The document summarizes colour vision and the mechanisms underlying it. It discusses that colour vision is mediated by three types of cone photoreceptors sensitive to red, green and blue wavelengths. The signals from the cones are processed in the retina through opponent colour coding systems and transmitted to the lateral geniculate nucleus and visual cortex via the optic nerve pathways. Theories like the trichromatic and opponent process theories attempt to explain the perception of different hues. Defects in colour vision occur due to abnormalities in cone photoreceptors.

1. Dr samarth mishra

2. -Colour sense is the ability of eye to discriminate b/w colours excited
by light of different wavelengths.
-It is a function of cones.
-So better appreciated in photopic vision.
-Three types i.e red, blue and green.
-It is a perceptual phenomenon.

3. The Physics of Light
Some examples of the reflectance spectra of surfaces
Wavelength (nm)
%PhotonsReflected
Red
400 700
Yellow
400 700
Blue
400 700
Purple
400 700
© Stephen E. Palmer, 2002

4. -Many factors determine the colour perceived.
-the spectral composition of light from the object is important.
-but the spectral composition of light from the visual surroundings
and the state of light adaptation of eye also contributes.
-in dim light all the colours are seen as gray; this is called purkinje
shift phenomenon.
-White objects reflect all colours to eye, black absorbs all colours so
no light to the eye.

5. HUE
INTENSITYSATURATION

6.  The eye, especially the retina
 The optic nerve
 The optic chiasma
 The optic tract
 The lateral geniculate body
 The optic radiation
 The visual cortex
 The visual association cortex.

7. -cone pigment just like rhodopsin has 11-cis retinal and opsin part.
cone pigment
11-cis retinal (opsin)
- 11-cis retinal is similar to rhodopsin, the opsin part known as
photopsin is different than the opsin part of rhodopsin

9. -all the three cone pigments have about 41 percent homology with the
rod pigment rhodopsin.
-thus the photochemistry of rhodopsin can be applied to the cone
pigments.
-the only difference being that the three different types of cones are
bleached by light of different wavelength.

10. -Similar to photochemical changes, the physiological process
concerned with colour vision are also same as for vision in general.
-the action potential generated in the photoreceptors is transmitted by
electronic conduction to the other cells of the retina across the
synapses of photoreceptors, bipolar cells and horizontal cells
-and then across the synapses of bipolar,ganglion and amacrine cells.

12. HORIZONTAL CELLS:
-showed two completely different kind of response.
a) luminosity response :there was a hyperpolarising response with a
broad spectral function.
b) chromatic response : which was hyperpolarising for part of the
spectrum and depolarising for the remainder of spectrum.
-this provided the first physiologic evidence for opponent colour
coding.

13. BIPOLAR CELLS:
-recordings shows a ‘centre surround’ spatial pattern.
-red light striking the centreof these cells caused hyperpolarisation.
-Green light in the surroundings caused depolarisation.

14. AMACRINE CELLS:
-the exact role of these cells in colour vision is not clear.
-they may act as
‘automatic colour
control’.

15. GANGLION CELLS:
-at this level first direct evidence in the visual system for colour
coding is seen.
-three distinct groups of ganglion cells W,X,Y seen.
-colour sensation is mediated by the ‘X’ ganglion cells.
-A single ganglion cell may be stimulated by a number of cones or by
a few cones.

16. -when all 3 cones stimulate , the resultant signal is white.
opponent colour cell:
 Some ganglion cells are excited by one colour type cone(i.e red)
and are inhibiteb by other(i.e green) or vice versa.
 This is called ‘ opponent colour cell’ system and is concerned in
the ‘successive colour contrast’.

17. Double opponent colour cell:
-have a system which is opponent for both colour and space.
-the double opponent cells have a receptive field with a centre and
surround.

18. -the response may be ‘on’ to one colour (e.g red) in the centre and ‘off’
to it in the surround.
-while the response may be ‘off’ to green in the centre and ‘on’ to it in
surround.
This systems indicate that the process of colour analysis begins
in the retina and is not entirely a function of brain.

19. DISTRIBUTION OF COLOUR VISION IN THE RETINA:
-trichromatic colour vision mechanism extends 20-30 degrees from the
point of fixation.
-peripheral to this red and green become indistinguishable,and in the
far periphery all colour sense is lost ,although cones are still
found in this region of retina.
-the centre of fovea ( 1/8 degree) is blue blind.
-when a red test object is brought from the periphery in the field of
vision ,the individual first becomes aware of a colourless object in
the periphery.
-then as the object is advanced ,it is seen successively as salmon pink
or yellow and eventually red.

21. LATERAL GENICULATE BODY:
-All LGB neurons carry information from more than one cone cells.
-from ganglion cells colour information is then relayed to
parvocellular portion of LGB.
30 % LGB neurons:  spectrally non opponent cells.
60 % LGB neurons:spectrally opponent cells.
-these cells are excited by some wavelengths & inhibited by others.

