Colour vision (CV)/ Physiology

1. Mechanism of colour vision: 2 theories: trichomacy, photochemistry of CV

2. Neurophysiology of CV
3. Phenomena a/w CV
4. Colour triangle based on photopigments – the colour metric: CIE, Munsell colour
5. Normal colour attributes
6. Colour blindness

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Colour sense: ability of eye to discriminate between colours excited by light of different WL. Colour
is a perceptual phenomenon.

COLOUR VISION:

1. Function of cones: better seen in photopic vision

2. Determination of colour: spectral composition of light from object, surroundings, state of
light adaptation.
3. Sensation of colour is subjective – individuals are taught it.
4. Three different cones: combined perform function of CV. Absorption spectrum of cone
pigments: BLUE 440 nm, GREEN 535 nm, RED 565 nm.
5. All colours: mixture of 3 primary colours:
Red 647 – 723 nm
Green 492 – 575 nm
Blue 450 – 492 nm
6. Attributes of CV: hue, intensity, saturation.
7. All colours have complementary colour – when properly mixed produces white sensation.
8. Perceived colour depends on other objects in the VF. Ie: red is seen as red in green/blue
light, as pale pink/white in red light.
9. Visible WL: violet  red. Shorter: UV light, longer than 750 nm, infrared. (UV & infrared –
beyond visibility). Humans actually can see UV light as blue cones retain some sensitivity
around 10 nm, but crystalline lens blocks all UV rays. After cataract, one can see some UV
rays.
10. Purkinje shift phenomenon: in dim light, all colours are seen as grey

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Theories of CV: 2 theories:

1. Trichromatic theory / Young- Helmholtz theory

Postulates: existence of 3 kinds of cones with different photopigments, maximally sensitive
to 1 of 3 primary colours. The sensation of any colour is determined by the relative
frequency of impulse from each of the 3 cones system: admixture of 3 primary colours in
different portions.
Noted to be correct cause: now demonstrated by identification & chemical characterization
of each of the 3 pigments by recombinant DNA technique: each having different absorption
spectrum.
 Concludes:
1. blue, green and red are primary colours
2. Cones with maximal sensitivity in yellow portion of spectrum are light at a lower
threshold than green.

Location
Rhodopsin Chromosome 3.
Cone Blue sensitive chromosome 7
Red & green sensitive arranged in a tandem array on the q arm of X
chromosomes.

Absorption spectrum of 3 cone pigments:

Colour sensitive aka Wavelength Absorb maximally in (nm) - peak

cone pigment sensitivity (WS)
Red Erythrolabe Long - LWS Yellow position, peak at 565 um.
Spectrum extends far enough into
long WL to sense red
Green Chlorolabe Medium - MWS Green portion, 535
Blue Cyanolabe Short - SWS Blue-violet 440

2. Opponent colour theory

i. Some colours are mutually exclusive.
ii. No such color reddish green.
iii. Difficult to explain on the basis of Trichromatic theory alone.

Both theories 1 & 2:

1. Colour vision is trichromatic at level of photorecptors

2. Colour opponency is explained by subsequent neural processing as discussed in
neurophysiology of colour vision.
Photochemistry of colour vision

1. Rhodopsin: 11 cis-retinal + opsin

2. Cone: 11-cis-retinal + photopsin
3. Photopsin: green & red sensitive cone pigments: 96% homology in AA sequence.
4. Each green & red homology with blue only 43%
5. Opsin (rod) + photopsin (cone): 41% homology
6. Bleaching cone & rod same, but cone in light of different WL.

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Neurophysiology of CV

Similar for vision in general. Photochemical changes in cone pigments followed by a cascade of
biochemical changes  produce visual signal: cone receptor potential, with sharp onset & offset.
Rod: sharp onset but slow offset.

Axn potential in photorecptors  electrical conduction  synapses of photoreceptors, bipolar,

ganglion & amacrine cells.

