2.8 Colour Perception Flashcards
Colour Perception is…
INDIRECT . we dont see colour, we code it
Light as a coloured stimulus:
we only see a small amount of the electromagnetic spectrum wavelength: HUE amplitude: BRIGHTNESS purity: saturation AND reflectance: lightness
reflectance
amount of light object reflects.
Crucial fo colour perception
newtons prism experiment
first to study colour in experiment
demonstrated white is all colours
colour is a property of..
light not of an object
object is simply reflecting the light and absorbing selected part of spectrum
perceived colour is a combo of…
nature of light falling on the surface (illumination)
nature of absorption and reflectance of surface
color wheel
developed by newton through his prism experiments
classifies on wavelength (hue) not brightness or purity/saturation
color solid
by munsell (1940s)
included saturation and brightness
series of colour wheels stacked on top of each other
3D stacks is brightness
each stack is a specific hue and middle of wheels is achromatic,
Achromatic light
colourless light
determines light purity
more achromatic light = less saturation
(middle of the colour wheels)
Colour mixing
subtractive (paint and filters ) and additive (light)
subtractive primaries
red yellow blue
additive primaries
blue red green
when mixing complementary colours
you get achromatic light
additive = opposite of colour wheel
additive = white
subtractive = dark brown
CIE Chromaticity space
quantifying amount of each primary (produces specific hue)
y axis: % of green
x axis: % if red
what is left = % of blue
when adding brightness: z axis (3d): brightness
computers also include saturation and brightness
Neural Coding
Perception of colour relies on a neural code. which is coded in electrical activity in visual system
Benham colour wheel
example of sensation not equaling perception
as the disc spins (black and white) you end up seeing colour. opposite direction give you its complementary colour
shows that perception of colour is influenced by:
- Speed
- Distance
- Direction
- Lighting conditions
Univariance
photoreceptors response is summarized by one variable that specifies amount of light aborbed.
Each photopigment responds to preferred wavelength
- but the neural response of photoreceptors does not specify wavelength
therefore given proper intensity, all wavelengths can effect a single receptor in some way
therefore we must have more than one type of photoreceptor (which must be solved through the theories)
Trichromatic Theory
Young and Helmholtz - it was an intuitive theory - human eye has 3 photopigments - relative activation of each determines the colour percieved .... % of cone activation = coding Red Green and Blue
Supported by:
- Rushton: composition of ingoing and returning light found 2 cone types
- Microspectrophotometry: using dissected retinal tissue determined how much light of each wavelength is absorbed found 3 cone types
Absoprtion Curve
relative responsiveness of each cone. Blue far off then green then red
Opponent process theory
Hering (1878)
“Why cant we see blueish-yellow or reddish-green
- There must be an opponent process
= 6 primary colours (3 types of receptors in pairs)
- 3 cones complementing 3 receptors (in pairs equaling 6)
Hue Cancellation
Some colours cancel each other out
because complementary adding = achromatic
—- must be a decrease and increase in activity
Achromatic system in Opponent Process
White Black
Codes brightness
increase in activity with light
Chromatic System in Opponent Process
Red/Green
Increase w/ red, decrease w/ green
Yellow/Blue
increase w/ yellow, decrease w/ blue
Truth about colour perception
@ receptor level: trichromatic theory
@ higher levels: opponent process theory
evidence: responses to coloured light at horizontal cell level (opponent responses to colour)
Short wavelength Cones
Excitatory: +B , -Y
Inhibitatory: +Y , -B
Medium wavelength cones
Excitatory: + G, -R , and +Wh - Bl
Inhibitatory: +R -G and -Wh +Bl
Long Wavelength Cones
Excitatory: +Y -B, and +R -G ,and +Wh -Bl
Inhibitatory: +B -Y, and +G -R, and -Wh +Bl
Colour opponency
oppoenent cells throughout visual system except receptors
EX: LGN cells generate both on/off responses to send info to higher processing
in cortex its more complex: double opponent process
LGN opponency
+ in the middle, - in surround / vise versa (on/off)
Visual cortex opponency
+- in middle and -+ in surround
Theory on hue perception
relative amount of activity in opponent cells
Theory of lightness perception
activity rate of black and white cells
Theory on saturation perception
which cells are more active
Trolands theory
explaining why benham color wheel doesnt work with theories
this is because single channel may be used to percieve color, with color information coded into neural “morse code”
Factors that effect colour vision
region of retina stimulated (better in fovea)
exposure duration (too quick: white patch, too long: adaptation)
Deficits (loss of cones)
- red/green : 8% genetic x linked
- yellow/blue genetic autosomal
achromatopsia
selective loss of colour vision associated with damage to temporal and/or occipital areas
Red/Green Colour Blind
dichromatic colou deficiency. absense of either red or green cones
Anamalous Trichomacy
one of the 3 cone types has abnormal absoprtion curve
constancy of colour
perceived colour of object is determined by light reflecting off of it
which depends on the light hitting in (the wavelength)
and sunlight is constantly changing
our brain fixes this through:
- light adaptation
- colour contrast