22. - The LGN is segregated into six layers.
- Two magnocellular (large cell)
achromatic layers (M cells).
- Four parvocellular (small cell)
chromatic layers (P cells).
- Within the LGN P-cell layers there are
two chromatic opponent types:
red vs. green and blue vs. green/red.

23. These have been classified into 4 types:
a) Cells having red and green antagonism (with +R/-G)
b) Cells having red and green antagonism (with +G/-R)
c) Cells with blue and yellow antagonism (with +B/-Y)
d) Cells with blue and yellow antagonism (with +Y/-B)

24. © Stephen E. Palmer, 2002
G+R-
G+R-
R+G-
R+G-
Red/Green
Y+B-
Y+B-
B+Y-
B+Y-
Blue/Yellow

25. -Colour information from the parvocellular portion of LGB is relayed
to the layer IV c of striate cortex.( area 17)
-it then passes to blobs ( in layers 2 and 3).
-these blobs are ‘centre surround’ cells. (like the ganglion cells and
LGB)
BLOBS
visual association area
lingual & fusiform gyri ( occipital lobe)

28. THEORIES OF COLOUR VISION:
A ) TRICHROMATIC THEORY:
-Suggested by young
-Subsequently modified by helmholtz (1802).
-Therefore, it is called
young-helmholtz trichromatic theory.
-Postulates the existence of 3 kinds of photopigment.
-This photopigment is sensitive maximally to only one of the three
primary colours.
Hermann von Helmholtz

30. -the 3 primary colours being 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 system.
-the correctness of the young-helmholtz trichromatic theory of colour
vision has now been demonstrated by the identification and
chemical characterisation of each of the three pigments by
recombinant DNA technique.

31. ERYTHROLABE :
-Red sensitive pigment is also known as erythrolabe. Or long
wavelength sensitive(LWS) cone pigment.
-It absorbs maximally in a yellow position with a peak at 565nm.
-But its spectrum extends far enough into the long wavelength to
sense red.

32. CHLOROLABE:
-Green colour pigment also called as chlorolabe or medium
wavelength sensitive (MWS) cone pigment.
- It absorbs maximally in the green portion with a peak at 535nm.

33. CYANOLABE:
-Blue sensitive cone pigment is also known as cyanolabe or short
wavelength sensitive (SWS) cone pigment.
-It absorbs maximally in the blue violet portion of the spectrum with
a peak at 440nm.
-Thus , the young-helmholtz theory concludes that blue,green and
red are primary colours ,but the cones with the maximal
sensitivity in the yellow portion of the spectrum are light at a
lower threshold than green.

35. B) OPPONENT COLOUR THEORY:
-it was proposed by hering (1878).
-he pointed out that some colours
appear to be ‘mutually exclusive’.
-there is no such colour as ‘reddish-green’.
-such phenomenon can be difficult to explain on the basis of
trichromatic theory alone.
Ewald Hering

37. - [yellow-blue] and [red-green] represent opponent signals
producing four colour primaries red,green,yellow and blue, and
not just three.
- [ white-black ] opponency proposed by him has been abandoned in
most modern version of the theory.

38. according to opponent colour theory:
a) Red- green opponent colour cells use signals from red and green
cones to detect red/green contrast within their receptive field.
b) 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 fields.

39.  Explains why people with dichromatic deficiency are able to
match test field using only 2 primaries .
 How we see yellow though there is no yellow cone
 Explains Colour after images

40. ZONE THEORY:
-Proposed by donder (1881)
It seems that both the theories are useful in that:
-the colour vision is trichromatic at the level of photoreceptorss
-while , colour opponency is explained by subsequent neural
processing,at the level of ganglon cell onwards.
Trichromatic
stage
Opponent-
Process stage

41. COLOUR VISION DEFECTS:
-Color can be described in terms of hue( determined by wavelength)
and saturation (determined by amount of white light mixed).
-those with colour vision defects see fewer hues than normal.
HUE
INTENSITYSATURATION

42. TYPES OF COLOUR VISION DEFECTS:
Trichromatism: can differentiate all colours
(normal sight)
RED BLUE GREEN
Anomalous
Trichromatism : can differentiate all colours but one colour has
reduced or displaced sensitivity.
A)Protanomaly: displaced
sensitivity

43. RED BLUE GREEN
B)Deuteranomaly: displaced sensitivity
C)Tritanomaly: displaced sensitivity
DICHROMATISM: receptors missing for one type of cone
A)Tritanopia: missing
B)Deuteranopia: missing
C)Protanopia: missing
Monochromatism: totally unable to differentiate colors of equal
(achromatism) brightness

44. Color Blindness
What does the world look like to a color blind person?
Normal
Trichromat
Protanope Deuteranope Tritanope

45. .