BY- DR.RAHUL PANDEY

Is the ability of the eye to discriminate between colours excited by light of different wave lengths.

The sensation of colour is subjective and it is a perceptual phenomenon. There are three different type of cones.

Red sensitive (725 – 647 nm)-L(Long)

Green sensitive (575 – 492nm)-M(Middle)

Blue sensitive (492 – 450 nm)- S(Short)

For any colour there is a complementary color and if properly mixed with it produces a sensation of white.

In Dim light all the colours are seen as gray. This is called “purkinje shift phenomenon

THEORIES OF COLOUR VISION

I. TRICHROMATIC THEORY: Also called as young - helmholtz theory It postulates the existence of

three kinds of cones Each cone containing a

different photopigment and maximally sensitive to one of three primary colours i.e. Red, Green and Blue.

Thomas Young

Helmholtz

A given colour consist of admixture of the three primary colour in different proportion.eg.-Computer moniters and Televisions.

This theory has now been demonstrated by the identification and chemical characterization of each of the three pigments by recombinant DNA technique.

RED SENSITIVE CONE PIGMENT – (Erythrolabe or long wavelength sensitive cone pigment): It absorbs maximally in a yellow position with a peak of 565 nm. But its spectrum extends far enough in to the long wavelength to sense red.

GREEN SENSITIVE GREEN SENSITIVE CONE CONE PIGMENTPIGMENT – – ((ChlorolabeChlorolabe or or medium wavelength medium wavelength sensitive cone sensitive cone pigment): It absorbs pigment): It absorbs maximally in the maximally in the greengreen portionportion with peak at with peak at 535 535 nm. nm.

BLUE SENSITIVE BLUE SENSITIVE CONE PIGMENT CONE PIGMENT (Cyanolabe):(Cyanolabe): short short wavelength sensitive wavelength sensitive (SWS) cone pigment (SWS) cone pigment absorbs maximally in absorbs maximally in the blue – violet the blue – violet portion of the portion of the spectrum with a peak spectrum with a peak at at 420 nm420 nm

Ewald Hering some colours are mutually exclusive Early recordings of the responses of

single neurons in primate retina and geniculate nucleus revealed that- Cells excited by red and inhibited

by green light or vice versa. These were thought to be the red/green opponent color channel of Hering.

blue/yellow channel of Hering. Hering's white/black channel

Ewald HeringEwald Hering

The trichromatic theory by itself was not adeqaute to explain how mixture of lights of different colours could produce lights and yet another colour or even to appear colorless. So both the theories are useful in that. The colour vision is

trichromatic at the level of photoreceptor and

Colour opponency is explained by subsequent neural processing.

Cones differ from rods only in opsin part c/a photopsin.

The green sensitive and red sensitive cone pigments- 96% homology of amino acid sequence.

Where each of these pigments has only about 43% homology with the opsin of blue sensitive cone pigment.

All three bleached by light of different wavelength.

Genesis of visual signal- The photochemical changes

in cone pigments is followed by a cascade of biochemical cone receptor potential.

Sharp onset and offset.

action potential generated in photoreceptors bipolar cells and horizontal cells

ganglion cells and amacrine cells.

synapses

synapses

It shows two complete different kind of response. Luminosity Response : hyperpolarising

response. Chromatic Response : hyperpolarizing

in a part of spectrum and depolarising for the remainder of the spectrum.

This two response provide the first This two response provide the first physiological evidence of opponent physiological evidence of opponent colour coding and it also represents the colour coding and it also represents the first stage in visual system where first stage in visual system where evidence of chromatic interaction has evidence of chromatic interaction has been found and where wavelength been found and where wavelength discrimination can occur. discrimination can occur.

BIPLOAR CELL : It shows the centre surround spatical pattern. Red light striking in the centre of this cell causes hyperpolarisation and green light in the surrounding causes depolarization.

AMACRINE CELLS: The exact role is not known

but they may act as an automatic colour control.

three types- W, X and Y X ganglion cell mediate the color sensation. A single ganglion cell may be stimulated by a number of cones or by a few cones.When all the three types of cones (Red, Green and Blue) stimulate the same ganglion cell the resultant signal is white.

Some of the ganglion cells are excited by one colour type cone and are inhibited by other. This system is called ‘Opponent colour cell” System and concerned in the successive colour contrast.

These ganglion cells have a system which is opponent for both colour and space. This system is called ‘Double opponent cell system and is concerned with the simultaneous colour contrast.

(S+L)-M=RED

+ -

M-(S+L)=GREEN

+

(S+M)-L=BLUE(L+M)-S=YELLOW

Trichromatic colour vision mechanism extends 20 – 30° from the point of fixation. Peripheral to this red and green become indistinguishable and in the far periphery all colour sense is lost.

