1. Colour vision is the ability to perceive differences between wavelengths of light in the visible spectrum. 2. There are two main theories of colour vision: the trichromatic theory which proposes three types of cone cells sensitive to red, green, and blue light, and the opponent-colour theory which proposes the visual system interprets colours in an antagonistic way such as red vs green. 3. Colour signals are processed through the retina, lateral geniculate nucleus, and visual cortex, with different cell types involved in colour coding and perception at each stage.

1. Physiology of Colour vision
Othman Al-Abbadi, M.D

2. Definition
• The ability to discriminate between colours
excited by light of different wavelengths.

4. Visible light
• Electromagnetic energy
is wavelength between
380 -760 nm

6. Theories of color vision
1. The Trichromatic theory, or Young–Helmholtz
theory; states that there’re 3 types of cones, each
containing a different photopigment & maximally
sensitive to one of three primary colours… which
means that any colour consists of admixture of
three primary colours.
2. The opponent colour theory; states that the
visual system interprets color in an antagonistic
way: red vs. green, blue vs. yellow, black vs. white.

7. Usefulness of colour theories
• Trichromatic theory 
colour vision at the level
of the photoreceptors
• Opponent colour theory
 neural processing of
colour

8. Photochemistry of colour vision
• Capture of a photon Irreversible photochemical
isomerization of
chromophores
Series of Protein
conformational changes
Active
pigmen
t
Catalyst
Transducin
Active form
cGMP
phosphodiestera
se
Decrease
cGMP
Decrease
cation
conductan
ce
Hyperpolarizati
on of the
photoreceptors

9. • Blue or short(SWS)
– contain cyanolabe, which absorbs the short wavelengths
best. Its maximal sensitivity is at 440 nm.
• Green or middle (MWS)
– contain chlorolabe, which is best stimulated by
intermediate wavelengths. Its maximal sensitivity is to a
wavelength of 535 nm
• Red or (LWS)
– contain erythrolabe, which preferentially absorbs quanta
of longer wavelengths. It is best stimulated by light of a
wavelength of 565 nm, but its spectrum extends to the
long wavelengths.

10. • The 3 cone type represent the 3 primary
additive colors
• Each pigment has a distinct absorption
spectrum
• The peak of absorbance vary but their
absorbance spectra overlap so that a
combination of 1 , 2 or 3 react to a given light
stimulus

12. Signal processing in the retina
• Color perception require photon absorption
by the 3 classes of receptors with different
spectral responses
• The chemical results then transmitted to
opponent processes

13. Signal processing in the retina
• Color perception
require photon
absorption by the 3
classes of receptors
with different spectral
responses
• The chemical results
then transmitted to
opponent processes

14. • Generation of action
potentials at the level of
cones

15. Horizontal cells
• Two types of responses ;
– Luminosity response :
Hyperpolarizing response with broad spectral
sensitivity
– Chromatic response :hyperpolarizing for part of the
spectrum and depolarizing for the remainder
Evidence for the opponent color coding

16. • Bipolar cells :
• Center surround spatial pattern
• Not completely understood.
• Amacrine cells :
• Automatic color control

17. Ganglionic cells
• First Direct evidence for colour coding.
• When all 3 types of cones stimulate the same ganglion
the resultant signal is white
• Opponent color cell :
– Some cells are stimulated by one color type and inhibited
by the other
– Successive colour contrast
• Double opponent color cell :
– Opponent for both color and space
– Simultaneous colour contrast

18. Processing of color signals in LGB
• All LGB neurons carry information from more than
one cone cell
• Color information carried by the ganglionic cells is
relayed to the parvocellular portion of the LGB
• Spectrally non opponent cells constitute about 30%
of all LGB neurons
• Spectrally opponent cells make 60% (2 types):
– R+/G-
– R-/G+
– B+/Y-
– B-/Y+

19. Distribution of color vision in the
retina
• Trichromatic color vision mechanisms extend 20 –
30 degrees from the point of fixation
• Peripheral to this red green become
indistiguishable
• In the far periphery all color sense is lost
• The very center of the fovea (1/8 degree) is blue
blind

