Colour vision Flashcards
What is colour vision?
The ability to discriminate between different colour excited by light of different wavelengths
Which photoreceptor has the function of colour vision?
Cones
When are cones activated?
In photopic vision= bright light
When are rods activated?
In scotopic vision =low light, all colours are seen as grey in low light
What is the purpose of colour vision?
Identifying objects Survival value eg. ripe vs unripe fruit Enriches perceptual experiences
Which area in the brain is damaged causing colour blindness?
Lingual Gyrus (area V4)
What is the clinical name for colour blindness?
Achromotopsia
What are the 3 types of cones? (each containing a photopigment)
S cones contain S pigment (short wavelength M cones contain M pigment (medium wavelength L cones contain L pigment (long wavelength
What do these pigments do?
Each cones contains a different photopigment which are photosensitive meaning they respond to particular wavelengths of light:
What light does S pigment absorb?
Blue
What light does M pigment absorb?
Green
What light does L pigment absorb?
Orange/Red
What is colour blindness caused by?
The absence of one of the pigments (S/M/L)
If one of the pigments is absent, what is this referred as? What is a person with normal colour vision called
Dichromacy or the patient is a dichromat Normal= trichromat
Where is the word dichromacy derived from?
The term is from di meaning “two” and chroma meaning “colour”. (as only 2 out of the 3 pigments are present)
What is the loss of S cones called (loss of BLUE vision)?
Tritanopia (very rare)
What is the loss of M cones called (loss of GREEN vision)?
Deuteranopia
What is the loss of L cones called (loss of RED vision)?
Protanopia
Which types of colour deficiency are congenital (x-linked)
Red-green deficiency (protanopia and deutronopia)
Which gender is congenital colour defect more common in and why?
Males as X linked disorder (Males have X,Y chromosomes whereas females have X,X so for male to have colour vision defext only one X chromosome needs to be defective but for female both have to be defective)
What do protanopes see?
They can’t see red, orange and green-it appears brown and faded
Purple is indistiguishable from blue as there is no red component
They see blue and yellow as normal
What do deautranopes see?
Same as protanopes
They can’t see red, orange and green-it appears brown and faded
Purple is indistiguishable from blue as there is no red component
They see blue and yellow as normal
What colour appear very clear to people with red-green defects?
Blues and yellows
What is Protanomalia and Deuteranomalia colour defects?
Protanopia is complete & Protanomaly is partial Deuteranopia is complete & Deuteranomaly is partial
Protanomaly and Deautronoamly happens when the cones are present but don’t function properly whereas in ProtANOPIA and DeautrANOPIA the cones are missing completely.
What is total colour blindness?
Tritanopia A failure in the appreciation of blue and yellow may be termed tyritanomalia if partial, and tritanopia if complete
What every day tasks are affected in patients with a colour vision defect?
Matching colours clothes, paints Ripe vs unripe fruit, judging if meat is cooked Recognising sunburn and skin rashes Road traffic signs
What difficulty to protans have on the road and why?
Due to their red light insensitivity, they will have trouble seeing low intensity red lights such as car and bicycle retroreflectors
What jobs can a person with a colour vision defect who fails the Ishihara not do in the UK?
-Pilot -Air traffic controller -Flight engineer or flight navigator in the armed forces or civil aviation authority -Firefighter -Train driver or railway signaller
What other careers will be difficult to pursue?
-Photography -Paint and textiles -Interior decorating -Electronics -Medical professionals report difficulty seeing body colour changes, blood in urine etc Optometrists report difficulty seeing disc pallor or red eyes or retinal haems You can still drive a car, motorbike, bus or lorry.
What do you do if you find a child has colour vision defect?
Inform px and parent and write letter fo school about the conditions and how they can help
How do you tell the parent?
Explain the problem, effect on education, career choice, reassure that it is not a disease and does not get worse.
Why do you need to write to the school?
Widespread use of colour in teaching materials can lead to unsupported children falling behind in class Write letter to school so they can make adjustments for the child
What adjustments can you suggest to the school?
Labelled crayons/felt tips To seat CVD pupils in good natural light
Give example how you would word the letter to the school
John has protanopia, a congenital colour deficiency. He sees colours but sees them differently to other children and will confuse some colours particularly reds, green and browns but also purples, oranges and yellows. These colours are more likely to be confused if they are pale or dull or viewed in dim lighting. Here are some suggestions that may help John: write suggestions
What are the types of tests to test colour vision?
Ishihara City D15 100 hue
What is the Ishihara test for?
Quick screening for red-green colour deficiency Most commonly used and also a required entrance test for many professions
What are the disadvantages of the Ishihara test?
Does not assess tritan problems Less useful for grading the severity of the defect or monitoring acquired defects After several years the colours fade so it loses its validity
How do you use Ishihara test?
Room light on, 75cm away at right angle to line of vision, answer should be given without more than 3 second delay 38 total plates: 25 with number and 26-38 are for non verbal patients
How can you use 26-38 for non verbal patients/can’t read numbers?
They trace the line between the to X’s Should be done within 10 seconds
How do you interpret the Ishihara results?
Normal colour vision reads all or makes 1 or 2 mistakes Colour defect: 6 or more errors Severe trichromats can’t see any numbers after 12
What is the first plate for?
everyone should see it even if they have a CVD, can catch out malingers if they say they can’t see
What do transformation Nos. 2-9 plates show?
The number is seen by those with normal colour vision A different number is seen by people with red-green colour deficiency. Those with total colour blindness and weakness are not able to read any numeral.
What do vanishing Nos. 10-17 plates show?
The number is seen by those with normal colour vision but cannot be seen by people with red-green colour deficiency.
What do hidden digit Nos. 18-21 plates show?
The number cannot be seen by those with normal colour vision AND those with total colour blindness but can be seen by people with red-green colour deficiency
What are plates 22-25 for?
Protan/deutan classification In protanopia and strong protanomalia, only see the number on the right In deuteranopia and strong deuteranomalia, only see the number on the left. If neither number is seen, protan/deutan classification must be made using another test
