Problem 3 - DONE Flashcards
colour perception
three steps of colour perception
- detection
- discrimination
- appearance
step 1 - colour detection
- wavelengths must be detected
- cone-photoreceptors
step 2 - colour discrimination
- we must be able to tell the difference between one wavelength (or mixture of wavelengths) and another
- principle univariance
- trichromatic theory of colour vision
- metamerism
step 3 - colour appearance
- opponent colours
step 1
cone-photoreceptors
- S-cones: short-wavelength pigment, with maximum absorption at 419-nm
- -> ‘blue cone’
- M-cones: middle-wavelength pigment, with maximum absorption at 531-nm
- -> ‘green cone’
- L-cones: long-wavelength pigment, with maximum absorption at 558-nm
- -> ‘red cone’
step 2
principle of univariance
=
- one photoreceptor type cannot make colour discriminations based on wavelength
- -> receptor does only know the total amount it has absorbed
- once a photon of light is absorbed –> identity of the lights wavelength is lost
- anything a photon can do: hyper polarise more or less
step 2
trichromatic theory of colour vision
= trichromacy = Young-Helmholtz theory
- theory that the colour of any light is defined in our visual system by the relationships of three numbers
- -> the outputs of three receptor types (three cones)
- colour vision depends on three receptor mechanisms, each with different spectral sensitivities
- based on colour-matching experiments
trichromatic theory of colour vision
colour-matching experiments
- we can come up with any colour we perceive by matching three light sources
- background: metamerism
trichromatic theory of colour vision
metamerism
- situation in which two physically different stimuli are perceptually identical
- metamers = any pair of stimuli that are perceived as identical in spite of physical differences
- -> reason (metamers look alike): both result in the same pattern of response in the three cone receptors
- background of colour matching experiments
step 2
mixture of lights
additive colour mixture = if light A and light B are both reflected from a surface to the eye, the effects of those two lights add together –> adding up the wavelengths of each light in the mixture
=> all the light that is reflected from the surface by each light when alone –> also reflected when lights are superimposed
=> more wavelengths are be reflected –> each light adds wavelengths to the mixture
step 2
mixture of pigments
subtractive colour mixture = if pigments A and B mix, some light will be subtracted by A and some by B –> remainder contributes to perception if colour
=> when mixed, both paints still absorb same wavelengths they absorbed when alone –> only wavelengths reflected are those that are reflected by both paints in common
=> fewer wavelengths are be reflected –> each paint subtracts wavelengths from mixture
step 3
opponent colour theory
- theory that perception of colour is based in the output of three mechanisms, each of them resulting from an opponent between two colours: red-green, blue-yellow, black-white
- colour vision is caused by opposing responses generated by blue and yellow and by red and green
- based on results of phenomenological observations
opponent colour theory
phenomenological observations
- perceptual pairing (B+ Y-; R+ G-)
- opponent neurones = respond with an excitatory response to light from one part of the spectrum and with an inhibitory response to light from another part
- -> single-opponent neurone = M+ L- receptive field; found in the retina, lateral geniculate nucleus and visual cortex; subtracts one type of cone input from another
- -> double-opponent neurone = side-by-side receptor field; found in the visual cortex; one region is excited by one cone type and inhibited by the opponent cones (R+ G-)
colour deficiency
= partial loss of colour perception
- monochromat
- dichromate
- trichromat
- -> anomalous trichromat
colour deficiency
monochromatism
- usually hereditary
- colour blindness
- poor visual acuity
- cone monochromat = individual with only one cone type –> characteristics of rod vision in both dim and bright lights
- rod monochromat = individual with no cones of any type; in addition are badly visually impaired in bright light
colour deficiency
dichromatism
- experience some colours (lesser range)
- sex-linked
types: - protanopia
- deuteranopia
- tritanopia
colour deficiency
trichromacy
- individual with three visual pigments –> third pigment increases number of colours that can be seen across visual spectrum
anomalous trichromat
- mixes wavelengths in different proportions
- not good at discriminating between wavelengths that are close together
adaptation and afterimages
- colour induction = generation of illusory sensations of colour without direct stimulation of the corresponding retina
- -> negative afterimage = afterimage whose polarity is the opposite of the original stimulus
colour constancy
= tendency that colours of objects/surface appear relatively unchanged (even if changed lighting conditions)
- why it occurs?
1. chromatic adaption - colour perception is changed by exposure to chromatic colour
- partial colour constancy = perception of object is shifted after adaptation
2. effects of surroundings
3. memory of colour
light constancy
= achromatic colours are perceived as remaining the same even under changing illumination
- why it occurs?
1. ratio principle - lightness is determined by ratio of reflectance of object vs. reflectance of surrounding objects
- -> ratio remains the same = perceived lightness remains the same
2. perception under uneven illumination - in three-dimensional scenes: illumination is uneven because of shadows
types of dichromatism
protanopia = colour deficiency due to absence of L-cones
- perceives short-wavelengths (blue)
- neutral point: 492nm
deuteranopia = colour deficiency due to absence of M-cones
- perceives blue at short-wavelengths; yellow at long-wavelengths
- neutral point: 498nm
tritanopia = colour deficiency due to absence of S-cones
- perceives blue at short-wavelengths; red at long-wavelengths
- neutral point: 570nm
–> neutral point = wavelength at which person perceives grey/no colours
three dimensions of colour
- hue/chromatic colours (blue, green, red)
–> dominant wavelengths
{1a. achromatic wavelengths (white, black, grey)} - brightness (value, intensity)
–> amplitude of wavelength - saturation (chroma)
–> spread of wavelength
population coding
overview lecture
= analysis of a stimulus by multiple types of receptors/neurones
- important for colour vision based on different cone types + colour vision based on opponent cells
- solution for univariance
1. perception of colour changes when relative pattern of activity of three receptor types changes
2. perception of colour does not change if relative pattern of activity is the same
3. response of a single receptor is ambiguous
reflectance curves
- shows plot of percentage of light reflected vs. wavelength
- -> selective refraction = wavelengths of chromatic colours/hues are more reflected than others
- -> colours in environment are created by selective reflection of some wavelengths from objects
transmission curves
- shows plot of percentage of light transmitted vs. wavelength
- -> selective transmission = only some wavelengths pass through the object/substance
understanding #the dress
- light reflection from an object depends on
- -> reflectance properties of object (= surface): constant
- -> illumination spectrum (= light conditions): variable (temporal + spectral)
- colours in picture vary along blue-yellow axis
- -> correct for cool illumination: white/gold
- -> correct for warm illumination: blue/black
