colour vision

Newton’s question

does the prism create colour or are some types of light fundamental?

what Newton found

•White light is a mixture of “wavelengths”,

•Refraction (bending)  depends on wavelength,

•Object colour depends on the lighting

•You can mix ‘primaries’ to produce any other colour

•In other words, he described the physics of colour

Thomas Young

proposed a universal phonetic alphabet

said there must be a limited number of colours e.g. the principles: red, yellow and blue - there are 7 primitive distinctions of colours but the different proportions in which they may be combined leads to an infinite variety of tints

trichromatic theory of vision

all colour sensations are produced by the activity of just 3 retinal photoreceptor types

adaptive optics

technique that allows to photograph the individual photoreceptors in the retina

if a subject adapts to a long-wavelength red light, the red cones become most bleached and when the retina is photographed after this in white light, the red cones don’t respond as well as they did before the adaptation so look darker

this way we can map out the individual red, green and blue cones in the retina

metamers

lights that look the same even though their spectra are different

they look the same because they drive the photoreceptors in the same way

key concepts

•Humans can match any colour by mixing three ‘primaries’ because… you have three classes of cones.

All colours are represented by the amplitudes of responses in these three cone classes.

•There are infinitely many spectra that will give rise to the same colours. These are called ‘metamers’

Opsin genes

humans have 23 chromosome pairs in almost all cells (diploid)

germ-line cells (gametes) are haploid: one of each type

the sex chromosomes: X and Y are special

S-cones (blue)

S cones: a fossil of the original colour vision system - not many of them: about 10% of cones

don’t see much form or motion through S-cones

colour blindness

generally caused by missing/abnormal opsin genes

almost always the genes involved are the L and M opsins because these are on the X chromosome, men are affected by colour blindness far more than women as they have no backup gene to rescue them if something goes wrong

effect of colour blindness

usually caused by loss of one cone type

knocks out a dimension of colour vision

if all your M cones become L, the L-M dimension disappears - you can still discriminate dark/light and yellow/blue but not red/green

opponent processing

absorption spectra not evenly spaced - L and M cones are especially close so they convey almost the same info

opponent channels

inefficient to send L, M and S signals straight to cortex - instead the retina computes 3 combos of those signals

•L+M,  L-M  and S-(L+M)

•Those are ‘black/white’, ‘red/green’ and ‘yellow/blue’

faulty opsin genes

losing either of the L or M opsins damages the opponent red/green system leaving blue/yellow

losing the S cone opsin leaves only the red/green system

colour blindness in animals

many animals only have 2 photoreceptors: L and S - like human dichromats these animals have one colour channel and one luminance channel

they can distinguish light/dark and blue/yellow but not red/green

colour blindness in humans

•Is not usually the absence of colour vision, Instead, people lose a dimension of colour vision.

•Most common is ‘anomalous trichromacy’ where discrimination is poorer along the red/green axis - then ‘dichromacy’ where L or M is absent

•Rarest type (<1 in 1000) is tritanopia where individuals lack S cones

genetics of colour blindness

X-Linked Recessive: The most common forms of colour blindness (red-green) are inherited in an X-linked recessive pattern. This means the genes responsible are located on the X chromosome.

tests for colour blindness

Ishihara plates - the red/green pattern is masked by random luminance noise so that small luminance cues due to printing/lighting errors can’t be used

when did trichromacy evolve

our primate ancestors were dichromats

L cone gene duplication occurred about 40 million years ago

primate colour vision

trichromatic primate colour vision has evolved to support frugivory: the eating of fruit

so primates are useful to trees just as honeybees are useful to flowers

in the brain

many areas respond to both colour and achromatic contrast e.g. the ventral and dorsal streams

the ventral stream

seems to be concerned with object identity and form

has strong representation of the fovea and a strong response to colour

the dorsal stream

seems to be involved in motion, action and location

some regions here like hMT respond very weakly to pure colour