21. Phenomena associated with colour
sense
1. Simultaneous colour contrast
2. Successive colour contrast
3. Phenomenon of colour constancy
4. Hierarchy of colour coded cells
1. Opponent colour cells ganglion cells & LGB
2. Double opponent cells layer IV of area 17
3. Complex & hypercomplex colour coded cells
layers II,III,V,VI of area 17

22. Colour metric
• To overcome the problem of each person have
their name of one single colour.
1. CIE system
2. Munsell system

23. Normal colour components
Hue: the perceptual pigmentation
difference experience with varying
wavelengths, this is what we are
actually referring to when we use
terms such as "green" or "blue."
Saturation: refers to the degree of
freedom from dilution with white.
(Purity of the color)
Lightness: depends on the luminosity of
the component wavelength.

24. 490 nm 590 nm

25. PhotopicScotopic

26. • STATE OF DARK
ADAPTATION (PURKINJE
EFFECT)
• When the eye is light
adapted (daytime),
yellow, yellow green,
and orange appear
brighter than do blues,
greens, and reds. The
cones' peak sensitivity
is to light of 555 nm.
• at dusk, although the
brightness of all colors
decreases, blues and
greens appear to gain in
relative brightness
when compared with
yellows and reds.

27. Color Blindness
• Normal individual is said to be Trichromate
• Congenital
• Acquired

28. Congenital color blindness
• X-linked recessive
• Males 3-4% & females 0.4%
• Two types:
– Dyschromatopsia: colour confusion
• Anomalous trichromatism
– Protanomalous
– Deuteranomalous
– Tritanomalous
• Dichromatism
– Protanopia
– Deuteranopia
– Tritanopia
– Achromatopsia
• Cone monochromatism
• Rod monochromatism

29. Deuteranopia

30. Protanopia

31. Acquired color blindness
• Damage to the macula or optic nerve
• Usually associated with central scotoma or
decreased V.A
• Blue-Yellow deficiency retinal lesions as CSR,
macular edema
• Red-Green defieciency optic nerve lesions as
optic neuritis, optic nerve compression
• Acquired blue color defects (blue blindness) may
occur in old age due to increased sclerosis of
crystalline lens.

32. Color vision testing
• 1. Pseudoisochromatic color confusion charts
• Most common.
• Using Ishihara plates.

35. • 2. Hue arrangement tasks
• Farnsworth munsell 100
hue test
• Farnsworth panel D 15
• Lanthonthy desaturated
D-15
• Farnsworth-Munsell tests
use Munsell color chips
mounted in caps. The
colors differ only in hue.
They have the same
saturation and brightness.
There are two tests: the
D-15 and the FM-100.
• The current model of the
FM-100 actually has 85
chips.)

36. D-15 Test
• The D-15 hues, which are selected from all parts of the color
wheel, are provided in a box. The reference cap (blue) is fixed to
the box. The examiner removes the other caps from the box and
arranges them in random order. According to the manual, the
examiner then states, “The object of the test is to arrange the
buttons according to color. Take the button which looks most like
the reference button and place it next to it, then …” After the test
is finished, the examiner flips over the box and records the order of
the chips. Trichromats arrange them from 1 to 15.
• Deutans arrange them as follows: 1, 15, 2, 3, 14, 13, 4, 12, 5, 11, 6,
7, 10, 9, and 8;
• protans arrange them as 15, 1, 14, 2, 13, 12, 3, 4, 11, 10, 5, 9, 6, 8,
and 7.
• The examiner then connects the numbers on the score sheet in the
order in which the patient has placed them

39. FM-100 Test
• n the FM-100, 85 hues,
which, if arranged in a
circle, would make a color
wheel, are divided into
four boxes.
• The dominant
wavelengths of box one
run from red to yellow;
box two, from yellow to
blue green; box three,
from blue green to
purple; and box four, from
purple back to red