Color vision and physiological processes

Color vision process, physiology, defects, interpretation, types of color vision defect, interpretation and management

1. RAJU KAITI
Consultant Optometrist
Nepal Eye HospitalNepal Eye Hospital

2. Objectives
To define color and color vision
To define color vision types
To understand the theory behind the color vision

3. What is cOlOr???
Color is that what one perceive due to the property of
different wavelengths.
An aspect of visual perception, characterized by the
attributes of hue ,brightness and saturation and resulting
from stimulation of the retina by visible photopic light
levels.
It is not a physical property.
Isolated visible wavelength i.e. monochromatic lights are
commonly named by the color sensation they evoke when
seen in isolation.

4. cOlOr…
There is no one to one relationship between wavelength and
color.
Lack of correlation between wavelength and color
Depends on number of parameters
Wavelength or band of wavelengths coming from the object
Wavelengths coming from other objects in the field of view
Wavelength that the observer was looking at before he looked
at the object

5. sOMe terMs
METAMERIC COLOR: spectrally different radiations
that produces the same color under the same viewing
conditions (metamerism)
COMPLEMENTARY COLORS : One of a pair of colors
which, when mixed additively produce white or grey
(achromatic sensation).
ACHROMATIC: A visual sensation resulting from a
stimulus having brightness, but devoid of hue or
saturation

6. intrOductiOn
Colour vision
Perception of Colour
induced by different
wavelength of visible
spectrum
Only present in daylight
(photopic vision)
Function of Cones
Absent at scotopic
vision

7. cOlOr perceptiOn
Cone cells in the human eye
Trichromatic color vision
Opponent mechanisms
Cone typeCone type NameName RangeRange Peak sensitivityPeak sensitivity
SShort hort
wavelengths wavelengths
of lightof light
β (Blue)β (Blue)400..500 nm400..500 nm440 nm440 nm
MMediumedium γ (Green)γ (Green)450..630 nm450..630 nm544 nm544 nm
LLongong ρ (Red)ρ (Red) 500..700 nm500..700 nm580 nm580 nm
The visual system combines the information from each type of receptor
to give rise to different perceptions of different colors
(wavelengths of light)

8. cOne sensitivity

9. cOlOr perceptiOn
Total cone population
 64% red cones
32% green cones and
4% blue cones.
Each type is most sensitive to a specific portion of the visual
spectrum.
The stimulation of cones in various combinations accounts for the
perception of colours.
E.g.:
Perception of yellow results from a combination of inputs from green
and red cones and minimum from blue cone

10. cOlOr visiOn
is the result of
Nature of the physical world,
The physiological response of the eye (more strictly the
retina) to light
The neural processing of the retinal response by the
brain

11. cOlOr visiOn
The human visual system can detect the range of light
spectrum from about 400 nm (violet)-700 nm (red).
Our visual system perceives this range of light wave
frequencies as a smoothly varying rainbow of colors.
 We call this range of light frequencies
visual spectrum.

15. perceptiOn Of cOlOr
Varies complexly as a function of :
Spectral composition of light
coming from the subject
Emanating from the surrounding objects
State of light adaptation in the subject prior to viewing any
given object
Color constancy -the phenomenon in which; apparent
color of an object does not seem to vary appreciably
when wavelength and intensity of light illumination of
object are altered.

16. cOlOr cOnstancy
Refers to the approximately constant color appearance
of objects as lighting conditions change.
Assist us in identifying objects as lighting conditions
vary.
Not absolute
Occurs over a wide range of photopic illumination
levels for the same light source.

17. cOlOr cOnstancy…
An object which looks yellow in white light will look
redder in tungsten light but is still described as yellow
because all other objects in the scene have undergone
similar transformation.
Also called relation color constancy
Is a function of color memory mediated by processes in
area V4 of the visual cortex.
Lesions in visual cortex involving V4 area may affect a
person’s ability to remember familiar color names.

18. cOncepts Of cOlOr visiOn
All color experienced by three psychological impressions:
1)hue
Strongest effect on color
Major determination of principle colors (RYGB)
Property of stimulus which it may share with one or more
particular sectors of rainbow.
Function of wavelength
200 varieties

19. BEZOLD –BRUCKE PHENOMENON
Most wavelength show a slight changes in hue as the
stimulus intensity changes
Bipartite field is used.
E.g. greenish-yellow test wavelength(550nm)-As intensity is
increased, it appears to be of a longer wave length(more
yellowish).So, it is necessary to reduce its wavelength to
maintain the initial hue.
Intensity-wavelength plot shows tilts line .
All stimuli that falls on this line, referred to as a hue contour
line, has the same hue.

