2. 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.
3. 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.
4. 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
5. Mechanisms of colour vision
Two theories proposed are :
1. Trichromatic theory
2. Opponent colour theory of Hering
6. Trichromatic theory
The 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.
7. 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
8. 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.
9. Opponent colour theory of
Hering
The 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.
10. 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
12. Neurophysiology of colour
vision
Processing and transmission
Horizontal cells : first physiological evidence
Of opponent colour coding
Ganglion cells : colour sensation by X
ganglion cells
13. 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.
14. 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.
