Chapter 9: Percieving Colour

Question 1

What is Cerebral Achromatopsia?

Answer 1

Colour blindness caused by brain injury.

Question 2

What are some different functions of colour perception?

Answer 2

Colour as a signal: we use colour to signal lots of things like when a fruit is ripe, when to stop a car at the light, whether a frog is poisonous or not

Colour enhances contrast

Colour is associated with emotions

Question 3

What determines an objects colour? What about a transparent object?

Answer 3

The wavelengths of light that are reflected from the object into our eyes.

Transparent objects’ colour is determined by which wavelengths are able to pass through. This is called selective transmission.

Question 4

What are Chromatic and Achromatic colours?

Answer 4

Chromatic colours are when some wavelengths are reflected more than others. This is called selective reflection. It includes colours like Blue, red, and green

Achromatic colours occur when light is reflected equally across the spectrum. It includes black, white, and grey. We differentiate between the shades of white, black and grey because they reflect all colours at different percentages (e.g. white reflects at ~80% while black reflects at ~5%)

Question 5

What is the difference between subtractive and additive colour mixtures.

Answer 5

Subtractive colour mixture: when you combine two different colours and they make a new colour based on the wavelength reflected that they have in common. An example of this would be mixing paint.

Additive Colour mixture: when you combine two different colours of light and all the wavelengths are reflected so that the colour you perceive is based on the combination of those wavelengths.

Question 6

What is the difference between spectral and non-spectral colours?

Answer 6

Spectral colours include ROY G BIV (red orange yellow Green Blue Indigo Violet) but Indigo has been disputed as a spectral colour because we actually have a hard time seeing between blue and violet.

Non-spectral colours are colours outside the spectrum made from a combination of ROY G BIV

Question 7

What are the three perceptual dimensions of colour?

Answer 7

Hue: the colour itself (Blue versus yellow)
saturation: the intensity of the colour, changed by the amount of white added (e.g. red to pink)
Value: aka lightness, the light to dark dimension of the colour. (Bright blue versus navy blue)

Question 8

What is the Trichromacy theory of colour vision?

Answer 8

Proposed by Thomas Young (1773-1829) with the following principle

• Colour Vision is based on 3 principle colours and on the activity of 3 different receptor mechanisms

Colour matching experiments support this theory.

Question 9

What are the different types of cones and their spectral sensitivity peak?

Answer 9

Short wave pigment: max sensitivity to 419nm, ~blue
Middle wave pigment, max sensitivity to 431nm, ~green
Long wave pigment, max sensitivity to 558nm, ~yellow

Question 10

What are metamers?

Answer 10

Metamers are when two physically different stimuli illicit the same pattern of response from receptors and therefor are perceptually identical.

Question 11

How does someone with monochromacy see colour?

Answer 11

This is colour vision with only 1 pigment, they have no functioning cones so they only see shades of grey.

Question 12

What is the principle of univarience?

Answer 12

The principle of univarience is the idea that, once a photon of light has been absorbed by a visual pigment molecule, the identity of the light’s wavelength is lost.

Cones respond to light intensity, not to wavelength.

This means that colour vision depends on the relative response of multiple cone types to perceive colour. We perceive colour through a pattern of firing among photoreceptors

Question 13

What is Dichromacy?

Answer 13

It is colour vision with only 2 pigments. There are multiple types, one for each missing pigment (e.g. protanopia is missing the L cone)

Question 14

What is the opponent process theory of colour vision? and what is key evidence for this theory?

Answer 14

There are 2 pairs of chromatic colours that act in opposition to each other.
Red – Green
Blue – Yellow

Phenomenological evidence: all other colours are made up of these 4 primary colours but the primary colours are pure (e.g. there is no yellow, green, or red in the colour blue)

Psychophyiscal evidence: you can add a certain amount of the opponent colour to cancel out the other colour. E.g. how much blue cancels out the colour yellow

Physiological evidence: we have opponent neurons

After images appear as the opposite colour

Question 15

What are Opponent neurons?

Answer 15

These are neurons that respond with an excitatory response to light from one part of the spectrum with an inhibitory response to light from another part.

These opponent cells have different receptive field layouts:

• Circular single opponent
• Circular double opponent
• Side by side single opponent

Single opponent cells respond to areas of colour while double opponent cells respond to patterns and borders

Question 16

What is colour constancy and how does it work?

Answer 16

Colour constancy: we perceive the colours of objects as being relatively constant even under changing illumination.

Colour constancy depends on chromatic adaption. This is when a light of a certain colour is exposed to us for a length of time, we become adapted to it and our sensitivity to that colour is reduced. For example, in yellow light we can still perceive other colours because exposure to the yellow light has led to an adaption to it.

Other factors that contribute to colour constancy include our brain’s ability to take illumination into account when processing colour and our previous knowledge of what an objects colour should be.

Question 17

What is lightness constancy and how does it work?

Answer 17

This is when we see blacks, whites, and greys as the same even under variable light conditions.

This occurs because we perceive achromatic colours by the proportion of light it reflects, not by the total amount. This works in even illumination conditions.

Question 18

How do we percieve lightness (achromatic colours) in uneven illumination?

Answer 18

• Reflectance edge: looking for where the reflectance changes on a surface (a change in the % of light being reflected)
• Illumination edge where the illumination changes on a surface

Using a number of sources of information, our brain is able to identify which type of edge it is, therefore determining if there has been a colour change or an illumination change.

Question 19

Why is colour vision more common in genetic Males than genetic females?

Answer 19

Colour vision is linked to the X Chromosome. Males only have 1 X chromosome so if there is a problem with the genes related to colour vision, they will have problems. Females have 2 X chromosomes which means that even if 1 chromosome has problems, the other can compensate.