2002/02/05PSYC202-005, Term 2, Copyright Jason Harrison, 20021 Psychophysics of colour perception Neurophysiology, psychophysics, and magic (or you only.

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”?

2002/02/05PSYC202-005, Term 2, Copyright Jason Harrison, 20021 Psychophysics of colour perception Neurophysiology, psychophysics, and magic (or you only.

Documents

copyright jason harrison

colour channels

colour matching

spectral colour

bad colour

cone blueyellow

yh colour theory redgreen

yh colour theory blueyellow