2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002
1
Psychophysics of colour perception
Neurophysiology, psychophysics, and magic
(or you only though you knew what colour was…)
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 2
Why have colour vision?
• Why don’t other mammals have colour vision?
• Why do bees have colour vision?• Is hue or luminance more useful?• Is hue or luminance more accurate?
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 3
The neurophysiology of colour vision• Rods and cones
– Rods are inhibited during day light– Cones are “blind” during night light
• Three different types of cones, each optimally responsive to a different wavelength1. L-Cone: long wavelength, red2. M-Cone: medium wavelength, green3. S-Cone: short wavelength, blue
• Each type responds to a range of wavelengths
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 4
Cone wavelength ranges
S M L
Wavelength (nm)
Rela
tive a
bso
rpti
on
400 500 600 700
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 5
Cone Distributions
• Where is thefovea?
• Why is blue a“bad” colour?
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 6
Young-Helmholtz theory of colour• 3 primaries needed to produce any
spectral colour• 3 cones found in the retina• simplest colour vision theory:
– cones transduce light and send signals directly to brain
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 7
Weaknesses of the YH colour theory• red-green are mutually exclusive
(opponency):– a red light can have a blue or yellow tint but
not a green tint– a green light can have a blue or yellow tint but
not a red tintpurple orange brown
cyan yellow-green brown
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 8
Weaknesses of the YH colour theory• blue-yellow are mutually exclusive
(opponency):– a blue light can have a red or green tint but
not a yellow tint– a yellow light can have a red or green tint but
not a blue tint
purple
orangegrey
cyan
yellow-green
grey
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 9
Patching up the YH colour theory• Hering theorized opponent channels
– one type of colour receptor– three colour channels:
• Red/Green, Yellow/Blue, White/Black
– also explains coloured afterimages– does not explain colour matching
• (requires 3 types of colour receptor)
• Modern opponent process theory:– combine Young-Helmholtz and Hering
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 10
Modern opponent process theory• different combinations of three cone
types are fed into the channels:– achromatic: M+L cone responses– blue-yellow: M+L versus S cone responses– red-green: L+S versus M cone responses
• chromatic channels are weighted so that they give a zero response to white
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 11
Colour opponent channels
• achromatic: – M+L cone
• blue-yellow: – M+L versus S cone
• red-green:– L+S versus M cone
S M L
400 500 600 700
Rela
tive e
ffici
en
cy
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What is colour?
• Property of objects?• Property of light?• Property of our brain?
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Physical dimensione.g. wavelengthP
sych
olo
gic
al d
imen
sio
ne.g
. colo
ur
?
Psychophysics of colour
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 14
Colour experiments
• determine number of distinguishable colours– JNDs
• determine number of “necessary colours”• organize the colours according to similarity
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 15
Colour similarity
• resulting ordering seems to be cyclic• where should white and black go?• where is purple?
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 16
Colour similarity
• white and black correspond to saturation and brightness
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 17
HSV: Hue, Saturation and Value
• Hue: spectral hues + mixed colours like purple• Saturation: purity of colour• Value: brightness or intensity
purple!
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high
low
high
low
HSV: Hue, Saturation and Value
Hue Saturation Value
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hue
HSV: a psychological colour space
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saturation
HSV: a psychological colour space
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 21
low
high
brightness
medium
HSV: a psychological colour space
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 22
white
black
HSV: a psychological colour space
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 28
Colour constancy
• Discounting the spectral distribution of the light source– Incandescent lights are reddish– Sun is yellowish– Sun setting is orange
• Similar to lightness constancy
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 29
Colour constancy demo
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 30
Colour constancy demo
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 31
Colour constancy demo
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 32
Congenital colour deficiency• Approximately 8% of the population (1 in 12)• Sex linked genes on X-chromosome
– mostly men (women possibly tetrachromats)
• Three types: missing or abnormal1.protanopia/protoanomaly: L-cones2.deuteranopia/deuteranomaly: M-cones3. tritanopia: missing S-cones
S M L
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Colour vision tests
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 34
Colour vision tests
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Normal colour vision
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 36
ProtanopiaL-cone, red/green deficient
2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002 37
DeuteranopiaM-cone, red/green deficient
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Tritanopia:S-cone, blue/yellow deficient
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Normal Tritanopia
DeuteranopiaProtanopia
Comparison
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• Bilateral damage to V4 results in achromatopsia
• “she looked like a rat”, Rama p. 73• Neurons in V4 respond to colours
(not wavelengths)
Cerebral Achromatopsia
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Benham, Fechner disks• Black and white patterns that produce colours
– flickering monochromatic light below 40Hz
• Colour is sensation NOT just physics• Does colour exist “out there”? Or “up here”?