20. Three wavelengths remain a constant hue as
intensity is increased-indicated by nontilted hue
contour lines.
Wavelengths are 478nm(blue),503nm(green) and
578nm(red)
These hues are called unique hues.
The wavelengths are called invariant wavelengths or
invariant points.

21. A simple rule-stimuli with wavelength that is shorter
than unique blue (blues and violets
below478nm)appear more blue as the intensity is
increased and stimuli longer than 478nm appear more
yellow as the intensity increased
Hue contour line of stimuli shorter than 478nm will
tilts to the left and longer than 478nm will tilts to right
as intensity is increased.
 physiological basis is provided by color opponent
theory(??..)

23. 2. SatURatiON:
Reflects how much a hue has been diluted by
grayness
At short and long wavelength,20 distinguishable
steps of saturation for each hue
In middle spectral, 6 distinguishable steps of
saturation

24. SatURatiON
Attribute possessed by chromatic stimulus by virtue of its
being resolvable into a single hue only.
Also called colometric purity
Ratio of luminance of test light to the sum of test and white
light
i.e. L√/(L√+Lw)
The more the white ,the less is the saturation and looks
faded and wasted.
E.g pink (can be converted to good deal of red)

25. 3.BRigHtNESS
Sensation shared with achromatic visual systems
Short wavelength do not contribute
Have 500 distinguishable steps of brightness for
every hue and grade of saturation
More than million graduations to detect the shape

26. These three attributes are mutually dependent.
Thus, the some stimulus may appear to be deep red
when its brightness is low, but orange when it is high.

27. COLOR CODiNg PROCESS
Color information at the retinal level is conveyed
to higher visual center in LGN and striate cortex
In LGN, color opponent system synapses
structures in parvocellular layers (both single and
double opponency).
Color information at LGN is transferred into
either blobs or inter blobs within striate cortex.

28. Colour vision deficiency, or colour
blindness
The inability to distinguish certain colours.
Occurs when one or more of the cone types is
missing or defective to any extent

29. tyPES Of CV DEfiCiENCy
Congenital
Acquired

30. CONgENitaL aCqUiRED
Other visual functions
(e.g: visual acuity, visual
field, electroretinogram)
are normal
Other visual abnormalities
are found
The defect is stable The defect may progress or
regress
The defect is symmetrical
in both eyes
The defect is often
asymmetrical.
Errors on tests are
consistent and reproducible
Test results may vary
The patient names colors
correctly
The patient may name
colors incorrectly

31. tyPES Of COLOR BLiNDNESS
On Verriout’s classification and findings from
Fransworth Munsell 100 Hue test
1.Type I Protan like : Red blindness
2.Type II Deutran like : Green Blindness
3.Type III Tritan like : Blue/ yellow Blindness
4.Any Combination of above

32. KOLLNER'S RULE
As a general rule,
Diseases involving optic nerve , inner retina, visual
pathways and visual cortex produce
Red / Green deficiencies resembling Protan/ Deutran
Where as diseases involving outer retinal diseases and
media changes
 Blue/ Yellow deficiencies resembling Tritan

33. ExCEPtiON tO KOLLNER’S RULE
Degenerative conditions of retina
Cone dystrophy and Stargardt's disease
Predominantly Red-Green defect.
Optic nerve diseases
Autosomal dominant optic atrophy and glaucoma
Predominantly Blue defect

35. tHEORiES Of COLOR ViSiON
Trichromatic theory
Opponent colors theory
Zone theory

36. tRiCHROMatiC tHEORy
Operates at the receptor level
Postulated by young and proposed after color matching
experiment by Helmholtz
Known as Young Helmholtz Maxwell theory
Based on
3 classes of cones receptors sub serving color vision
“color match in the visible spectrum possible by appropriate mixing of
three primary colors”

37. 3 classes of cones
1
st
class
SWS,4%
Blue cones
Most sensitive to blue violet wavelength around 435nm
2
nd
class
MWS,32%
Green cones
Most sensitive to blue violet wavelength around 530nm

38. 3
rd
class
LWS,64%
Red cones
Most sensitive to greenish-yellow wavelength around
565nm
“They are overlapped so no individual class of cones can
be stimulated in isolation by any one wavelength”

39. The most direct evidence of presence of three classes of
cones is given by microspectrophotometry.
Three classes of cones in human retina are with different but
overlapping sensitivities.
Occurs because the sensitivity of all visual pigments falls off
sharply on the long wavelength side of the peak, but much
less sharply on the short wavelength side of the peak .

