Anuraag Singh

Colour vision is the ability of the eye to

discriminate between colours excited by lights

of different wavelengths.

Colour Vision - Function of cones

Colour - Perceptual phenomenon

- Spectral composition of light from

the object and visual surroundings

also matter.

Sensation of colour is a subjective phenomenon i.e Taught since childhood

All colours are a result of admixture of Primary colours in different proportions-

Red ( 723-647)

Green ( 575-492)

Blue ( 492-450)

For any colour there is a complementary colour

- when both mixed properly produce a sensation of white

1.Trichromatic Theory

• It postulates the existence of three kinds of cones

- Each containing different photopigment

- Each sensitive maximally to one of three primary

colours ( Red, Green , Blue )

-Any given colour’s sensation is due to relative

frequency of impulse from each of three cone

system

•

1. Red sensitive cone pigment/erythrolabe/LWS

- Absorbs light maximally in yellow position (peak at 564 nm)

- Its spectrum extends to long wavelength to sense

Red

2. Green Sensitive Cone pigment/chlorolabe/MWS

- Absorbs light maximally in green portion ( peak at

534 nm )

3. Blue sensitive cone pigment/cyanolabe/SWS

-Absorbs maximally in Blue-violet portion with a peak at

420nm

• Postulated by Ewald Hering

• Some colours are mutually exclusive

• The cone photoreceptors are linked together to form

three opposing colour pairs: blue/yellow, red/green,

and black/white

• Activation of one member of the pair

inhibits activity in the other

• No two members of a pair can be seen at the same

location to be stimulated

• Never "bluish yellow" or "reddish green“colour

experienced

• Cone pigment = 11 cis-retinal + opsin part

• 11 cis-retinal part is similar to rhodopsin

• Priciples of photochemistry of rhodopsin can

be applied to cone pigments

• Difference being that three cone pigments

are bleached by light of different wavelength.

Opsins of green and red sensitive cone

pigments show 96% homology ( amino

acid sequence )

Both show 43% homology with blue

pigment

All three i.e Red,Green,Blue show 41%

homology with rod pigment rhodopsin.

Photochemical changes

Biochemical changes

Visual signal

( Cone Receptor potential )

Action potential generated is transmitted as electronic conduction to other cells of retina.

A slow graded potential is recorded in these cells

Increase in amplitude of response with increasing

luminosity is also recorded

These cells show two different response

1. Luminosity response:- Hyperpolarizing response with

a broad spectral function

2. Chromatic response:-Hyperpolarizing for a part of a

spectrum and Depolarizing for the remaining part of

spectrum

This represents first stage in visual system where

chromatic interaction’s evidence is noted.

Shows Centre-Surround spatial pattern

Red light striking in centre caused hyperpolarisation and green light in surroundings caused depolarisation.

Amacrine cellsAutomatic colour control

There are three distinct groups of ganglion cells:-

1.W

2.X

3.Y

Colour sensation is mediated by ‘X’ type

A single ganglion cell may be stimulated by a number

of cones

When all three types of cones stimulate same

ganglion the resultant signal is White.

Two type of colour opponent system is found in

ganglion cells:

1. Opponent colour cell system

Some ganglion cells are excited by one colour type

cone and inhibited by other

Two types of colour opponent ganglion cells:-

1.Red – green opponent colour cells use signals from

red and green cones to detect red/green contrast.

2.Blue – yellow opponent colour cells obtain yellow

signal from summated output of red and green cones

which is contrasted with output from blue cones.

This system is concerned in successive colour system

(Phenomenon of coloured after images )

Example:-when one sees at green spot for several

seconds , then looks at grey card , one

sees a red spot on the card

2. Double opponent colour cell system

These ganglion cells have opponent for both colour

and space.

Example:- Response may be ‘on’ to red in centre and

‘off’ to it in surround, while ‘off’ to green in centre

and ‘on’ to it in surround.

This system is concerned with ‘Simultaneous colour

contrast’ – Phenomenon of perception of a particular

coloured spot against coloured background ( grey

spot appear greenish in red surround and reddish in

green surround ).

Trichromatic vision extends 20 to 30 degrees from point of

fixation.

