PHYSIOLOGY OF COLOUR VISION WITH VARIOUS THEORIES

Dr samarth mishra

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-Colour sense is the ability of eye to discriminate b/w colours excited by light of different wavelengths.

-It is a function of cones.

-So better appreciated in photopic vision.

-Three types i.e red, blue and green.

-It is a perceptual phenomenon.

The Physics of Light

Some examples of the reflectance spectra of surfaces

Wavelength (nm)

% P

hoto

ns R

efle

cted Red

400 700

Yellow

400 700

Blue

400 700

Purple

400 700

© Stephen E. Palmer, 2002

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-Many factors determine the colour perceived.

-the spectral composition of light from the object is important.

-but the spectral composition of light from the visual surroundings and the state of light adaptation of eye also contributes.

-in dim light all the colours are seen as gray; this is called purkinje shift phenomenon.

-White objects reflect all colours to eye, black absorbs all colours so no light to the eye.

THREE ATTRIBUTES OF COLOURS

HUE

INTENSITYSATURATION

the visual system consists of:

The eye, especially the retina The optic nerve The optic chiasma The optic tract The lateral geniculate body The optic radiation The visual cortex The visual association cortex.

Photochemistry of colour vision

-cone pigment just like rhodopsin has 11-cis retinal and opsin part.

cone pigment

11-cis retinal (opsin)

- 11-cis retinal is similar to rhodopsin, the opsin part known as

photopsin is different than the opsin part of rhodopsin

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-all the three cone pigments have about 41 percent homology with the rod pigment rhodopsin.

-thus the photochemistry of rhodopsin can be applied to the cone pigments.

-the only difference being that the three different types of cones are bleached by light of different wavelength.

Neurophysiology of colour vision

-Similar to photochemical changes, the physiological process concerned with colour vision are also same as for vision in general.

-the action potential generated in the photoreceptors is transmitted by electronic conduction to the other cells of the retina across the synapses of photoreceptors, bipolar cells and horizontal cells

-and then across the synapses of bipolar,ganglion and amacrine cells.

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HORIZONTAL CELLS:

-showed two completely different kind of response.

a) luminosity response :there was a hyperpolarising response with a broad spectral function.

b) chromatic response : which was hyperpolarising for part of the spectrum and depolarising for the remainder of spectrum.

-this provided the first physiologic evidence for opponent colour coding.

.BIPOLAR CELLS:

-recordings shows a ‘centre surround’ spatial pattern.

-red light striking the centreof these cells caused hyperpolarisation.

-Green light in the surroundings caused depolarisation.

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AMACRINE CELLS:

-the exact role of these cells in colour vision is not clear.

-they may act as ‘automatic colour control’.

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GANGLION CELLS:

-at this level first direct evidence in the visual system for colour coding is seen.

-three distinct groups of ganglion cells W,X,Y seen.

-colour sensation is mediated by the ‘X’ ganglion cells.

-A single ganglion cell may be stimulated by a number of cones or by a few cones.

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-when all 3 cones stimulate , the resultant signal is white.

opponent colour cell:

Some ganglion cells are excited by one colour type cone(i.e red) and are inhibiteb by other(i.e green) or vice versa.

This is called ‘ opponent colour cell’ system and is concerned in the ‘successive colour contrast’.

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Double opponent colour cell:

-have a system which is opponent for both colour and space.

-the double opponent cells have a receptive field with a centre and surround.

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-the response may be ‘on’ to one colour (e.g red) in the centre and ‘off’ to it in the surround.

-while the response may be ‘off’ to green in the centre and ‘on’ to it in surround.

This systems indicate that the process of colour analysis begins

in the retina and is not entirely a function of brain.

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DISTRIBUTION OF COLOUR VISION IN THE RETINA:

-trichromatic colour vision mechanism extends 20-30 degrees from the point of fixation.

-peripheral to this red and green become indistinguishable,and in the far periphery all colour sense is lost ,although cones are still found in this region of retina.

-the centre of fovea ( 1/8 degree) is blue blind.

-when a red test object is brought from the periphery in the field of vision ,the individual first becomes aware of a colourless object in the periphery.

-then as the object is advanced ,it is seen successively as salmon pink or yellow and eventually red.

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LATERAL GENICULATE BODY:

-All LGB neurons carry information from more than one cone cells.

-from ganglion cells colour information is then relayed to parvocellular portion of LGB.

30 % LGB neurons: spectrally non opponent cells.60 % LGB neurons:spectrally opponent cells.

-these cells are excited by some wavelengths & inhibited by others.

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- The LGN is segregated into six layers.

- Two magnocellular (large cell) achromatic layers (M cells).

- Four parvocellular (small cell) chromatic layers (P cells).

