COLOUR VISION

Jagdish Dukre

Colour Vision

It is the ability of the eye to discriminate between

different colours excited by light of different

wavelengths.

Colour vision is a function of the cones

and thus better appreciated in photopic vision. In

dim light (scotopic vision), all colours are seen grey

and this phenomenon is called Purkinje shift.

Colour can be specified using three

properties:

(1) hue, which is closely related to wavelength, and

which is used to name a colour;

(2) saturation, which

describes the intensity of a colour; and

(3) brightness,

which indicates the intensity of light emitted or

reflected by the surface.

Distribution of colour vision in

Retina

Central 1/8 deg. Blue blind

Uptil 20-30 trichromatic

40-70 deg. Red green blind (dichromatic)

Thereafter monochromatic

Mechanisms of colour vision

Two theories proposed are :

1. Trichromatic theory

2. Opponent colour theory of Hering

Trichromatic theoryThe trichromacy of colour vision was originally suggested by

Young and subsequently modified by Helmholtz. Hence it

is called Young-Helmholtz theory.

It postulates the existence of three kinds of cones,

each containing a different photopigment which is

maximally sensitive to one of the three primary colours

viz. 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 systems.

characterization of each of the three pigments by recombinant DNA technique,

each having different absorption spectrum as below

Red sensitive cone pigment or erythrolabe or long wave length sensitive (LWS)

cone pigment, absorbs maximally in a yellow portion with a peak at 565 mm.

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

Green sensitive cone pigment orchlorolabe or medium wavelength sensitive

(MWS) cone pigment, absorbs maximally in the green portion with a peak at 535 nm.

Blue sensitive cone pigment Or cyanolabe or short wavelength sensitive (SWS) cone

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

The Young-Helmholtz theory concludes that

blue, green and red are primary colours.

It has been studied that the gene for

human rhodopsin is located on chromosome 3,

for the blue-sensitive cone is located on ch. 7

The genes for the red and green sensitive cones

are arranged in tandem array on the q arm of the X

chromosomes.

Opponent colour theory of

HeringThe opponent colour theory of Hering points out that some colours appear to be

‘mutually exclusive’.

There is no such colour as ‘reddish-green’, and such phenomenon can

be difficult to explain on the basis of trichromatic

theory alone.

According to apponent colour theory, there are

two main types of colour opponent ganglion cells:

Red-green opponent colour cells use signals from red and green cones to detect

red/green contrast within their receptive field.

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 field.

In fact, it seems that both theories are

useful in that:

The colour vision is trichromatic at the level

of photoreceptors, and

Colour apponency occurs at ganglion cell

onward

Associated Phenomenon

Simultaneous colour contrast

Successive colour contrast

Neurophysiology of colour

vision Processing and transmission

Horizontal cells : first physiological evidence

Of opponent colour coding

Ganglion cells : colour sensation by X

ganglion cells

Ganglion cells

Opponent colour cells

Double opponent colour cells

Have receptive field with centre & surround

It is ‘on’ to one colour in centre and

‘off’ to its complementary colour.

Thus analysis of colour begins in retina

itself.

Processing in LGB

received in parvocelluar portion

60% are opponent neurons

then relayed to 4c of striate cortex

Then to layers 2 & 3 (blobs)

to the lingual and fusiform gyri of occipital

lobe.

Thank you …