The very centre of fovea is blue blind.

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

Colour information carried by ganglion cell is relayed to the parvocellular portion of LGB.

Spectrally non opponent cell which give the same type of response to any monochromatic light constitute about 30% of all the LGB neurons.

Spectrally opponents cells make 60% of LGB neurons these cells are excited by some wavelength and inhibited by others and thus appear to carry colour information

Colour information

parvocellular portion of the LGB

layer IVc of striate cortex (area 17).

blobs in the layers II and III

thin strip in the visual association arealingual and fusiform gyri of occipital lobe.( specialized area concerned with colour)

Analysis of colour signals in the visual cortex

SIMULTANEOUS COLOUR CONTRAST: perception of particular

coloured spot against the coloured back ground.

The colour of the spot tends to be complementary towards the colour of the surround.

function of double opponent cells .

Successive colour contrast is the effect of previously-viewed color fields ("inducing fields") on the appearance of the currently-viewed test field. it is a phenomenon of coloured after image. It is function of opponent cell of visual system.

An afterimage or ghost image is an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased

In which the human eye continue to perceive the colour of a particular object unchanged even after the spectral composition of the light falling on it is markedly altered.

Computational mechanism of brain is responsible for this phenomenon.

HUE- Identifiction of color

BRIGHTNESS- Intensity of color

SATURATION- Purity of color

HUE : Is the dominant spectral colour is determined by the wavelength of the particular colour. Hue is that aspect of colour describe with the names such as red, blue, green etc.

BRIGHTNESS: depends upon the luminosity of the component wavelength.

In photoptic vision-peak luminosity function at approximately 555 nm and in scotopic vision at about 507 nm.

The wavelength shift of maximum luminosity from photoptic to scotopic viewing is called ‘ Purkinje Shift Phenomenon’

SATURATION : it refers to degree from freedom to dilution with white.

It can be estimated by measuring how much of a particular wavelength must be added to white before it is distinguishable from white.

The more the wavelength require to be added to make the discrimination, the lesser the saturation.

COLOUR BLINDNESS Is the inability to perceive difference

between some of the colours that other people can distinguish.

The first major study of colour blindness was published in 1794 by John Dalton, who was colour-blind.

colour blindness is sometimes called “Daltonism”,

Defective perception of colour (anomalous) and absent of colour perception is anopia.

It may be- Congenital Acquired

John Dalton

X – linked recessive Affecting males more (3 – 4%) than

female (0.4%) Types

Dyschromatopsia Achromatopsia

Dyschromatopsia: colour confusion due to deficiency of mechanism to perceive colours. 2 types:

Anomalous trichromatism Dichromatism

Here the mechanism to appreciate all the three primary colour is present but is defective for one or two of them.

TYPES- PROTANOMALOUS: PROTANOMALOUS:

DEUTERANOMALOUS: DEUTERANOMALOUS:

TRITANOMALOUS:TRITANOMALOUS:

Red- green deficiency is most Red- green deficiency is most commoncommonBlue deficiency is Blue deficiency is comparatively rarecomparatively rare

B. DICHROMATE COLOUR VISION: Means faculty of perceive one of the three primary colours is completely absent.Protanopia: complete red

colour defectDeuteranopia: complete defect

of green colourTritanopia: Absence of blue of

colour appreciation

PROTANOPIA. TRITANOPIA DEUTERANOPIA

Extremely rare condition

2 types cone

monochromatisn rod

monochromatisn

Cone Monochromatism: Presence of only one primary colour.

visual acquity of 6/12 or better.

very rarecomplete or incomplete autosomal recessive

trait.Characterized by:

• Total color blindness• Day blindness (V.A.

about 6/60)• Nystagmus • Fundus is usally

normal

Red, green and blue cone sensitivity

vs. wavelength curves

Red or green cone peak sensitivity is shifted. orRed or green cones absent

B RG

437 nm 564 nm533 nm

NORMAL CONE SENSITIVITY CURVES(TRICHROMAT)

5% of Males

B RG

437 nm 564 nm

Deuteranomaly(green shifted toward

red)

1% of Males (there is no green curve)

B R

437 nm 564 nm

Deutan Dichromat(no green cones; only

red and blue)

B RG

437 nm533 nm

1% of Males

Protanomalous (red shifted toward green)

1% of Males (there is no red curve)

B G

437 nm533 nm

Protan Dichromat(no red cones; only

green and blue)

Why do colors that

look different to us appear the same to

color deficient

individuals?

Consider a green vs. yellow light…

B RG

Large difference

in stimulation of green

and red cones

Small differenc

e in stimulati

on

The two spots appear different in color

because R-G is large for one, and small for

the other.

Each spot produces the same R-G stimulation and thus looks the same!

