How we perceive colour

23 important questions on How we perceive colour

How do we perceive colour?

1. Detection - wavelengths must be detected; light is differently absorbed by 3 photopigments in the cones
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?

We see white when we are stimulated by equal intensities of all the wavelengths of the visible spectrum and we see colour when we are stimulated by only a portion of this spectrum.

What is one way to interpret Isaac Newton's results?

That the perception of colour occurs when some wavelengths are subtracted from white light
---> 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?

The colour of an object depends on composition of light contained in the
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?

3 = S-cones, M-cones, and L-cones

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?

It gives us our overall ability to detect wavelengths from 400nm to 700nm

(p. 118 perceptual textbook)

What is the trichromacy theory?

Young and Helmholtz ---> proposed that our perception of colour was based on the relative activation of three receptors for long, medium and
short wavelengths (red, green and blue).

What experiment supports the trichromacy theory?

Colour matching experiments in which any colour can be produced by a combination of the three primary colours (red, green
and blue). This principle underlies all modern visual displays (e.g. tvs and cinemas).

What is the physiology of the trichromatic theory?

The human retina responds to different wavelengths of light:
– 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?

Failure to make one of the cone pigments
i.e. only 2 cones instead of 3

What are people with only two cones known as?

Dichromats
– protanopia (loss of long wavelength red)
– deuteranopia (loss of middle wavelength green)
– tritanopia (loss of short wavelength blue)

What is the opponent-process theory?

Hering
---> 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?

A cell type found in the retina, lateral geniculate nucleus, and visual cortex. The cell subtracts one type of cone input from another

(p. 125 perceptual textbook)

What is an example of the opponent process theory?

The negative retina after image (see slide 2, p. 6)

---> 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?

Another method that demonstrates the opponent colour theory (Hurvich & Jameson, 1957)

---> 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?

1. Chromatic adaptation
2. Memory

What is chromatic adaptation?

The human visual system's ability to adjust to changes in illumination in order to preserve the appearance of object colours.
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?

We use our knowledge (memory) of what colours are supposed to look like to determine the actual colour of objects.

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?

V1
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?

The fact that an infinite set of different wavelength intensity combinations can elicit the exact same response from a single type of photoreceptor. One photoreceptor type cannot make colour discriminations based on wavelengths.

---> 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?

---> problem of variance
---> there is only one type of rod receptor

(p. 119 perceptual textbook)

How do rods make a contribution in seeing colour?

Rods make a small, important contribution to colour vision, but only in fairly dim light
(p. 125 perceptual textbook)

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