Week 6 Colour Perception

Question 1

What types of cells have narrow and broad broadband detectors?

Answer 1

Tuning properties in cells have either narrowband or broadband detectors

Narrowband detectors = sensitive to a narrow range of the stimulus dimension - ORIENTATION

Broadband detectors = sensitive to a broad range of the stimulus dimension - COLOUR

Question 2

Why do we need broadband sensitive cells for colour?

Answer 2

1. We would need more cone types over R, G and B - If colour had narrowband sensitive cells, you would need a lot more cone types to encode the full spectrum of colours - which wouldn’t align with the size of the eyes and would look at neighbouring regions over the same receptive field.
2. Spatial acuity is reduced due to the separation of many cones in the retina - eg. red cones would be further away to green cones if we had many more cones. Further separation of cones would impair the eye’s ability to focus on looking at really close regions next to one another, so spatial acuity would be reduced.
3. With narrowband you would get confused about metamers due to having more specific cells tuned to specific colours.

Question 3

Why do we need narrowband sensitive cells for orientation and how does this occur?

Answer 3

To discriminate fine orientations to determine seconds moving around a clock or telling if something is hung straight.

A given cone can feed into many simple cells tuned to different orientations

Question 4

Why is the percept of colour a psychological construct unlike orientation?

Answer 4

1. The percept of colour is mapped onto different wavelengths but is entirely arbitrary, ie. there is nothing intrinsically blue about a wavelength of 450 nm
2. While orientation often refers to the physical perception of an object or self
3. The psychological quality of colour is something the brain has invented in order for us to be able to discriminate different wavelengths apart, ie. telling ripe from off fruit apart, giving us a behavioural advantage in evolution

Question 5

What would happen to narrow vs. broadband tuning if stimuli were mixed?

Answer 5

Broadband mixing - perceptive one colour through colour mixing (unimodal population response, perceiving single colour average)

Narrow band mixing - able to discriminate 2 distinct enough different orientations together (bimodal population response, piercing transparency)

Question 6

What is white light?

Answer 6

The combination of all of the colours

Question 7

What is additive colour mixing?

Answer 7

Additive mixing = the addition of wavelengths so the resultant colour is the sum of the component spectrum, mix everything = white!

Question 8

What are the two types of summation in additive colour mixing?

Answer 8

Spatial summation = distinct colours that are REALLY close together like little bundles of little red, green and blue pixels that are too small to resolve for the brain’s spatial acuity, The brain averages the colours out as a blend of colours - how an old colour TV works

Temporal summation = a colour wheel spinning at more than 15 cycles per second so the two colours are not perceived, but the colours are averaged together by the brain

Question 9

What is subtractive colour mixing?

Answer 9

Subtractive colour mixing is creating a new colour by the removal of wavelengths from a light with a broad spectrum of wavelengths

Example: combining filters or mixing paint, everything = black!

Question 10

How does subtractive colour mixing work in example?

Answer 10

1. Adding colour filters together, so less wavelengths can get through all of the filters
2. Mixing paints/hues together means each paint absorbs some of the colour that the other paint reflected, wavelengths are subtracted and left with the only wavelengths that both colours reflect

Question 11

What is the minimum amount of colours needed to produce the percept of white and other colours?

Answer 11

White can be made from yellow and blue, all wavelengths or red, green and blue

Minimum number of primary colours to make any colour = 3
Minimum number of primary colours to make white = 2

Question 12

What can trichromatic vs. monochromatic people in colour?

Answer 12

People with standard colour perception (trichromatic) can generate any colour with three primary colours, chosen correctly
because of having 3 cone types

Monochromatic people have 1 cone type of receptor, so they can only discriminate differences in luminance intensity

Question 13

How could we change the intensity of the light to make a wavelength response of 500 nm go to 550 nm?

Answer 13

Decrease intensity of light, makes the wavelength longer = makes 500 nm to 550 nm (greener)
Increasing intensity of light, shortens the wavelength = makes 550 nm to 500 nm (bluer)

Question 14

How do dichromatics have a neutral point in wavelength colours? Why is the neutral point perceived white?

Answer 14

The neutral point is crossover of A and B, where the sensitivity curves of the two receptors intersect

The colour at the neutral point is white because the rate of activation is 1:1 between the 2 photoreceptors!

Question 15

How does the magenta-seeing-green dots illusion work?

Answer 15

This illusion is the transient impulse flicker across magenta dots, when
Troxler fading occurs with fuzzy dots

Using Hering’s opponent model people with trichromatic colour vision encode relative activation through 3 opponent mechanisms and adapt to moving colour objects, you see its complete complementary colour over time. If you adapt to magenta dots you will see green dots as a result.

Question 16

What are 2 characteristics of trichromats?

Answer 16

1. Three different primary sensitivity peaks with no neutral point
2. The firing rate would increase but the ratio of red to green to blue cones would be constant

Question 17

Why is the presence of tetrachromats debated in humans?

Answer 17

1. Tetrachromats divide the colour spectrum into more distinct hues
   - above 580 nm and two long wavelength sensitivity cones,
2. There is a question of if you even could have 4 cone types, can you perceive more distinct hues?

Question 18

What are 2 exceptions to the trichromatic theory of colour?

Answer 18

1. Colour aftereffects = staring at cross with red and green dots shows opposite aftereffects colours (indicates colour opponency)
2. Colour naming = asking people to assign a name from monochromatic lights you can’t describe all colours or combinations using red, green and blue, you need other colours and shades

Question 19

What is the Opponency model of colour?

Answer 19

The opponency model of colour states that colour is mediated through 3 opponent mechanisms:

1. Red - green
2. Blue - yellow
3. White (light) - black (dark)

Question 20

What is 3 pieces of evidence that support the opponency model?

Answer 20

1. Present a red monochromatic light
   Add monochromatic green until the mixture cancels out the perception of red and green = R-G mechanism
2. We have colour-opponent cells in the LGN that are called ‘double duty cells’, they are sensitive to both luminance and colour
3. Colourafterfeccts, seeing green after magenta

Question 21

What is the core principle of trichromatic theory?

Answer 21

Our perception of colour is based upon the relative activation of the 3 cone types - R,G,B / L, M and S cones

Question 22

How does both the trichromatic and opponency model both explain colour perception?

Answer 22

1. Our perception of colour is based upon having 3 cone types and relative activation, explained by the trichromatic theory
2. How the brain encodes colour is done through opponent mechanisms (G-R, B-Y, W-B), opponency model

We encode colour through our double duty cells, which are sensitive to both chromatic & luminance cells

Reflected by subcortical pathways such as the M-cell pathway (only sensitive to luminance, colourblind) and P-cell pathway (sensitive to luminance and colour)