How we perceive colour
23 important questions on How we perceive colour
How do we perceive colour?
2. Discrimination - we must be able to tell the difference between one wavelength from another; cone-opponent theory
3. Appearance - we want to assign those perceived colours to lights/surfaces/objects and not to change dramatically as the viewing conditions change
(p. 118 perceptual textbook)
Following on from Isaac Newton's discovery, when do we see white and when do we see colour?
What is one way to interpret Isaac Newton's results?
---> This occurs naturally when light from the sun is scattered by particles of air in the atmosphere to create the perception of blue sky and yellow sun.
---> Short wavelength light (blue) is scattered more than long wavelength
light (red). This effect becomes most pronounced when the light has to travel further causing middle wavelength light (green) to be scattered (sunset).
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Why are objects the colour they are?
illuminant (i.e. the sun, artificial light) and the type of light an object reflects.
i.e. whatever is absorbed and reflected determines the colour of the object
Example
The red apple contains colourant particles (pigments) that absorb ‘green’ and ‘blue’ wavelengths of light and allow ‘red’ to be reflected, whereas the yellow banana contains pigments that absorb ‘blue’ light reflecting ‘red’ and ‘green’ wavelengths.
---> Light that is reflected at the interface of a surface has the same colour as the incident light.
How do pigments and light not combine in the same way?
- Pigments in blue paint absorb ‘red/yellow’ light and reflect ‘blue’ (and some ‘green’) light. Pigments in yellow paint absorb ‘blue’ light and reflect ‘yellow’ (and some ‘green’) light. When painters mix blue and yellow, the pigments absorb ‘blue’ and ‘yellow’ light. The only light that continues to be reflected is ‘green’.
- In contrast, when different coloured lights combine, the effect is additive making more of the spectrum visible. ‘Blue’ and ‘yellow’ light combine to produce ‘white’ light.
How many cone receptors do we have? What does this mean?
i.e. L-cones are more sensitive to long wave lengths than other cones are
---> these cones differ in the photopigment they have and as a result, they differ in their sensitivity to light of different wave lengths
(p. 118 perceptual textbook)
What does the combination of sensitivities of the 3 cones allow us to do?
(p. 118 perceptual textbook)
What is the trichromacy theory?
short wavelengths (red, green and blue).
What experiment supports the trichromacy theory?
and blue). This principle underlies all modern visual displays (e.g. tvs and cinemas).
What is the physiology of the trichromatic theory?
– red (long wavelength)
– green (middle wavelength)
– blue (short wavelength)
Rushton’s principle of Univariance ---> If there wasn’t trichromacy and if there was a single mechanism to signal all colours, this single mechanism would have a spectral sensitivity curve. It would be maximally responsive to a particular wavelength and less sensitive to others. i.e. if there was only one cone, there would be the same firing rate for every wavelength
= we need 3 cones
(slide 2, p. 3 lecture notes)
What can colour blindness result from?
i.e. only 2 cones instead of 3
What are people with only two cones known as?
– protanopia (loss of long wavelength red)
– deuteranopia (loss of middle wavelength green)
– tritanopia (loss of short wavelength blue)
What is the opponent-process theory?
---> the theory that perception of colour is based on the output of 3 mechanisms, each of them resulting from an opponency between two colours: red-green, blue-yellow, and black-white
---> i.e. different colours were processed in opposition to each other
He suggested three processing channels:
– red versus green [L vs M]
– blue versus yellow [S vs (L&M)]
– dark versus light (black vs light)
(slide, 1 p. 4 lecture slides)
(p. 128 perceptual textbook)
What is a cone-opponent cell?
(p. 125 perceptual textbook)
What is an example of the opponent process theory?
---> afterimages have an opposite polarity to the original stimulus. Light stimuli produce dark negative afterimages. Colours are complementary e.g. red produces green; yellow produces blue.
(p. 133 perceptual textbook)
What is hue cancellation?
---> the colours in the colour circle are represented by 2 pairs of opposing colours (blue vs yellow & red vs green). Therefore, a colour could be a reddish yellow or a bluish green, but not a reddish green or a bluish yellow
= supports Hering's opponent colour theory
(p. 128 perceptual textbook)
What are 2 explanations of colour constancy?
2. Memory
What is chromatic adaptation?
For example in a scene illuminated by ‘red’ light, cones that are maximally responsive to ‘red’ light gradually become less sensitive causing objects in the scene to be perceived as being less red.
How does memory explain colour constancy?
When looking at an object one is usually confident of its colour: bananas are yellow, apples are green etc.
---> Previous knowledge of objects could help to resolve potentially confusing images in the eye.
What brain areas are responsible for colour?
Blobs in primary visual cortex contain colour selective neurons. However, the responses of these neurons only take into account changes in wavelength over a small region of the retina. As we have seen, this often bears little relationship to the actual colour of an object.
V4
Many neurons in V4 have large receptive fields capable of summing information from a large region of the image. Most importantly, some V4 neurons display unchanging responses to surfaces irrespective of changes in the wavelength composition of light. These response appear to reflect the colour constancy that is apparent in human observers.
What is the problem of univariance?
---> trichromacy theory provides a solution - with 3 cone types, we can tell the difference between lights of different wavelengths
(p. 119 & 120 perceptual textbook)
Why is their a lack of colour in dimly lit scenes?
---> there is only one type of rod receptor
(p. 119 perceptual textbook)
How do rods make a contribution in seeing colour?
(p. 125 perceptual textbook)
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