Chapter 9: Introduction to Colour

Question 1

Why is the presence of colour important?

Answer 1

• Aesthetic importance (ex. brightly coloured flowers)
• evolutionary importance (ex. Identifying plump berries by their bright colours)

Question 2

What is a spectral power distribution?

Answer 2

• Every light source has one
• The intensity of light at each wavelength for that specific light source.
• Can vary for many different lights

Question 3

Heterochromatic light vs. monochromatic light?

Answer 3

• Heterochromatic light = the light source contains more than one wavelength
• Monochromatic light = the light source contains only one wavelength (ex. lasers)

Question 4

T/F: White light is a form of monochromatic light.

Answer 4

• FALSE
• White light contains equal intensities and frequencies of all wavelengths of light

Question 5

What’s spectral reflectance?

Answer 5

• The amount of light of each wavelength that a surface reflects

Question 6

What’s selective reflection?

Answer 6

• The degree to which some wavelengths are reflected or absorbed by an object
• Determined by the molecular structure of objects
• The more wavelengths an object reflects, the muddier the colour of the object will appear

Question 7

What’s spectral transmission?

Answer 7

• The amount of light of each wavelength that a surface transmits through

Question 8

What two factors play roles in our perception of colour?

Answer 8

• Our perception of colour is determined by the light source’s spectral power distribution and the objects spectral reflectance.

Question 9

What are the three dimensions of colour?

Answer 9

• Hue (often referred to simply as colour)
• Saturation (the vividness/purity/intensity of a colour)
• Brightness (how white-ish or black-ish the colour is)

Question 10

What are the physical correlates of the three perceptual dimensions of colour?

Answer 10

• Hue = wavelengths of light
• Saturation = Degree of heterochromaticity
• Brightness = Overall amount of light reflected

Question 11

What are non-spectral purple hues?

Answer 11

• Hues that must be created by combining wavelengths (heterochromatic) as opposed to the other monochromatic (spectral) hues

Question 12

What’s subtractive colour mixing?

Answer 12

• Occurs in materials such as paints
• Each component of the mixture absorbs (subtracts) some wavelengths of white light
• Only the wavelengths that are reflected by all components of the mixture will be used to determine the final colour
• Same goes for selectively transmitted wavelengths (only the wavelengths transmitted by all components of the mixture will be transmitted by the final mixture)

Question 13

What’s additive colour mixing?

Answer 13

• Occurs when combining lights on a screen.
• Adding together all the wavelengths of the different lights being reflected
• All of the light reflected by each light alone will also be reflected when mixed.

Question 14

Who developed the trichromatic colour theory and how did they do it?

Answer 14

• Developed by Thomas Young (came up with the idea) and Herman Von Helmholtz (popularized the idea)
• Performed metameric colour-matching experiments (metamers = any two stimuli that are physically different but perceptually the same) - participants were given a test colour (monochromatic light) and then were provided three lights and had to combine these lights to match the test colour (additive colour mixing)
• Any three colours could be used, they just couldn’t be the same as the monochromatic light

Question 15

What were the results from the Young-Helmholtz metameric colour-matching experiments?

Answer 15

• People needed a combination of three wavelengths in the comparison field to match any monochromatic light
• Able to create some colours with 2 wavelengths, but not all of them
• Concluded, we must have three types of colour receptors that are sensitive to different wavelengths of light

Question 16

What are some of the physiological evidence supporting trichromatic colour theory?

Answer 16

• George Wald found a way to measure the absorption spectra of photopigments in the retina. Discovered there are three types of cones each with their photopigments. The difference between all three is the opsin part of the molecular structure.
• Retinal Densitometry - hi-resolution images of the retina where you can identify the three types of cones

Question 17

What are the relative ratios of the three types of cones among humans?

Answer 17

• For most people, about 5% of their cones are s-cones (short-wavelength cones)
• The proportion of medium and long wavelength cones can vary greatly

Question 18

What’s the principle of univariance?

Answer 18

• Once a photon is absorbed, it causes the same response (i.e., an action potential) regardless of its original wavelength.
• In other words, all info about wavelength is lost once the receptor fires
• That’s why we must have three different receptors since they create enough variation for us to differentiate between hues

Question 19

What would happen if we only had one type of cone?

Answer 19

• This would mean that all wavelengths could cause the same firing rate just by adjusting the intensity of the light
• Monochromatic light and heterochromatic light would cause the same pattern of firing
• Would only see shades of grey

Question 20

What’s monochromatism?

Answer 20

• A colour deficiency where you have no functioning cones (i.e., rod monochromats)
• Means the person is very light-sensitive and have very poor acuity
• Colour-blind (see only shades of grey)
• Occurs 1 in 100 000

Question 21

What’s dichromatism?

Answer 21

• A colour deficiency where you have only two types of cones, so you have a limited range of colours
• Not colour-blind necessarily but colour-deficient
• Can only create metamers with 2 comparison lights
• 3 different types

Question 22

What are the three types of dichromatism?

Answer 22

• Protanopia = Missing the L-cone, sex-linked on the X chromosome
• Deuteranopia = Missing the M-cone, sex-linked on the x chromosome
• Tritanopia = Missing the S-cone, extremely rare. Sometimes called blue-green colour deficiency

Question 23

Why do the effects of protanopia and deuteranopia look very similar?

Answer 23

• Because there’s a lot of overlap between the spectra of the L-cones and M-cones

Question 24

What’s anomalous trichromatism?

Answer 24

• Still have all 3 cone types but you’re using slightly different combinations to create metamers
• Means you’re not as good at discriminating colours
• Caused by a shifted M-cone and L-cone spectra so they are even closer together

Question 25

Who was Ewald Hering and why was he important?