Cells Response
Horizontal 1. Luminosity response: hyperpolarizing response with a broad spectral
function
2. Chromatic response: hyperpolarization for part of spectrum & depolarizing
for remainder.
Bipolar Red light striking centre  hyperpolarization, green light in surroundings causes
depolarization.
Ganglion Colour coding. 3 distinct groups of ganglion cells with diff fx: W, X, Y.
X – mediates CV.
Can be stimulated by single/ few cones:
1. All 3 cones stimulate same ganglion cell  resultant in white colour

2. Opponent colour cell: some GC excited by 1 colour type (ie red) and
inhibited by another (ie green)  successive colour contrast.

Located in ganglion cells (18) & LG neurons (12, 13)

Two main types of colour opponent in ganglion cells:

i. Red- green opponent colour cells: uses signals from red & green
cones to detect red/green contrast within receptive field
ii. Blue- yellow opponent colour cells obtain a yellow signal from the
summated output of red and green cones, which is contrasted with
the output from blue cones within receptive field.

3. Double opponent colour cell: for both colour & space. Concerned with
simultaneous colour contrast. Have a receptive field with a centre &
surround. Response maybe ‘on’ to one colour in centre, while ‘off’ to it in
surround. Indicates the process of colour analysis begins in the retina & not
entirely a function of the brain.
Located in layer IV of striae cortex (19, 20).
Retina Trichromatic CV extends 20-30 degrees from point of fixation.
Peripheral to it, red & green indistinguishable, far periphery – all colour sense is lost
though cones are still found in this region.
Very centre of fovea (1/8 degree): blue blind.
Confrontation: object from periphery, individual first becomes aware of a colourless
object in periphery then as it advances, seen as salmon pink/ yellow eventually as
RED.
LGB 1. All LGB neurons carry info from more than 1 cone cell.
2. Colour info: relayed from ganglion cells  parvocellular portion of LGB
3. Spectrally nonopponent cells which give the same type of response to any
monochromatic light constitutes 30% of all LGB neurons.
4. Spectrally opponent cells: 60% of LGB neurons. These cells are excited by
some WL & inhibited by others, thus carry colour info. 4 types:
Red & green antagonism : +R/-G, +G/-R
Blue & yellow antagonism: +B/-Y, -B/+Y
Striae cortex From LGB  layer IVc of striae cortex (area 17)  blobs in layers II & III.
(centre- surround cells)  thin strips in visual association area  specialized area
concerned with colour: lingual and fusiform gyri of occipital lobe.

Phenomena a/w colour sense:

1. Simultaneous colour contrast

- Perception of particular coloured spot against coloured background
- ie: grey spot appears greenish in a red surround, reddish in a green surround.
- In general, a colour spot tends to be towards the complementary of the colour of the
surround.
- Function of double opponent cells of visual system
2. Successive colour contrast
- Colour of after image tends to be near complementary of primary image.
- Ie: looks at green spot for several seconds, then looks at grey card, one sees red spot on
card.
- Function of opponent cells in visual system.

3. Phenomenon of colour constancy

- Human eye continues to perceive colour of a particular object unchanged even after
spectral composition of light falling on it is markedly altered.
- Ie: new- fallen snow thought as white even looked by moonlight, highnoon or in blush of
setting sun.

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Colour Triangle Based on Photopigments: 3 dimensional graphic representation of all the colours
drawn on the basis of trichromacy of colour mixture. The 3 axes of this colour space, mutually at
right angles are scaled to represent various amounts of pigment absorption by each of 3 cone
pigments.

Colour metric: making sense that two individuals mean the same colour when naming it:
internationally in industry.

1. CIE colour space system (International Commission of Illumination) – for a precise ID of

colours for such items as textiles, paints, food colouring etc. Based on amounts of 3
1`colours to match a specified colours.

2. Munsell colour system: all colours are represented in a cylinder in terms of hue, value and
chroma (HVC). Covers colours used in medicine and industry.
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Normal colour attributes

1. Hue: dominant spectral colour is determined by WL of particular colour. Depiction of regions

of visible spectrum where ability to discriminate between adjacent WL is at its maximum.