40. R-G cones do nearly all the work in color vision
R>G>B order of dominance creates a “warm” color bias in
our color experience.
The photo pigment in
 all the cones (and rhodopsin, the photo pigment
contained in rods)
 are variations on a single primitive opsin receptor
molecule,
which evolved by substituting or adding amino acids
within the basic opsin structure

41. Each molecule of cone photo pigment consists of
chromophore and opsin.
The chromophore, which is identical for all cone photo
pigments, is retinal(an aldehyde of vitamin A).
Light quanta are absorbed by the chromophore initiating
the series of events leading to vision.
It is the opsin, virtually inert chain of amino acids that
determines the absorption characteristics of the photo
pigment molecules.

42. Each class of cones has a different opsin. The genes for the
photo pigment of M and L cones are situated on the X-
chromosome.
So, CV deficiencies in which either the M or L cone is missing
are inherited in sex-linked manner.
The gene for S cone photo pigment is on chromosome 7 and
for rhodopsin is found on chromosome 3.
The M and L cone photo pigment genes are exceedingly
similar, showing 98% homology.
The homology of the S cone photo pigment to the M and L
cone photo pigment genes is 40%.

43. TrichromaTic Theory…
One of the important empirical aspects of this theory
is that
Color match in the visible spectrum possible by
appropriate mixing of three primary colors
Which primary color used is not important
But mixing two of them until that produce the third

44. color maTchinG
Almost any wavelength or band of wavelengths can be
matched by a mixture of three well-chosen
monochromatic lights(usually 400-450,510-520 and 630-
700nm)
Hold under various states of adaptation
Breaks in certain circumstances –at very high intensities
Self screening occurs when the density of photo pigment
in a receptor is sufficiently large than at the near end of
the receptor.

45. color maTchinG…
Not much self-screening at human cone receptor.
At high intensities more pronounce
Color matches in fovea also break in other areas retina
Partly due to macular pigment and
Partly due to absent of rods there

46. color maTchinG eXPerimenTs
RETINAL DENSITOMETRY
A very dim measuring light is directed onto retina .
The amount of reflected light is measured.
Reflected light<incident light (because of retinal
absorption)
Same procedure is repeated across the spectrum to obtain
absorption for the retinal receptors.
Esp. for M and L cone photo pigments-Done in areas
having only one cone(in fovea of red-green dichromate)

47. Not sensitive to allow determination of the photo
pigment absorption characteristic in tails of the curves.
Retinal densitometry fails in the tails
Tails of photo pigment spectra are critical for predicting
color matching data.

48. MICROSPECTROPHOTOMETRY
Technically difficult procedure.
Retinal tissue is back illuminated with monochromatic
light .
Light is directed towards a single cone.
Difference between amount of light incident and
transmitted through cone is determined.
Repeated across the spectrum to obtain a cone absorption
specgtrum.

49. Why rod cells noT
conTribuTe To color
vision???
CV occur at photopic condition ,rhodopsin pigments
saturate at lower luminosities
Temporal phase difference b/w rod and cone system-75-
100ms lag of rod in dark-adapted state
However, interaction between rod-cone systems is
indicated in dichromats for CV processing

50. Cones- fundamental units of visual information,
not the photo pigments
The idea that our perception of millions of colors
depends on just three distinct color receptors is
called the trichromatic theory of color vision
This framework is used in
Color stimuli specification
Interpretation of color by eye from light
Insufficient to explain CV

51. oPPonenT colors Theory
Ewald hering(1878).
Contradicts the Young –Helmholtz trichromatic
theory.
Explains four physiological color primaries, R, G, Y, B
Explains the phenomena of after-image (Y-B)
An additive mixture of red and green light gives
yellow, not a reddish green.

