Colour vision

The Electromagnetic Spectrum.

• how we see colour?..

• how colour is described?..

• what colour blindness is?..

COLOUR VISION:Definition: Colour vision is the ability of the

human being to identify & distinguish different

colours.

• Rods & cones: light sensitive receptors

– Rods – Night vision & vision in shades of gray.

– Cones – daylight or bright light vision, colour

vision & acuity of vision

• Any colour has: hue, intensity & saturation.

– Hue – colour that is red, yellow, blue etc.

– Intensity – bright or pale

– Saturation – degree of freedom from dilution with

white

• In man cone vision sensitivity 1/100th – 1/1000th

that of rod vision.

• The peak sensitivity of scotopic vision is ≈ 500 nm

and photopic vision is ≈ 560 nm.

What We See • Hue: identification of colour

• Brightness: intensity of colour

• Saturation: purity of a colour

Brightness

Colors are described by

their brightness

Hue will change with

intensityINTENSITY

Saturation

Among monochromatic lights,

short wavelengths (blue) and long

wavelengths (red) appear the

most saturated. Wavelengths of

around 575 nm (yellow) appear

the least saturated. To illustrate

this phenomenon, ask yourself

which hue: blue, red or yellow, is

the most similar to white?

hue is less

saturated

Day light visible spectrum consists of VIBGYOR.

Violet has shortest of wavelength of 450nm while

Red has longest wavelength i.e. 720nm

Ultraviolet & infrared: beyond visibility.

370-470 Reddish-Blue (violet)

470-475 Blue

475-480 Greenish-Blue

480-485 Blue-Green (Aquamarine)

485-495 Bluish-Green

495-535 Green

535-555 Yellowish-Green

555-565 Green-Yellow

565-575 Greenish-Yellow

575-580 Yellow

580-585 Reddish-Yellow

585-595 Yellow-Red (Orange)

595-730 Yellowish-Red

Light absorption by the respective pigments of the

rods & colour cones of the human retina

Red (570nm), Green (535nm) & blue (445nm)

• Primary colours are 3:

– Red(720- 650nm), Green(575-490nm) & blue(490-450nm)

– White – is mixture of all colours (equal stimulation of all

red, green, blue)

– Black is absence of colour but it is a positive sense so blind

eye does not see black, it sees nothing.

• Cones contain a pigment called as cone pigment:-

Photopsin + Retinal. Retinal portion is same in both rods

and cones only opsin part is diff. in cone pigment.

Colour Theory: Trichromatic Theory

• Young & von Helmholtz both proposed that the eye detects 3

primary colors. Eye uses 3 types of cones that respond to light

in the wavelengths of red/green/blue

• 3 types of cones in human i.e. red cones (erythrolabe 575

nm), green cones (chlorolabe 535nm), blue cones

(cyanolabe 430nm).

• All other colors can be derived by

combining these 3.

– Human eye can detect almost all

gradations of colour when only red,

green & blue monochromatic lights are

appropriately mixed in diff combination.

OPPONENT-PROCESS THEORY

• A competing theory of

color vision, which

assumes that the visual

system treats pairs of

colors as opposing or

antagonistic.

• 2 kinds of colour

processors which respond

VS

VS

VS

e.g. when ‘red’ fires ‘green’ is inhibited

Demonstration of the degree of stimulation of the diff colour

sensitive cones by monochromatic lights of 4 colours: blue, green,

yellow & orange.

R G B

Orange - 99: 42 : 0

Blue - 0 : 0 : 97

Yellow - 83 : 83 : 0

Green - 31 : 67:36

Rods Cones

• high sensitivity;

specialized for night vision

• more sensitive to scattered

light

• high amplification; single

photon detection

• more photopigment

• slow response, long

integration time

• lower sensitivity;

specialized for day vision

• more sensitive to direct

axial rays

• less amplification

• less photopigment

• fast response, short

integration time

Rods Cones

• low acuity; not present

in central fovea

• achromatic; one type of

rod pigment

• high acuity, concentrated in

central fovea

• chromatic; 3 types of cones,

each with a different pigment

that is sensitive to a different

part of the visible spectrum

• Photochemistry of Colour Vision:

Cones contain a colour pigment photopsin, found

in outer segment.

In the outer segment there are large no. of disc &

each of the disc is actually infolded shelf of a cell

membrane – 1000 disc in each cone.

Rhodopsin & colour pigments- Photopsin are :

conjugated proteins, incarporated into membranes

of the discs in the form of transmembrane

proteins.

Inner segment has cytoplasm with cell organelles

imp are mitochondria.

The synaptic body is the portion of the rod or cone that connects with

subsequent neuronal cells: horizontal & bipolar cells.

• Cones contain a pigment called as cone

pigment- Photopsin + Retinal.

• For rods it is: Rhodopsin + Retinal

• Thus Retinal portion is same in both

rods and cones, only opsin part is diff

in cone pigment.

