Chapter 9: Perceiving Colour Flashcards
cerebral achromatopsia
a loss of colour vision caused by damage to the cortex
mr i
a successful artist who suffered from cerebral achromatopsia following an automobile accident
colour deficiency
a condition in which people see fewer colours than people with normal colour vision and need to mix fewer wavelengths to match any other wavelength in the spectrum due to the genetic absence of one or more types of cone receptors.
are most people with colour deficiency bothered by their condition?
Most people who are born with this condition aren’t distributed by it because they have never experienced normal colour vision
functions of colour perception
- Signalling functions
- Facilitate perceptual organization
- Recognizing and identifying things we can see easily
- Cue to emotions signalled by facial expressions
why do some scientists think colour evolved?
to allow humans and monkeys to detect ripe fruits
object colour experiment (Tanaka & Presnell, 1999)
participants were able to recognize appropriately coloured objects more rapidly than inappropriately colour objects
ambiguous emotions experiment (Thorstenson et al., 2019)
found that when asked to rate the motions of ambiguous-emotion faces, participants were more likely to rate the face as expressing disgust when coloured green and as expressing anger when red
Newton, 1704 experiment
Light entered through a hole in a window shade and then passed through a prism. The colours of the spectrum were then separated by passing them through holes in a board. Each colour of the spectrum then passed through a second prism. Different colours were bent by different amounts, demonstrating that white light is a mixture of differently coloured lights, which was split into its individual components by the prism
key findings from Newton’s coloured beam experiment
1) the second prism didn’t change the colour appearance of any light that passed through it 2) the degree to which beams from each part of the spectrum were bent by the second prism was different
wavelength of violet rays
400-450 nm
wavelength of blue rays
450-490 nm
wavelength of green rays
500-575 nm
wavelength of yellow rays
575-590 nm
wavelength of orange rays
590-620 nm
wavelength of red rays
620-700 nm
how is the colour of objects determined for opaque objects?
the wavelengths of light that are reflected from the object into our eyes
how is the colour of objects determined for transparent objects?
the wavelengths of light that are transmitted from the object into our eyes
chromatic colours
colours with a hue, such as blue, yellow, red, or green
selective reflection
when an object reflects some wavelengths of the spectrum more than others
achromatic colours
colours without a hue, like white, black, and gray
reflectance curve
a plot showing the percentage of light reflected from an object vs. wavelength
selective transmission
when some wavelengths pass through visually transparent objects or substances and others do not
what is selective transmission associated with?
the perception of chromatic colour
transmission curve
plots the percentage of light transmitted through a liquid or object at each wavelength
what colour is perceived from short wavelengths?
blue
what colour is perceived from medium wavelengths?
green
what colour is perceived from long and medium wavelengths?
yellow
what colour is perceived from long wavelengths?
red
what colour is perceived from long, medium, and short wavelengths?
white
what happens to wavelengths when paints are mixed?
When mixed, both paints still absorb the same wavelengths they absorbed when alone. So, the only wavelengths reflected are those that are reflected by both paints in common
Subtractive colour mixture
the creation of colours that occurs when pairs of different colours are mixed together
what happens when blue and yellow paint are mixed together?
a blob of blue paint absorbs all of its long-wavelength light, while a blob of yellow paint absorbs all of the short-wavelengths. When mixed together, the only wavelengths that survive are some of the medium wavelengths, which are associated with green
what happens to wavelengths when lights are mixed?
When coloured lights are superimposed, all the light that is reflected from the surface by each light when alone is also reflected when the lights are superimposed
what happens when blue and yellow light are mixed together?
The light from a blue spot and from a yellow spot are both reflected in the observer’s eye. The added-together light contains short, medium, and long wavelengths, resulting in the perception of white
additive colour mixture
the creation of colours that occurs when lights of different colours are superimposed
spectral colours
colours that appear in the visible spectrum
nonspectral colours
colours that do not appear in the spectrum because they are mixtures of other colours.
how many colours can humans differentiate?