B RGSmall

difference in

stimulation

Look the same!

Small difference in stimulation

Deuteranomaly(the green sensitivity curve is shifted toward the red)

Color Deficiency Males Females

Protanopia 1% 0.01%

Deuteranopia 1% 0.01%

Protanomaly 1% 0.01%

Deuteranomaly 5% 0.4%

Overall (red-green)

8% 0.5%

Tritanopia 0.008%

0.008%

Tritanomaly Rare Rare

Rod monochromatis

m

Rare Rare

Cone monochromatis

m

Rare Rare

ACQURIED COLOUR BLINDNESS Koelhar formulated that lesions in the outer layers

of retina give rise to a blue yellow defect, while lesion in the inner layer & optic nerve may produce red-green defect.

Blue yellow impairment: is usually seen in Central serous retinopathy Diabetic retinopathy Macular oedema Myopia Retinitis pigmentosa

Red green deficiency Optic neuritis Leber’s optic atrophy

Acquired blue colour defect: crystalline lens absorbs shorter wavelength in young, less than 400 nm and in old people up to 550 nm are absorbed. It results in defective colour vision on shorter wavelength side.

DRUG CAUSING CVDBlue-yellow: chloroquine, indomethacin, oral

contracaptives. Estrogens, Digitalis & ButazolidinRed green: Ethyl alcohol & EthambutolMixed type: Di & Tri cyclic anti depressants.

Gene rhodopsin - chromosome 3.

Gene for blue sensitive cone - chromosome 7, AD

The genes for red and green sensitive cones are arranged in tandem array on the ‘q’ arm of x chromosome,XR.

Tritanopia and tritanomaly – rare,no sexual selectivity.

DEUTERANOMALY AND PROTANOMALY

Is probably due to the arrangement of the genes for the green and red sensitive cone pigments.

They are located near each other in a head to tail tandem array on the ‘q’ arm of the X chromosome and are prone to recombination during development of germ cell.

PSEUDOISOCHROMATIC COLOUR TEST:

most commonly employed tests- eg.-

ISHIHARA PLATES and

HRR(HARDY,RAND,RITTLER) plates Ideal for paediatric testing of

congenital color blindness.

designed in four ways

1st plate- for demonstration and

malingerers.

(2-9) plate- Transformation plates: normal person sees one figure and a CVD sees another.

(10-17)plate-Vanishing plates: normal person see the figure while a CVD person will not

Pseudoisochromatic colour plates (18-21)plate-Hidden-digit

plates: normal person does not see a figure while a CVD will see the figure.

(22-25)plate-Diagnostic plates: seen by normal subjects, CVD one number more easily than another. Protans only see the no. on the right side and deutans only see the no. on the left.

75 cm ,day light,right angle,3 sec.

For illitrate patients

subject has to name the various colours shown to him by a lantern.

TYPES: Farnsworth lantern Optec 900 Holmes Wright Type A and

B lantern Beyne lantern Edridge green lantern is

most popular test.

MOST SENSITIVE.Subject has to arrange 85 colour chips in ascending order. The colour vision is judged by the error score. The results are recoded in a circular graph.

Normal patternAbnormal patterns

Abridged versionPatients are asked to arrange 15 coloured caps in sequential order based on similarity from the pilot colour cap .

10 Plates ,35 cm,daylight,right angle.

It is also a spectroscopic test where a centre coloured plate is to be matched to its closest hue from four surrounding colour plates.

The subject is asked to make a series of colour matches from a selection of skeins of coloured wools.

GOLD STANDARDExtraordinarily sensitive. In this test the observer is asked to mixed red and green colours in such a proportion that the mixture should match the yellow colour disc. Indication of defect is relative amount of red and green required.

MOST RELIABLE means to distinguish acquired from inherited color vision defect.

Not commercially available.

Color blindness

no yes

Red-green Blue-yellow

Protan Deutan

Genetic Acquired

Anamolous Anamoly

mild moderate severe

Currently No treatment. Some filters may help to distinguish the colours

but not in the identification of colours. The purpose of this is to eliminate certain lights

and modify the light reaching the eyes so that the receptors receive correct information.

Future direction-Viral mediated gene therapy

Tinted contact lenses

Filtered goggles

REFFERENCESDiagnosis of Defective

Colour Vision – Jennifer Birch 2nd ed.

Clinical neuro-ophthalmology-Ulrich schifer

Ophthalmology – Myron Yanoff & jay s. Duker 2nd ed

Clinical ophthalmology – Jack J. Kanski 6th ed.

Adler’s Physiology of Eye 19th ed.

Parson’s Basic Diseases of The Eye- 20th ed.

Anatomy and physiology of Eye- A.K. Khurana 2nd ed.