Depicts 2 regions approx. 490 nm &

590 nm of visible spectrum where
the ability to discriminate between
adjacent WL is at its maximum

2. Saturation: degree of freedom from dilution with white. Measured by how much a particular
WL must be added to white before it is distinguishable from white. The more WL required,
the lesser the saturation.

3. Lightness: or brightness of colour depends on luminosity of component WL. Photopic –

peak luminosity function approx. 555 nm, scotopic (dim) 507 nm.
Purkinje shight: WL shift from maximum luminosity from photopic to scotopic viewing.
GRAFT: Luminosity curve.

Colour Blindness
 1. Normal colour vision individual: trichomate.
2. Defects in colour vision: defective (anomalous) or absent (anopia).
3. Maybe congenital or acquired.

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Congenital: X linked recessive, males (3-4%) > females (0.4%). Types:

1. Dyschromatopsia: means colour confusion dt deficiency of mechanism to perceive colours.

1. Anomalous trichromatic CV: mechanism to appreciate all 3 primary colours present, but
defective in 1 or 2 of them.
2. Dichromatic CV: faculty to perceive 1 of 3 primary colours is completely absent.
Anomalous trichromatic CV Defective appreciation Dichromatic CV
Protanomalous Red Protanopia
Deuteranomalous Green Deutranopia
Tritanomalous Blue Tritanopia

2. Achromatopsia: extremely rare: cone & rod monochromatism

Cone monochromatism Rod monochromatism

Characterized by presence of only 1 primary Maybe complete or incomplete.
colour  so person is TRULY colour blind. Inherited as autosomal recessive trait.
VA usu 6/12 or better Characterized:
1. Total colour blindness
2. Day blindness (VA ~ 6/60)
3. Nystagmus
4. Fundus: normal

Inheritance of colour blindness

1. 8% males, 0.4% females

2. Rare: tritanomaly & tritanopia
3. Males 2% colour blind are dichromats: protanopia/ deuteranopia
4. Males 6% anomalous trichromates: red or green sensitive pigment shifted in its spectral
sensitivity.
5. X-linked colour blindness skips a generation and appears in males of every 2nd generation.
6. Other X linked: haemophilia, duchenne’s muscular dystrophy.
7. Why common deutranomaly & protanomaly – cause of arrangement of genes near head-t-
tail tandem array on the q arm of the X chromosome & are prone to recombination (unequal
crossing over) during development of germ cells.

Acquired colour blindness

 1. May follow damage to macula or ON.
2. Usu a/w central scotoma or decreased VA

Type Cause
Blue-yellow impairment Retinal lesions such as CSR, macular odema, shallow RD
Red-green deficiency ON lesion ie optic neuritis, Leber’s optic atrophy & compression of
optic nerve
Acquired blue color defect Older age dt ↑ sclerosis of crystalline lens, it is owing to physical
(blue blindness) absorption of blue rays by ↑amber coloured pigment in the nucleus.

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Tests for colour vision

Purpose:

1. Screening defective CV from normal

2. Qualitative classification of colour blindness ie protans, deutrans, tritans
3. Quantitative analysis of degree of deficiency ie mild, moderate, marked.

Tests Function
1. Pseudo-isochromatic chart test: Patterns of coloured & grey dots which reveal one
- Ishihara plates pattern to normal individual & another to the colour
- HRR: Hardy-Rand- Rittler plates deficient.
Quick methods for screening.
2. Lantern test Subject has to name various colours shown to him by
- Edridge-Green lantern a lantern & judged based on mistakes.

3. Farnsworth-Munsell 100 hue test Spectroscopic test which subjects has to arrange
coloured chips in ascending order Judged on error
score. Greater score: poorer vision.
4. City University colour vision test Spectroscopic test where central coloured plate is to
be matched to its closest hue from 4 surrounding
plate.
5. Nagel’s anomaloscope test Observer is asked to mix red & green colours in such
a proportion that mixture should match the given
yellow coloured disc. Judgement is made from the
relative amounts of red & green colours & the
brightness setting used by the observer.
6. Holmgren’s wool test Asked to make a series of colour- matches from a
selection of skeins on coloured wools.