Cone

s

Plasma

membrane

Sacs discs

s

Cilliary

neck

mitochondr

ia

Nucleus

Incident

light from

lensSynaptic

Terminal

Inner

Segment

Outer

Segment 30 nm

Generation of hyperpolarization receptor potential by rhodopsin

decomposition decreased flow of Na +ions into outer segment of rod

Photopsin /

Colour Pigment /

Retinal – visual cycle in the rod or cone.

Excitation of cones by hyperpolarization : Metarhodopsin IIexcites electrical changes in cones→ releases retinal (retinene)→activates G protein → activatesphosphodiesterase → catalysesconversion of c GMP → 5’- GMP→ closure of Na channels betweencone cytoplasm & extracellularfluid → ↓intracellular Na conc. →hyperpolarization response inbipolar cells→ signal in optic nerve

Action Potential in Gn

cells, Ap travels to V.Cx. For visual perception

+ve

removal of inhibition

(or in effect, excited)

(Formation of

metarhodopsin)

inhibitory

Three types of ganglion cells W, X, Y.

W ganglion cells:

40%, < 10 µm diam.

slow velocity: 8 m/sec

For directional movt., crude

rod vision

X ganglion cells:

55% , 10-15 µm diam,

Med. Velocity:14 m/sec

For Colour Vision,

Y ganglion cells:

5% of total, 35 µm,

fast velocity: 50m/sec.

Respond to rapid changes

in visual fields

Thomas Young & Von Helmholtz posulated the

theory of Colour Vision in humans: 3 kinds of cones

each containing diff photo pigment & maximally

sensitive to one of the 3 primary colours.

3 types of cones in human i. e.

red cones (erythrolabe – 575 nm)

green cones (chlorolabe – 535nm)

blue cones (cyanolabe – 430nm).

Light absorption by the respective pigments of the

rods & colour cones of the human retina

Red (570nm), Green (535nm) & blue (445nm)

Perception of colour

• At retinal level by ganglion cells

• At lateral geniculate body

• At visual cortex

Colour Contrast Mechanism by Ganglion

Cells:

• Some ganglion cells are excited by only one colour type of cone but

inhibited by second type. e.g. red causing excitation ( by direct

excitatory route thru depolarizing bipolar cells) , green inhibition

(by indirect inhibitory route thru hyperpolarizing bipolar cells) &

vice a versa.

• Similarly blue cones & combination of red & green cones for

yellow colour act as contrast for each other.

• Therefore by this colour contrast mechanism, colour differentiation

begins at retina itself & is not entirely a function of the brain.

– Thus, each colour contrast type of ganglion cell is

excited by one colour but inhibited by the

• Visual pathway-

• Ganglion cells→→optic nerves→→optic chiasma→→ crossing

of nasal half to opposite side →→ optic tract →→ synapse at

LGB →→ Geniculo calcarine fibers →→ Primary visual cortex.

• Analysis of visual detail, colour & conscious vision: From

primary visual cortex (Brodmann’s area no.17) into visual area II

(Brodmann’s area no.18) →→ medial, inferior & ventral region

of occipital and temporal cortex.

• Visual cortex has got 6 layers(I – VI).

– Geniculocalcarine fibers mainly terminate in layer IV.

– This layer is subdivided into a, b, cα & cβ.

– The output of Y ganglion cells end on cα layer of IV.

– The output of X ganglion cells end on a & cβ layer of IV.

Area no I & II Area no III, IV,

V & VI

Colour Blobs: Interspersed among the

primary visual column of some of the

secondary visual areas are special

column like areas called colour blobs.

They receive lateral signals from the

adjacent visual columns & are activated

specifically by colour signals. Therefore

these blobs are primary areas to decipher

the colour.

In colour blobs there are center surround

cells.

They are called double opponent cells →

stimulated by green center & inhibited by

green surround→ same blob inhibited by

red center & stimulated by red surround.

•At retinal level : by Ganglion cells

By this colour contrast mechanism, colour

differentiation begins at retina itself & is not entirely a

function of the brain.

Thus, each colour contrast type of ganglion cell is

excited by one colour but inhibited by the “opponent

colour”.

•At lateral geniculate body level : by parvocellualr

pathway

Layers1&2 magnocellular -large

neurons

input Y ganglion cells-rapid

conduction

pt. to pt. transmission poor, color

blind

Only black & white, movement,

location,

Spatial organization

Layers 3,4,5,6 parvocellular –

neurons –small to medium

Input X ganglion cells

moderate velocity of conduction

pt. to pt. transmission accurate,

color.

• Visual cortex has got 6 layers(I – VI).

– Geniculocalcarine fibers mainly terminate in layer IV.

– This layer is subdivided into a, b, cα & cβ.

– The output of Y ganglion cells end on cα layer of IV.

– The output of X ganglion cells end on a & cβ layer of IV.

Analysis of visual detail, colour & conscious

vision: From primary visual cortex (Brodmann’s

area no.17) into visual area II (Brodmann’s area

no.18) →→ medial, inferior & ventral region of

occipital and temporal cortex.

Area no I & II Area no III, IV,

V & VI

Colour Blobs: ……..

