Colour Perception

Question 1

List some examples of what we use colour for

Answer 1

• bananas- its relatively difficult to decide out of these bananas which one you would like to eat- once seeing it in colour, its easier to see the ripeness and preference.
• In the animal kingdom- red is used as a signal of sexual readiness
• red soles on shoes- expensive shoes
• show affiliation by wearing football teams colours
• use colour to pick out objects- car in carpark
• interior design- aesthetics
• marking in red/ green pen
• stop and go sigs
• Looking at other people- we can get a sense of health/ emotion status based on how they look

Question 2

Where does colour come from?

Answer 2

• electromagnetic spectrum- theres a little slice in the middle of the spectrum called visible light. This is the slice which our eyes are sensitive to.
• If you take white light- you can split it with a prism and you will get lights of lots of different wavelengths
• It’s not just in outside physics, the brain is doing a lot of work to interpret those signals from the outside world and turn it into a full colour experience.

Question 3

What are photoreceptors?

Answer 3

Photoreceptors are sensitive to light, when light hits them they begin to fire action potentials

Question 4

Cone photoreceptors:
Human Trichromacy- what are the 3 cone types?

Answer 4

Three cone types, maximally sensitive at short (S), middle (M) and long (L) wavelengths

These 3 cone types can give us vision because they have different spectral sensitivities.

Question 5

Cone photoreceptors:
- graph
- photo of retinal cone mosaic

Answer 5

• black curves- pattern of sensitivities relative to the wavelength of light
• Retinal cone mosaic- photo of someones retina where they have falsely coloured the different types of cone.

Question 6

Which cone type do we have fewer of?

Answer 6

We generally have fewer short wavelength cones, they’re more spread out across the retina.

Question 7

Why do we have these 3 cone types?

Answer 7

It is possible to have colour vision of 2 cones. However humans have evolved to have 3. Evolution of 2 cones (short, long) to three (short, middle, long).

Question 8

Evolution of cone types:
How did trichromacy evolve?

Answer 8

Related to foraging for ripe fruit/berries

Dichromatic (two cone types): Short and Long-
30-40 mil years ago
Theres certain colour discriminations you can’t make with only these cone types.

Trichromatic (three cone types): dichromatic L cone split into medium and long.
This means you can start to detect things like ripe berries against green leaves.

Question 9

Evolution of cone types
What does the way our cones are separated help us determine the difference of?

Answer 9

The way our cones are separated is just right to determine the difference between oxygenated blood and deoxygenated blood. they argued this has a social function because from the amount of oxygenated and deoxygenated blood, you can determine the health status of other people.

Question 10

Evolution of cone types

Answer 10

Reflectance spectrum- how much light of different wavelengths is reflected by the 2 types of blood

You can discriminate between oxygenated and deoxygenated blood by having some difference in the oval part of the spectrum.

diagram (e): indicates that along the 2 dimensions of colour, one is yellow/blue which quantifies how much blood. the opposite one is green/red and this tells you how much oxygen is in their blood. This tells you there are measurable spectral differences in humans with different ethnicities and in other species as well.

Question 11

Bare skin: socio-sexual signals from blood oxygenation

Answer 11

If you look at the primates (our nearest relatives in evolutionary tree) the monochromatic primates (one cone type) tend to live a nocturnal life (they don’t really need colour they just make use of whats out there). Dichromatic primates have 2 cone types.

Monochromatic and dichromatic tend to have fur all over their bodies and faces. The trichromatic primates usually have some skin exposed. This means some areas of their skin signal their health status and perhaps emotional status.

Question 12

What is the difference between monochromatic, dichromatic and trichromatic primates?

