COLOUR VISION & COLOUR BLINDNESS

- AKSHAY.B.K.

• It is the ability of the eye to discriminate between different colours excited by light of different wavelengths.

COLOUR VISION

THE PHYSICS OF LIGHT

400 500 600 700

ELECTROMAGNETIC SPECTRUM

VISIBLE SPECTRUM

10-14 meters 106 meters

Wavelength (nm)

CosmicRays

GammaRays X-rays UV Infra-

RedMicro-waves TV RadioLight

Light: Electromagnetic energy whose wavelength is between 400 nm and 700 nm. (1 nm = 10 meter)

•Colour perception by the eye is due to two receptors present in the layers of retina .

RODS (scotopic vision)CONES (photopic vision)

cone

rod

•These cells contain photosensitive pigments.•But colour vision is mainly attributed as function of cone cells.

LAYERS OF RETINA

VISUAL PATHWAY

THEORIES OF COLOUR VISION

1) TRICHROMATIC THEORY (YOUNG-HELMHOLTZ THEORY)

•This theory proposes the existence of three kinds of cone cells ,each containing a different photopigment which is maximally sensitive to one of the three primary colours ( 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.

•Three different photosensitive pigments:1. RED SENSITIVE CONE PIGMENT2. GREEN SENSITIVE CONE PIGMENT3. BLUE SENSITIVE CONE PIGMENT

FIG: THOMAS YOUNG & VON HELMHOLTZ

RED SENSITIVE CONE PIGMENT also known as erythrolabe/long wavelength sensitive cone pigment(LWS).Max absorption in yellow portion at 564 nm.

GREEN SENSITIVE CONE PIGMENT also known as Chlorolabe/medium wavelength sensitive cone pigment (MWS). Max absorption in green portion at 533nm.

BLUE SENSITIVE CONE PIGMENT also known as cyanolabe/short wavelength sensitive cone pigment(SWS). Max absorption in blue-violet region at 437nm.

2) OPPONENT COLOUR THEORY OF HERING

According to this theory ,some colours appear to be mutually exclusive i.e there is no such colour as reddish-green and such phenomenon can be difficult to explain on the basis of trichromatic theory alone.

Therefore HERING proposed the existence of two main types of colour opponent ganglion cells:

1. RED-GREEN OPPONENT COLOUR CELLS They use signals from red and green cones to detect red/green contrast within their respective field.

2. BLUE-YELLOW OPPONENT COLOUR CELLS They obtain a yellow signal from the summed output of red and green cones , which is contrasted

with the output from blue cones within the respective field.

fig: EWALD HERING

RED GREEN OPPONENT CELLBLUE YELLOW OPPONENT CELL

TESTS FOR COLOUR VISION OBJECTIVES OF THE TESTS:

•Screening defective colour vision from normal.•Qualitative classification of colour blindness.(protan,deuteran,tritan)•Quantitative analysis of deficiency(mild/moderate/marked).

Different colour vision tests:

•PSEUDO-ISOCHROMATIC CHARTS.•EDRIDGE-GREEN LANTERN TEST.•FRANSWORTH-MUNSELL 100 HUE TEST.•CITY UNIVERSITY COLOUR VISION TEST.•NAGEL’S ANOMALOSCOPE.•HOLMGREN’S WOOLS TEST.

Optical illusion.

1.PSEUDO-ISOCHROMATIC CHARTS

•Test done using ISHIHARA PLATES/CHARTS.•Patterns of coloured and greydots which reveal one pattern to the normal individuals and another to colour deficients.•Quick and easy method.•Ishihara charts are mainly used to detect red-green deficiency.

ISHIHARA CHARTS

INTERPRETATION OF ISHIHARA TEST RESULTS:

•The concise version of ishihara chart contains 14 plates.•Scoring is made on the basis of first 11 slides.

•10/11 is considered normal.•7/11or less is abnormal.•Those scoring 8 or 9/11 need further testing.(fransworth munsell hue test etc..)•Patient scoring 10/11 need not be assesed further.

HRR PLATES

Other tests based on same principle:

•HARDY –RAND-RITTLER PLATES (HRR).•AMERICAN OPTICAL COLOUR PLATE TEST.

WHAT DO THE PLATES MEAN???