Interspersed among the

primary visual column of

some of the secondary visual

areas are special column like

areas called colour blobs.

They receive lateral signals

from the adjacent visual

columns and are activated

specifically by colour signals.

... these blobs are primary

areas to decipher the colour.

APPLIED:

• Colour Blindness : inability to perceive one or more

different colour is called as colour blindness

• if there is weakness for particular colour : anamoly

• complete absence is anopia

Classification of colour blindness:

Trichromats Dichromats

MonochromatsAll 3 types of cones 2 types of cones Only one type of cone

are present. are present. present and only

a) Protanomaly: red weakness shades of grey are

b) Deuteranomaly: green weakness appreciated.

c) Tritanomaly: blue weakness

a) Protanopia : red blindness

b) Deuteranaopia : green blindness

c) Tritanopia: blue blindness

Monochromatism and

Achromatopsia

•Monochromats have only one

cone pigment instead of the 3

primary color based pigments.

•Achromats have only rods and no

cones and are truly color-blind.

This form is extremely rare. Such

people are able to distinguish

objects only by brightness and

usually have very poor vision due

to the lack of cones.

Anomalous

Trichromacy (most

common) - the

affected person

has all 3 (thus tri-

chrom) cone

pigments but one

is abnormal.

•Protanomaly. A Protanomal has abnormal red-

sensitive cones and requires excess red to match

the yellow standard.

•Deuteranomaly. A Deuteranomal has abnormal

green-sensitive pigment and needs excess green

to match the yellow standard.

•Tritanomaly. Very rare. Abnormal blue-sensitive

cones. Has difficulty in distinguishing blues from

yellows and my use excessive blue to match the

yellow standard.

Anomalous Dichromacy

- the affected person

has only 2 cone

pigments and thus

cannot distinguish

certain colors.

•Protanopia - lack of red-sensitive cone

pigment.

•Deuteranopia - lack of green-sensitive cone

pigment.

•Tritanopia. Very-very rare. Lack of blue-

sensitive pigment. Can't distinguish blue from

yellow. Usually an inherited syndrome with

optic atrophy.

• Red green colour blindness : inheritance is

X linked disorder because the gene for

this pigment is located on short arm of X

chromosome

- Only males are sufferer , females are

carrier.

• Blue colour blindness : very rare , gene is

located on 7th chromosome

PRACTICAL:• Colour vision is to be tested before

employment in

– 1. Dye industry

– 2. Textile industry

– 3. Military services

– 4. Paints and printing industry

– 5. Before issuing driving license

• It is tested by

– 1. Ishihara Chart

– 2. Edridge Green Lantern

Normal person

read this as 74

whereas red green

colour blind person

will read this as 21.

Normal person

read this as 42

wheras red colour

blind person will

read this as 2

while green colour

blind will read this

as 4.

• This picture to the right shows

the (Vertical Pattern)

Edridge-Green Lantern, a

funnel-shaped colour

perception test lantern

(Vertical pattern) with

rotating colour discs, fitted for

electrical illumination.

Crucially its readings can be

taken independently of the

colour vision of the examiner.

Welcome to the world of colour blindness:

Normal Protanomaly Deuterano

maly

Normal Protanope Deuter

anope

world of colour blindness:

Thank You!

Veena

A quick colour blindness test…

Both normal and those

with all colour vision

deficiencies should

read the number 12.

Normal vision

should read the number

29.

Red - green

deficiencies should read

the number 70.

Total colour

blindness should not read

any numeral

Normal colour

vision should read the

number 5.

Red - Green

colour deficiencies

should read the number

2.

Total colour

blindness should not be

able to read any

numeral.

Normal colour vision

should read the number

6.

The majority of

those with colour vision

deficiencies cannot read

this number or will

read it incorrectly.

Normal colour

vision and those with

total colour blindness

should not be able to read

any number.

The majority of

those with red-green

deficiencies should read

the number 5.

Understanding Colour

Transformations

• red green deficiencies are very similar in terms

of perception

– red blind = protanope

– green blind = deuteranope

standard

protan

deutan

standard

protan

deutan

Fig. illustrates that the after image of a white object is the

opposing color to blue which is yellow. By fixating on the black

spot on the left for a minute and then glancing over to the right,

one sees yellow, not black, candles on the tree

• The most common test for color blindness uses

the Ishihara charts, which are plates

containing figures made up of colored spots on

a background of similarly shaped colored spots.

The figures are intentionally made up of colors

that are liable to look the same as the background

to an individual who is color blind.

• Some color-blind individuals are unable to

distinguish certain colors, whereas others have

only a color weakness.

• The prefixes "prot-," "deuter-,“ and "trit-" refer

to defects of the red, green, and blue cone

systems, respectively.

• Individuals with normal color vision are called

http://www.college-optometrists.org/

A portion of the spectrum is expanded at the bottom of the figure.

Layers of RetinaLayers of retina

1. Pigment layer

2. Layer of rods &

cones

3. Outer limiting

membrane

4. Outer nuclear

layer

5. Outer plexiform

layer

6. Inner nuclear

layer