Answer 12

Monochromatic primates: one cone type
Dichromatic primates: two cone types- short & long
Trichromatic primates: three cone types- short, medium & long

Typically humans are trichromatic

Question 13

Genetic colour vision deficiency:
Monochromats

Answer 13

• Only one cone (or no cones, only rods).
• relatively unusual

Question 14

Genetic colour vision deficiency:
Dichromats
- what are the 3 types of dichromacy

Answer 14

1- Protanopia - lack L cone (i.e. long-wavelength)
2- Deuteranopia – lack M cone (i.e. medium-wavelength)
3- Tritanopia - lack S cone (i.e. short-wavelength)

Question 15

Genetic colour vision deficiency:
Anomolous trichromats

Answer 15

• Deuteranomoly (M cone shifted towards L)
• Protanomoly (L cone shifted to M)

This affects overall 8% men, <1% women genetic deficiency. More common in men due to being linked to sex chromosome.

Also acquired colour vision deficiency (ageing, drugs, hormones).

Question 16

A cure for colour vision deficiency?
(Mancuso et al., 2009 – the Neitz’s lab)
Task and results

Answer 16

Gene therapy turns dichromat into trichromat!

By using gene therapy (injecting viral DNA into eye) you can inject a new type of cone.

Dichromatic male squirrel monkeys

Red opsin gene, virus & DNA injected into some cones. You can inject a new type of cone

Task:
* rewarded for identifying coloured square

Results:
* Can see colours previously not seen!
* They found the squirrel monkeys showed behavioural responses- they can do tasks they couldn’t do before. They have a new dimension of colour.
* Brain able to use new signal even though circuitry not in use early on in life

Question 17

Human Tetrachromacy

Answer 17

• Some women have four cone types!
• Usual 3 cone types & shifted red or green cone type
• Does an extra cone mean they can see more colours?
• Psychophysical tests & genetic analysis
• Only one woman behaviourally tetrachromatic
• Still need cortical processing of extra signal.

Question 18

List 3 cone-opponent channels

Answer 18

• L/(L+M): “red-green”
• S/(L+M): “blue-yellow”
• L+M: “black-white”

Question 19

Cone opponency:
What are they more accurately expressed as?

Answer 19

L/(L+M): “cherry-teal”- compares output of long wavelength cone from the combination of the long and medium

S/(L+M): “violet-lime”- compares output of short wavelength cone from the combination of the long and medium

L+M: achromatic (or luminance axis)- sum of long and medium wavelength cone

Question 20

List how the 3 mechanisms that are present in the LGN connect up to these 3 cone opponent channels

Answer 20

Mechanisms connected to the systems:

Parvocellular = L/(L+M) (cherry-teal)

Koniocellular = S/(L+M) (violet-lime)

Magnocellular = luminance (black-white)

Question 21

Colour-opponent cells in the LGN
- R+/G LGN cell
- G+/R LGN cell

Answer 21

R+/G LGN cell:
- L-cones excited, cel fires
- M-cones excited, cell inhibited

G+/R LGN cell:
- M-cones excited, cell fires
- L-cones excited, cell inhibited

Question 22

Colour after-effects (related to cone opponent processing)

Answer 22

Keep staring at black dot, the areas of your retina (where light is falling on to it), were adapting these cone opponent mechanisms.

• Stare at black cross
• image went bright
• inverted colour version of the original image

Question 23

Lilac chaser illusion:
What is the definition of adaption and opponent coding?

Answer 23

Adaptation: prolonged exposure to a sensory stimulus reduces sensitivity.

Opponent coding: After a short period of staring at the central cross the gap from the missing magenta spot is replaced by its opponent colour – green.

Exposed for a long time to the same stimuli removes the dots.

Question 24

Describe how, in a trichromatic human, signals from the cones are combined into the channels of colour (3 marks)

Answer 24

Cone signals are combined into cone-opponent channels; cone opponency; retino-geniculate pathway

Signals from the L cones are compared to the L and M combined; L/(L+M)

Signals from the S cones are compared to the L and M combined; S/(L+M)

Cone signals are combined into cherry-teal; violet-lime channels/axes (accept even if only one is given, but mentioning both doesn’t get two marks)

Signals from the Land M combined form the luminance axis; L+M; light-dark

Question 25

Features of V1

Answer 25

V1 as well as having orientation selective neurons also has colour selective neurons.

