2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002

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?


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


Cone wavelength ranges

S M L

Wavelength (nm)

Rela

tive a

bso

rpti

on

400 500 600 700


Cone Distributions

• Where is thefovea?

• Why is blue a“bad” colour?


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


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


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


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


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


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


What is colour?

• Property of objects?• Property of light?• Property of our brain?


Physical dimensione.g. wavelengthP

sych

olo

gic

al d

imen

sio

ne.g

. colo

ur

?

Psychophysics of colour


Colour experiments

• determine number of distinguishable colours– JNDs

• determine number of “necessary colours”• organize the colours according to similarity


Colour similarity

• resulting ordering seems to be cyclic• where should white and black go?• where is purple?


Colour similarity

• white and black correspond to saturation and brightness


HSV: Hue, Saturation and Value

• Hue: spectral hues + mixed colours like purple• Saturation: purity of colour• Value: brightness or intensity

purple!


high

low

high

low

HSV: Hue, Saturation and Value

Hue Saturation Value


hue

HSV: a psychological colour space


saturation



low

high

brightness

medium



white

black



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


Colour constancy demo






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


Colour vision tests


Colour vision tests


Normal colour vision


ProtanopiaL-cone, red/green deficient


DeuteranopiaM-cone, red/green deficient


Tritanopia:S-cone, blue/yellow deficient


Normal Tritanopia

DeuteranopiaProtanopia

Comparison


• 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


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”?

2002/02/05 PSYC202-005, Term 2, Copyright Jason Harrison, 2002

Technology

colour channels

spectral colour

hz colour

colour matching

colour experiments

bad colour

colour similarity white

normal colour vision