Peripheral to this red and green become indistinguishable.

In far periphery all colour sense is lost.

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

All LGB neurons carry information from more than one cone

cell.

Colour information is transmitted from ganglion cells to

parvocellular portion of LGB.

Two type of LGB neurons:-

1. Nonopponent cells ( 30% of total LGB neurons )

Give same response to any monochromatic light

2. Opponent cells ( 60% of total LGB neurons )

Excited by some wavelengths and inhibited by others

1. +R/-G :- Red and green antagonism

2. +G/-R :- Red and green antagonism

3. +B/-Y :- Blue and yellow antagonism

4. +Y/-B :- Blue and yellow anatgonism

Colour Signal from parvocellular portion

Layer 4c of striate cortex ( area 17 )

Blobs in layers 2 and 3

( blobs have centre surround cells)

Visual association area

Lingual and fusiform gyri of

occipital lobe

Color coded cells are arranged in a hierarchy.

Opponent color cells are found among ganglion cells of retina

and LGB.

Double opponent cells are found in layer 4 of striate

cortex(area 17)

Hue

Identification of colour,dominant spectral colour is determined

by the wavelength of particular colour

Brightness

Intensity of colour,it depends on 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 photopic to

scotopic viewing is called Purkinje Shift Phenomenon’

Saturation

It refers to degree of 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.

It describes what mixtures of wavelength can be substituted

for each other without changing the colour.

A colour triangle can be drawn to describe

trichromacy of colour mixture.

Three axis are scaled to represent various amounts of

pigment absorption by each of the three cone

pigments.

A particular colour is represented by a line or a

vector.

Length of line identifies brightness.

Angle or direction of line represents hue.

All colours seen by human eye fall inside the colour

triangle.

1. CIE (Commission Internal de Eclairage)

CIE colour space system is based on amounts of three

primary colours necessary to match a specified colour.

CIE chromaticity diagram is constructed by placing the

three reference wavelengths say 450 at X , 520 at Y and

650 at Z of the triangle.

*2.The Munsell colour system

All the colours are represented in a cylinderin terms

of hue and chroma.

Hue dimension i.e dominant spectral colour is

located on circumference of the cylinder.

Value dimension i.e lightness is indicated by moving

up or down the cylinder

THANK YOU

*Standard source A is a defined tungsten lamp

run at a defined current and voltage.

*Standard source C is a substitue for daylight

and consist of a tungsten lamp with a defined

blue filter in front.

*Point E represents a source radiating equal

amount of energy in equal intervals of

wavelength throughout the spectrum.

*Colour blindness is also called “Daltonism”

*Defective perception of colour -anomalous

* Absence of colour perception is anopia

*It may be-

*Congenital

*Acquired

1.Monochromacy /Total colour blindness

When two or all 3 cone pigments are missing.

Types:-

1.Rod monochromacy

2.Cone monochromacy

2.Dichromacy - When one of the 3 colour pigment is

absent

Protanopia - RED retinal photoreceptors absent

Deuteranopia -GREEN retinal photoreceptors absent.

Tritanopia -BLUE retinal photoreceptors absent

3.TRICHROMACY [Anomalous Trichromacy]

Colour vision deficiency rather than loss

Protanomaly - RED colour deficiency [Hereditary,

Sex linked, Male1%, ]

Deuteranomaly - GREEN colour

deficiency[Hereditary, Sex linked, Male 5% ]

Tritanomaly - BLUE colour deficienc [ Rare,Not

hereditary ]

*Gene rhodopsin - chromosome 3.

Gene for blue sensitive cone - chromosome 7

The genes for red and green sensitive cones

are arranged in tandem array on the ‘q’ arm of

x chromosome so defect is inherited as x-

linked recessive

Tritanopia is inherited as an autosomal

dominant defect

Congenital colour blindness is two type

Achromatopsia

Dyschromatopsia

More common in male (3-4%)than female(0.4%)

It is x-linked recessive inherited condition.

Cone monochromatism:

Presence of only one primary colour

So person is truely colour blind

Rod monochromatism:

Complete or incomplete

Inherited as autosomal recessive trait

Total colour blindness

Day blindness(visual acquity is about 6/60)