2.3 million
3 dimensions of colour
hue, saturation, value
hue
the experience of a chromatic colour
saturation
the intensity of colour
value (lightness)
the light-to-dark dimension of colour
colour solid
a solid in which colours are organized in an orderly way based on their hue, saturation, and value
Munsell colour system
Depiction of hue, saturation, and value developed by Albert Munsell in the early 1900s in which different hues are arranged around the circumference of a cylinder with perceptually similar hues placed next to each other.
how did Newton believe vision occured?
rays of light falling upon the bottom of the eye excite vibrations in the retina, which are propagated along the fibres of the optic nerve, causing seeing
Thomas Young’s criticism of Newton
Newton’s idea of a link between each size of vibration and each colour won’t work because a particular place on the retina can’t be capable of the large range of vibrations required
trichromacy of colour vision
colour depends on the activation of three different receptor mechanisms
Young-Helmholtz theory
another term for the trichromacy theory of colour vision
colour-matching procedure
the procedure used in a colour-matching experiment where the experimenter presents a reference colour that is created by shining a single wavelength of light on a reference field. The observer then matches the reference colour by mixing different wavelengths of light in a comparison field.
Maxwell’s colour-matching findings
any reference colour could be matched provided that observers were able to adjust the proportions of 3 wavelengths in the comparison field
Microspectrophotometry
A technique in which a narrow beam of light is directed into a single visual receptor. This technique makes it possible to determine the pigment absorption spectra of single receptors
Adaptive optimal imaging
a technique that makes it possible to look into a person’s eye and take pictures of the receptor array in the retina
Aberrations
imperfections on the eye’s cornea and lens that distort light on its way to the retina
Cone mosaic
arrangement of short, medium, and long-wavelength cones in a particular area of the retina
Metamerism
a situation in which two physically different stimuli are perceptually identical because they both result in the same pattern of response in the three-cone receptors
Metamers
the two identical fields in a colour-matching experiment
Monochromatism
a rare form of colour blindness where people have no functioning cones. it is usually hereditary and occurs in about 10/ 1 million people
Monochromats
people with no functioning cones; their vision is only created by rods, so they see shades of grey
how many wavelengths do monochromats need to match any wavelength in the spectrum?
1
Principle of univariance
absorption of a photon causes the same effect, no matter what the wavelength is (Any two wavelengths can cause the same response by changing the intensity)
how many types of receptors does one need to perceive chromatic colour?
more than 1
how much 480 nm light does the pigment absorb?
10%
how much 600 nm light does the pigment absorb?
5%
ratio of pigment 1 to pigment 2 for 480 nm light
10:2
ratio of pigment 1 to pigment 2 for 600 nm light
5:10
Dichromats
people who see just two types of cone pigment. they see chromatic colours, but can’t distinguish among all colours
dichromats vs. trichromats
They can see chromatic colours, but confuse some colours that trichromats can distinguish
trichromats
people who see all three types of cone pigment. they can discriminate among more wavelengths across the spectrum
methods for determining colour deficiency if there are two receptor types
- colour-matching procedure
- Ishiara plates
Ishihara plates
a display of coloured dots used to test for the presence of colour deficiency. The dots are coloured so that people with trichromatic colour vision can perceive numbers in the plate, but people with a colour deficiency cannot perceive these numbers, or perceive them differently than trichromats
Unilateral dichromat
a person with trichromatic vision in one eye and dichromatic vision in the other
why do we study unilateral dichromats?
Studying them allows us to understand the nature of dichromat’s colour vision
neutral point
the wavelength at which dichromats perceive grey
protanopia
a condition in which patients are missing the long-wavelength (red) pigment.
colour perception in protanopes
As a result, the protanope perceives short-wavelength light as blue, perceives a grey neutral point at 492 m, and perceives yellow on the long-wavelength end of the spectrum.
prevalence of protanopia
This condition is inherited through a gene located on the X chromosome and affects 1% of males and 0.02% of females.
deuteranopia
a condition in which patients are missing the medium-wavelength (green) pigment.
colour perception in deuteranopes
As a result, deuteranopes perceive blue at the short-wavelength end of the spectrum and see yellow at the long-wavelength end, with a neutral point at around 498 nm.
