Chapter 6: colour perception

week 6

visual spectrum

the band of wavelengths from 400 to 700 nm that people with the most common classes of cones vision can detect

heterochromatic light

white light

many wavelengths

monochromatic light

light consisting of one wavelength

spectral reflectance

ratio of light reflected by an object at each wavelength

hue

colour quality of light

corresponds with colour named roygbiv

quality

value that changes but doesn’t make the value bigger or smaller

saturation

purity of the light

brightness

perceived intensity of the light present

lightness

psychological experience of the amount of light that gets reflected by a surface

additive color mixing

creating a new colour by adding one set of wavelengths to another set of wavelengths

subtractive colour mixing

new colour is made by removing wavelengths from a light with a broad spectrum of wavelengths

metameter

psychophysical colour match between two patches of light that have different sets of wavelengths

s cone

cone with its peak sensitivity to short wavelength light

around 420 nm (blue)

M cone

cone with peak sensitivity to medium wavelength light

535 nm (green)

L cone

peak sensitivity to long wavelength light 

565nm (yellow)

univariance

any single cone system is color blind

different wavelength and intensity can result in the same response from the cone system

trichromatic theory of colour vision

colour of any light is determined by the output of the three cone systems in our retinae 

opponent process theory of colour perception

color perception comes from 3 opponent mechanisms

red green

blue yellow

black white

afterimages

visual images are seen after actual visual stimulus has been removed

complementary colors

colours on the opposite side of the colour wheel that when added together in equal intensity give a white or gray or black

simultaneous colour contrast

a phenomenon that occurs when our perception of one colour is affected by a colour that surrounds it

hue cancellation

observers cancel out the perception of a particular colour by adding light of the opponent colour 

unique colours

colours that can be described only with a single colour term

red green blue yellow

cone opponent cells

neurons that are excited by the input from one cone type in the centre but inhibited by the input from another cone type in the surround

colour opponent cells

neurons that are excited by one colour in the centre + inhibited by another colour in the surround

OR

neurons that are inhibited by one colour in the centre and excited by another colour in the surround

double opponent cells

cells have a centre which is excited by one colour and inhibited by the other

in the surround: reversed pattern

habituation

learning process in which animals stop responding to a repeated stimulus

ex. pigeons don’t care about cars honking

dishabituation

after habituation has occurs, changing the stimulus requires hte organism to respond again

colour deficiency

the condition of individuals who are missing one or more of their cone systems

protanopia

lack of L cones

red green deficiency 

sex linked and more common in men

deuteranopia

lack of M cones

red green deficiency 

more common in men 

tritanopia

s cones lack

blue yellow colour deficiency

rare

not sex linked

anomalous trichromacy

all 3 cone systems are intact

1+ has an altered absorption pattern 

different metameric matches than in the most common type of trichromatic individuals 

unilateral dichromacy

presence of dichromacy in one eye but trichromatic vision in the other

cortical achromatopsia

loss of colour vision due to damage to the occipital lobe

constancy

the ability to perceive an object as the same under different conditions

lightness constancy

the ability to perceive the relative reflectance of objects despite changes in illumination

colour constancy

the ability to perceive the colour of an object despite changes in the amount and nature of illumination

gelb effect

intensely lit black object appears to be gray or white in a homogeneously dark space