Question 26

Colour at the cortex:
- what are responsive to colour and where
- what are the other areas of visual cortex that process colour
- sent to?
- where is colour info sent along?

Answer 26

Patches of cells (“blobs”) responsive to colour at primary visual cortex (V1)

Other areas of visual cortex process colour e.g., V2, V4/V8

Sent to temporal cortex (ventral processing stream – “what” pathway)

Colour info is sent along the ventral processing stream (bottom of the brain/ what pathway)

Question 27

Cerebral Achromatopsia
- damage to _______ means?

Answer 27

Damage to small cortical region, loss of colour perception

Humans with lesions in extrastriate visual cortex (e.g., V4/V8)

Functioning cones, can record activation at V1 in response to colour

BUT, things don’t appear coloured

Can affect one visual field

Illustrates importance of cortical processing

e.g., see Cowey & Heywood, 1997

Question 28

Memory colour

Hansen, Olkkonen, Walter & Gegenfurtner (2006) Nature Neuroscience

Explanation, task and results

Answer 28

Some objects have a typical colour (e.g. banana) that we learn from experience and therefore expect.

Bananas are typically yellow, so to make a banana look grey it is necessary to add a little extra blue, to counteract the memory colour.

Task:
Gave people a picture of a banana on a screen and asked pp’s to adjust the banana until it looks grey (by pressing key).

Results:
Pp’s overshoot and go off into the blue direction. This is because the brain is expecting the banana to be yellow so when you get to grey, it adds a bit of yellow so you have to add a bit of blue to counteract this.

The error is not present for a simple circle of colour.

Question 29

Aesthetic response to colour
What are the 2 theories?

Answer 29

1) Biological Components Theory (Hurlbert & Ling, 2007)- you can account for colour preference by waiting how much you care about the different cone opponent processes

2) Ecological Valence Theory (Palmer & Schloss, 2010)

Question 30

Ecological Valence Theory
(Palmer & Schloss, 2010)

Answer 30

Colour preference is due to colour-object associations

eg. blue with water, or clear blue skies

WAVE (how good/bad objects associated with that colour are)

WAVE follows a similar colour pattern to actual colour preferences that people give when you ask them to rate colours.

If you take all of the things a colour is associated with and you rate how much you like those things, then you can predict how much they will like the colour.

Question 31

Aesthetic preference for colour patterns

Answer 31

Colour in natural scenes tends to be dominated by blue-yellow variation.

This is also the type of colour pattern which people like the most.

They showed that across the sample of people, people tend to prefer points that are further away from the centre. They rate discomfort lower (how much they like an image)

This is probably because natural scenes vary in this blue yellow direction.

Question 32

Colour constancy
Purves & Lotto (2002)

Answer 32

2 rubrics cubes - one on yellow background, one on blue

The blue tiles on the top of the left cube and the yellow tiles on top of the right cube are physically the same colour.

And that colour is grey.

Colour constancy: the brain’s ability to subtract the illumination, and recover the true surface colour.

Consider the left side (yellow background)- for the light from a surface to be grey under yellow illumination the surface must actually be blue- since yellow and blue are opponent colours.

Brain adapts out overall colour thats imposed on it

The brain is able to keep the colour of objects the same.

Question 33

The Dress:
What are the differences in perception attributable to?

Answer 33

Blue & Black and White & Gold

Show there is a split

Differences in perception are attributable to differences in colour constancy and the assumption about illumination.

The reason we have individual differences is because peoples’ brains are making different assumptions about the illumination situation.

Question 34

The Dress:
Witzel, Racey & O’Regan (2017)

Answer 34

Show you can give people who haven’t seen the dress before a picture of the dress in the shade (so they tend to see it as white and gold) or a picture where the dress looks like it’s in direct sunlight. in this case more people report it as being blue or black.

This has shown if you see it in the original image which is ambiguous, you’re brain has to make a decision and you get individual differences. But you can cue people by giving them additional cues as to what the illumination situation is.