.

Answer 40

Two tones of colour are presented to the subject. The subject must match the two tones, which is typically yellow, by mixing red and green light.

The correct yellow tone is monochromatic, whereas the yellow tone presented to the subject must be matched by mixing red and green.

Question 41

Why is yellow chosen in Rayleigh match tests?

Answer 41

S cones cannot perceive yellow, so the test specifically singles out L and M cones. This is useful in measuring anomalous trichromacy/dichromacy.

Yellow is always seen as a mix of red and green light. We do not have a special cone sensitive to yellow, only cones sensitive to red, blue, and green.

Question 42

What do the results of a Rayleigh match tell us?

Answer 42

The proportion of red and green spectral lights needed to match yellow separates subjects into different groups based on a normal distribution. Subjects on either side or the middle of the distribution will not agree with each other’s colours. There is no overlap between the groups.

Those on the left of the distribution are often dichromats (either too little green or too little red is added).

Those on the right of the distribution often have a form of anomalous trichromacy (either too much red or too much green is added).