prevalence of deuteranopia
This condition is inherited through a gene located on the X chromosome and affects 1% of males and 0.01% of females.
tritanopia
a condition in which patients are missing the short-wavelength (blue) pigment.
colour perception in tritanopes
Tritanopes see blue at the short wavelength end of the spectrum, red at the long wavelength, and a neutral point at 570 nm.
prevalence of tritanopia
This condition affects 0.002% of males and 0.001% of females.
what sex is more likely to be dichromats
Males are more likely to be dichromats because they only have 1 X chromosome
Anomalous trichromatism
trichromats that mix wavelengths in different proportions from other trichromats. They are not as good at discriminating between wavelengths that are close together
opponent-process theory
States that there are two pairs of chromatic colours: red-green & blue-yellow
who proposed the opponent-process theory
Herring, 1878
what colours were considered primary colours in the opponent-process theory?
Identified red, green, blue, and yellow as the primary colours and stated that other colours were made up of a combination of these colours
colour circle
arranges perceptually similar colours next to each other around its perimeter such that complementary colours are across from one another
what perceptual property does the colour circle account for?
Only accounts for hue, without considering variations in saturation or value
complementary colours
colours that when combined, cancel each other out to create white or grey
hue scaling
a procedure in which participants are given colours from around the hue circle and told to indicate the proportions of red, yellow, blue and green that they perceive in each colour. Herring found that each of his primary colours was perceived as pure
unique hues
name given by Hering to what he proposed were the primary colours
why wasn’t Herring’s theory widely accepted
- The trichromatic theory was championed by Helmholtz who was more prestigious
- Hering’s phenomenological evidence, which was based on describing the appearance of colours couldn’t compete with Maxwell’s quantitative colour-mixing data
- There was no neural mechanism known that could respond in opposite ways
hue cancellation
a procedure in which a subject is shown a monochromatic reference light and is asked to remove or cancel one of the colours in the reference light by adding a second wavelength. This procedure was used by Hurvich & Jameson in their research on the opponent-process theory
opponent neurons
a neuron that has an excitatory response to wavelengths in one part of the spectrum and an inhibitory response to wavelengths in the other part of the spectrum
significance of opponent neurons in colour perception
provided physiological evidence for the opponency of colour vision
can the perception of opponent neurons be linked to the perception of colours?
no
DeValois, 1960
recorded opponent neurons in the LGN of monkeys
Circular single-opponent cortical neurons
firing rate increases when a medium-wavelength light is presented to the centre and decreases when a long-wavelength is presented to the surround
Circular double-opponent cortical neurons
firing increases when medium-wavelength light is presented to the centre and long-wavelength light is presented to the surround. Firing decreases when the long-wavelength light is presented in the centre and when the medium-wavelength light is presented in the surround
Side-by-side double-opponent cortical neurons
firing increases when a vertical medium wavelength is presented to the left side and when a vertical long wavelength is presented to the right side. Firing decreases when a vertical long-wavelength bar is presented to the left side and when a vertical medium-wavelength is presented to the right side.
what does research suggest is an alternative function of opponent neurons?
Opponent neurons might instead indicate the difference in responding to pairs of cones
Maxwell’s colour-matching conclusion
colour vision depends on three receptor mechanisms, each with different spectral sensitivities
evidence for the trichromatic theory
- Researchers measured absorption spectra of visual pigments in receptors (1960s) and found pigments that maximally responded to red, yellow, and blue
- Later, researchers found genetic differences for coding proteins for the three pigments (1980s)
what wavelength do short pigments respond maximally to?
419 nm
what wavelength do medium pigments respond maximally to?
531 nm
what wavelength do long pigments respond maximally to?
558 nm
colour perception is based on ___
the response of the three different types of cones
evidence for the opponent-process theory
- colour afterimages
- hue scalling
where are opponent neurons found?
in the retina and the LGN
which theory of colour vision is correct
both the trichromatic theory & opponent-process theory are correct. The trichromatic theory explains the responses of the cones in the retina.
The opponent-process theory explains neural response for cells connected to the cones further in the brain.