- Within the LGN P-cell layers there are two chromatic opponent types: red vs. green and blue vs. green/red.

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These have been classified into 4 types:

a) Cells having red and green antagonism (with +R/-G)

b) Cells having red and green antagonism (with +G/-R)

c) Cells with blue and yellow antagonism (with +B/-Y)

d) Cells with blue and yellow antagonism (with +Y/-B)

© Stephen E. Palmer, 2002

G+R-

G+R-

R+G-

R+G-

Red/Green

Y+B-

Y+B-

B+Y-

B+Y-

Blue/Yellow

VISUAL CORTEX:-Colour information from the parvocellular portion of

LGB is relayed to the layer IV c of striate cortex.( area 17)

-it then passes to blobs ( in layers 2 and 3).

-these blobs are ‘centre surround’ cells. (like the ganglion cells and LGB)

BLOBS

visual association area

lingual & fusiform gyri ( occipital lobe)

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Mechanism of colour visionTHEORIES OF COLOUR VISION:

A ) TRICHROMATIC THEORY:

-Suggested by young

-Subsequently modified by helmholtz (1802).

-Therefore, it is called young-helmholtz trichromatic theory.

-Postulates the existence of 3 kinds of photopigment.

-This photopigment is sensitive maximally to only one of the three primary colours.

Hermann von Helmholtz

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-the 3 primary colours being red, green and blue.

The sensation of any given colour is determined by the relative frequency of the impulse from each of the three cone system.

-the correctness of the young-helmholtz trichromatic theory of colour vision has now been demonstrated by the identification and chemical characterisation of each of the three pigments by recombinant DNA technique.

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ERYTHROLABE :

-Red sensitive pigment is also known as erythrolabe. Or long wavelength sensitive(LWS) cone pigment.

-It absorbs maximally in a yellow position with a peak at 565nm.

-But its spectrum extends far enough into the long wavelength to sense red.

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CHLOROLABE:

-Green colour pigment also called as chlorolabe or medium wavelength sensitive (MWS) cone pigment.

- It absorbs maximally in the green portion with a peak at 535nm.

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CYANOLABE:

-Blue sensitive cone pigment is also known as cyanolabe or short wavelength sensitive (SWS) cone pigment.

-It absorbs maximally in the blue violet portion of the spectrum with a peak at 440nm.

-Thus , the young-helmholtz theory concludes that blue,green and red are primary colours ,but the cones with the maximal sensitivity in the yellow portion of the spectrum are light at a lower threshold than green.

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B) OPPONENT COLOUR THEORY:

-it was proposed by hering (1878).

-he pointed out that some colours appear to be ‘mutually exclusive’.

-there is no such colour as ‘reddish-green’.

-such phenomenon can be difficult to explain on the basis of trichromatic theory alone.

Ewald Hering

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- [yellow-blue] and [red-green] represent opponent signals producing four colour primaries red,green,yellow and blue, and not just three.

- [ white-black ] opponency proposed by him has been abandoned in most modern version of the theory.

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according to opponent colour theory:

a) Red- green opponent colour cells use signals from red and green cones to detect red/green contrast within their receptive field.

b) Blue-yellow opponent colour cells obtain a yellow signal from the summed output of red and green cones, which is contrasted with the output from blue cones within the receptive fields.

SIGNIFICANCE Explains why people with dichromatic deficiency

are able to match test field using only 2 primaries .

How we see yellow though there is no yellow cone

Explains Colour after images

.ZONE THEORY:

-Proposed by donder (1881)

It seems that both the theories are useful in that:

-the colour vision is trichromatic at the level of photoreceptorss

-while , colour opponency is explained by subsequent neural processing,at the level of ganglon cell onwards.

Trichromaticstage

Opponent-Process stage

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COLOUR VISION DEFECTS:

-Color can be described in terms of hue( determined by wavelength) and saturation (determined by amount of white light mixed).

-those with colour vision defects see fewer hues than normal.

HUE

INTENSITY

SATURATION

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TYPES OF COLOUR VISION DEFECTS:

Trichromatism: can differentiate all colours(normal sight) RED BLUE

GREEN

Anomalous Trichromatism : can differentiate all colours but

one colour has reduced or displaced

sensitivity.

A)Protanomaly: displaced sensitivity

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RED BLUE GREEN

B)Deuteranomaly: displaced sensitivity

C)Tritanomaly: displaced sensitivity

DICHROMATISM: receptors missing for one type of cone

A)Tritanopia: missing

B)Deuteranopia: missing

C)Protanopia: missing

Monochromatism: totally unable to differentiate colors of equal

(achromatism) brightness

Color Blindness

What does the world look like to a color blind person?

NormalTrichromat

Protanope Deuteranope Tritanope

Thank you..

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