Answer 25

• Questioned the accuracy of the trichromatic theory.
• Used phenomenological studies to study perception (focused on subjective experience)

Question 26

What were the types of studies Ewald Hering conducted?

Answer 26

• Sorting tasks = People tend to sort coloured cards into 4 piles as opposed to three
• Description tasks = Can describe all spectral hues using 4 colour terms (blue, green, yellow, red)
• Afterimages caused by simultaneous colour contrast = surrounding an area with colour changes the perception of the surrounded area (observed specifically in afterimages)

Question 27

What’s Hering’s Opponent-Process theory?

Answer 27

• We still have three receptor types, each responding oppositely (inhibitory vs. excitatory) to opposite pairs of colours
• Red-green, blue-yellow, black-white (4 colours in three receptors)
• The colours can either elicit an excitatory response or an inhibitory response

Question 28

What is the physiological evidence supporting the opponent-process theory?

Answer 28

• Opponent neurons = different rates of firing depending on the visual stimulus
• Discovered in the LGN in the 1950s

Question 29

What’s the dual process theory?

Answer 29

• Unites both the trichromatic theory and the opponent-process theory (both are correct)
• have three cones on the retina
• Opponent-process neurons do exist just not at the level of the receptors. Instead, found in ganglion cells, the LGN, and the visual cortex
• Different patterns of firing are coded in the firing rates of the opponent-process neurons

Question 30

How is the colour blue perceived under the opponent-process theory?

Answer 30

• The medium and long wavelengths must first go through an amacrine cell, which will send signals to the yellow component of the opponent neuron.

Question 31

What kinds of opponent cells are found in the visual pathway?

Answer 31

• They can have either center-surround (LGN) or side-by-side (visual cortex) organization

Question 32

Single-opponent receptive fields vs. double-opponent receptive fields?

Answer 32

• Single-opponent - Excitation for one colour in the excitatory region, inhibition for one colour in the inhibitory region. More common in ganglion cells and the LGN. Useful for colours within an area.
• Double-opponent - Excitation and inhibition for different colours in each region of the receptive field. More common in the visual cortex. Useful for identifying the edges and boundaries between colours

Question 33

What’s colour constancy?

Answer 33

• The tendency to perceive a surface as having the same colour, despite changes in illumination.
• Changes in illumination cause changes in spectral power distribution
• Ex. think of the rubrics cubes under the blue and yellow light

Question 34

How do you calculate the wavelengths reflected in the eyes?

Answer 34

• Determined by multiplying the spectral power distribution by the spectral reflectance curve
• i.e., multiplying the values on the y-axis
  The wavelengths reflected into the eyes are also referred to as the relative intensity of the wavelengths

Question 35

How does colour constancy work?

Answer 35

• The mind ‘estimates’ the effect of the SPD
• This process is often referred to as discounting the illuminant
• 3 processes used for estimating the SPD: chromatic adaptation, effects of surroundings, and memory colour

Question 36

What’s chromatic adaptation?

Answer 36

• Receptors get tired after repeatedly firing to prolonged exposure to a stimulus
• Occurs with all senses, except for pain (the opposite occurs)
• Sensory neurons begin to fire less and less in response to the same stimulus
• Can’t make ourselves continue experiencing the feeling
• For vision, looking out coloured ski goggles is an example. When we take them off, strong after-image effects occur (other receptors that didn’t respond as strongly to the stimulus are ready to fire at full strength once the old stimulus is removed)

Question 37

How do the effects of the surroundings impact colour constancy?

Answer 37

• Colour constancy works best when there are multiple objects are also present in the scene
• Even better if it’s different objects of different colours
• This means colour constancy is poor for objects in isolation
• Not very good consensus on how surroundings are used (many theories)

Question 38

What did Uchikawa et al (1989) study come up with in regard to the effects of surroundings on colour constancy?

Answer 38

• They concluded that colour constancy can only occur when the observer can detect the surroundings’ lighting conditions. When they can, we can typically observe colour constancy (i.e., perceive close to the true colour, SPD still influences perception)

Question 39

What’s memory colour and how does it help with colour constancy?

Answer 39

• Prior knowledge of the typical colours of objects can also influence the perception of colours.
• Can influence the perception of familiar objects
• Also helps the visual system discount the illuminant

Question 40

What did the Hansen et al (2006) study discover regarding memory colour?

Answer 40

• They showed pictures of fruits or spots of light against a grey background
• Participants were responsible for adjusting the intensity of the fruit or spots of light until it matched the grey background
• Discovered that people who had to adjust the fruit found that there always remained a faint colour compared to the spots of light group

Question 41

What did the Bornstein et al (1976) study discover regarding infant colour vision?

Answer 41

• They used a habituation task to assess 4-month-old colour vision
• The same stimulus is repeatedly shown to a baby. As it becomes more familiar, they look at it less. They used this trick to assess colour vision and colour categorization
• habituated children to one wavelength, tested against one of two novel wavelengths
• One of the two wavelengths was a completely different colour, while the other was just a different shade but the wavelengths were still equidistant from one another
• Discovered that dishabituation occurred for 480 nm, but not for 540 nm
• Suggests that colour categorization is the same for babies and adults
• babies also understand that they are crossing a colour boundary, even though the wavelengths are equidistant

Question 42

What are the different forms of colour blindness commonly referred to? Are any of these slightly misleading?

Answer 42

• Protanopia and deuteranopia = red-green colour blindness (can’t see these colours)
• Tritanopia = commonly referred to as blue-yellow colour blindness, but should instead be referred to as blue-green colour deficiency (makes sense since the s-cones cover the blue and green segments)