52. oPPonenT colors Theory
This opponent process creates
the four unique hues red, green, yellow and blue.
The brightness or lightness of a color is determined by
the luminosity channel i.e. pair of black-white.
These six fundamental color sensations can combine
to give any visible colors.

53. Proposes that color is processed by bipolar hue
channels.
By bipolar we mean that at any instant, each
channel can signal only one of the two attributes
it is capable of coding .

54. eXPlanaTion of color afTerimaGes
by The oPPonenT-Process Theory..
When one member of the color pair is "fatigued" by
extended inspection, inhibition of its corresponding pair
member is reduced.
This increases the relative activity level of the
unfatigued pair member and results in its color being
perceived.

55. Explain thE phEnomEnon of aftEr imagE EffEct
   Complementary Afterimage

56. opponEnt colors thEory
After-image: visual sensation persisting after the
original stimulus has been removed.
Formation of after-image is still obscure,but no doubt
in retinal origin.
Complementary after-image
Homochromatic after-image
Positive after-image
Negative after-image

58. opponEnt colors thEory
It describes;
The perceptual qualities of color vision;
That is derived from the neural processing of the receptor
signals in two chromatic and an achromatic channel.
Explains that;
Mixtures of lights of different colors could produce lights
of yet another color or even appear colorless.
Red + Green = Yellow
Blue + Yellow = White
Thus, color seems to be mutually exclusive or opponent of
one another.

59. opponEnt colors thEory
Relates to sensation and not to relations between
stimuli and is based on the occurrence of metabolic
changes in three retinal substances which mediate red-
green, yellow-blue and white-black sensatins depending
on whether the substances are anabolised or
catabolised.

60. singlE opponEnt color cElls:
In such color cell, stimulation by yellow light increases
tonic firing .
In the same cell, stimulation by blue light inhibits or
completely eliminates the firing rate.
White light such cell produces no response as it both
inhibits and excites the cell.
This system is concerned with successive color contrast.

61. sUccEssiVE color contrast
More commonly described as colored after images .
When one stares at a red spot for several seconds then
looks at a gray card ,one sees a green spot on the card.
When green is followed by white ,the white appears
reddish, because. of successive color contrast
Not much important phenomena in color vision .

62. DoUblE opponEnt cElls:
Cells opponent for both color and space.
Have receptive cells that have both centers and
surrounds in there receptive fields.
The centers of the field may be stimulated by red
light and inhibited by green light.
Surround shows opposite properties.
This system is concerned with simultaneous color
contrast.

63. simUltanEoUs color contrast
Is usually demonstrated by observing the color of a
spot in a surround.
The general rule is that the color of the spot tends
towards the complementary of the color of surround.

64. noncolor opponEnt cElls
These neurons respond to all spectral stimuli with
excitation.
Not inhibited by any wavelength.
Don’t have color coding capabilities.
It is monochromatic??
A monochromat can adjust the intensity of any two
wavelengths such that they appear identical.

66. ZonE thEory
In 1881Donder proposed-color vision is processed in a
series of zones in the visual pathway.
Trichromacy occurs at one level and opponency on
other.
At receptor level, vision is trichromatic and mediated
by 3 classes of cones.

67. ZonE thEory ctD…
Electrical signals from the cones are processed in
neural layers of the retina.
Two opponent color channels and a luminance channel
in ganglion cell level.
Processed electrical signals

68. trichromacy with color opponEnt
intEractions:
James and Hurvich
Both theories should be combined to
explain fully the perception of color.
Thus color vision must occur in;
1
st
stage-cones level, CV is trichromatic
2
nd
stage-signals are transformed into opponent
color form

69. L and M wavelength cone system should interact in
visual system to form Green-Red opponent system
S and L wavelength cone system should interact in
visual system to form Blue-Yellow opponent system

70. conclUsion
The cells specifically sensitive to color(hue) exist only
in visual cortex.
The cells of the retina and LGB initiate the color-coding
process.
Color vision could be best explained by combining
trichromacy with color opponent interactions.
“this hybrid is the two stage model of color vision.”

71. Trichromacy describes 3 types of cones, color matching
and color vision up to receptor level.
The findings of color opponent neurons in visual system
tells us that receptoral information is coded in an
opponent fashion at postreceptoral levels.
Three classes of cones are wired together at
postreceptoral levels such that they are spectrally
antagonistic(early at the level of horizontal cells).

72. THANK YOU!