•Plate 1 is for explaining the test.•Plates 2-5 , red-green deficient patients may see different numbers.•“Plate -9”is special i.e is only red-green defecient persons can appreciate the number in the plate.•Plates 12-14 are those for red-green deficient persons.(to differentiate between protanopia and deuteronopia)

2.EDRIDGE GREEN LANTERN TEST

• Subject has to name the various colours shown to him by a lantern & judgement is made by the mistake he makes.

3.FRANSWORTH-MUNSELL 100 HUE TEST

• Spectroscopic test.•Subject has to arrange coloured chips in the ascending order•The colour vision is judged by the error score.

4.CITY UNIVERSITY COLOUR VISION TEST

•Spectroscopic test.•Central coloured plate is to be matched to its closest hue from four surrounding colour plates.

5.NAGEL’S ANOMALOSCOPE•Observer is asked to mix red and green colours in such a proportion that the mixture should match the given yellow coloured disc. Judgement about defect is made from the relative amount of red and green colours and the brightness setting used by the observer.

6.HOLMGRENS WOOLS TEST

•In this the subject is asked to make a series of colour matches from a selection of skeins of coloured wools.•Judgement is made on the basis of colour matches made by the subject.

COLOUR BLINDNESS•Individuals with normal colour vision are called trichromates.•In colour blindness, mechanism to appreciate one or more primary colours is either defective(anomalous) or absent (anopia).

Broadly classified in two types:

1. CONGENITAL COLOUR BLINDNESS 2. ACQUIRED COLOUR BLINDNESS

A. CONGENITAL COLOUR BLINDNESS

•It is an inherited condition.

• X linked recessive disorder.

•Males (3-4%) are more affected than females (0.4%).

classified in to two types:

1. DYSCHROMATOPSIA.2. ACHROMATOPSIA.

1.Dyschromatopsia it is confusion of colours due to deficiency of mechanism to perceive colours.

Classified in to three types:

i. ANOMALOUS TRICHROMATIC COLOUR VISION: Here the mechanism to appreciate all the three primary

colours is present but is defective for one or two of them.

It is further classified in to three types:a)PROTANOMALOUS: Defective red colour appreciation.b)DEUTERANOMALOUS: Defective green colour appreciation.c)TRITANOMALOUS: Defective blue colour appreciation.

ii)DICHROMATIC COLOUR VISION: In this condition the person affected will not be able to perceive one of the three primary colours. Such individuals are called DICHROMATES.

Classified in to three types:a) PROTANOPIA- complete red colour defect.b) DEUTERANOPIA-complete green colour defect.c) TRITANOPIA-complete blue colour defect.

RED-GREEN DEFICIENCY It is more commonly seen. this defect is a source of danger in certain occupations

( drivers , sailors, police etc…)

iii) BLUE CONE MONOCHROMATISM (BCM) •It is a condition of complete absence of red and green cone function.

2.ACHROMATOPSIA (2nd variety of congenital colour blindness)

• Extremely rare condition .•There is rod cell monochromatism.•It may be of complete type or incomplete type.•Inherited as an autosomal recessive trait.•Both sexes (M/f) are affected equally.

•It is characterized by:

TOTAL COLOUR BLINDNESS.DAY BLINDNESS ( VISUAL ACUITY 6/60)NYSTAGMUS.FUNDUS IS USUALLY NORMAL. ACHROMATOPSIA- BLACK AND WHITE VISION!!!!

B. ACQUIRED COLOUR BLINDNESS

•It may follow damage to macula or optic nerve.•Usually associated with a central scotoma or decreased visual acuity.

BLUE-YELLOW IMPAIRMENT:• Seen in retinal lesions such as CSR, macular oedema, shallow retinal detachment.

RED-GREEN DEFICIENCY:• Seen in optic nerve lesions(optic neuritis),lebers optic atrophy , compression of optic nerve.

ACQUIRED BLUE COLOUR DEFECT :Usually occurs in the old age due to increased sclerosis of the crystalline lens.

TREATMENT?????

Colour Blindness CureCuring colour blindness is currently impossible. 99% of colour blind males and females are colour blind as a result of defective genetics on the X chromosome. To cure this colour blindness would require some form of gene therapy, repairing the damaged chromosome.

Colour Blindness CorrectionRecent developments in light filtering lenses have made it possible to provide colour blind people with a greater ability to distinguish between certain shades that otherwise look the same. Many critics claim they are tailored specifically to passing the Ishihara tests for colour blindness and have no real world value.

Fig : light filtering glasses.

THANK YOU…..