lecture 31 - colour vision assessment I Flashcards
what is normal trichromatic colour vision?
. humans with normal trichromatic colour vision possess three distinct classes of cone photoreceptors which contain three pigments that absorb in different parts of the visible spectrum
. theses contain short (S), middle(M) and long(L) wave sensitive photopigments with peak absorption wavelength at 440 , 540 and 570 nm respectively
what results in differences in colour vision?
. genetic mutations in L and M cones genes that can cause shift in peak sensitivity that results in differences in colour vision
what are things that can explain differences in colour matching experiments ?
- macular pigment spectrum can explain some differences in colour matching experiments
- changes in L to M cone ration
- changes in optical density of cone photoreceptors , this makes some photoreceptors more or less efficient at absorbing photons
- post-receptoral amplification of cone signals that perceive the formation of colour channels can also cause significant changes in chromatic sensitivity
what are the classification of anomalies ?
- congenital
- shift in cone peak sensitivity
- no cone present - acquired
- damage to cone receptors or nerve fibres in the retina, optic nerve and visual cortex
- this is a result of a disease such as diabetes or hypertension
what are the characteristics of congenital anomalies ?
- present from birth
- stable throughout life in terms of classification and severity
- affects both eyes equally
- affects males more than females
- RG most common
- deficiency can vary in severity from minimal to severe
- available clinical tests are designed to detect only RG deficiencies
- normal visual function
- differences in luminosity discrimination amongst colour deficients
how are congenital anomalies classified?
- they are usually classified according to the photoreceptor that is affected
1- normal trichromat have three photoreceptors S, M and L
2- anomalous trichromat- peak sensitivity of one cones has shifted
3- dichromat - cone is missing
4- cone monochromat - one type of cone
5- rod monochromat - one type of rod
what are anomalous trichromat ?
- have three photoreceptors but the peak sensitivity of one of them is abnormal and slightly shifted
- S cone shifted- tritanomalous
- M cone shifted- deuteranomalous
- L cone shifted - protanomalous
what is a dichromat ?
- when a specific cone is missing
- tritanope - S cone missing
- deuteranope - M cone missing
- protanope - L cone missing
what is a cone monochromat ?
- only have one type of cone
- colour blind
what is rod monochromat ?
- only have one rod
- colour blind
what are the most typical types of colour deficiency ?
- deuteranomalous
- deuteranope
- protanomalous
- protanope
what is dichromatism ?
- normal trichromat - has three cones ( S , M , L )
- tritanope - missing S cone
- deuternaope - missing M cone
- protanope - missing L cone
what is the prevalence of congenital deficiencies ?
- RG are most common (8% of men and 0.4% women)
- YB deficiencies are very rare
why do men have congenital deficiency more than women ?
- if a male parent is missing an M cone pigment and female parent is normal
- possible combination of offspring
1. male with normal colour vision
- male with normal colour vision
- female missing M cone on x chromosome ( potential carriers )
- female missing M cone on x chromosome ( potential carriers )
- if the carrier female and a male with a missing M cone have offspring
1. colour deficient male
- colour normal male
- colour deficient female
- colour normal female
. as long as a female has a whole set of photoreceptors on one X chromosome , females will have normal colour vision
how does luminance discrimination can vary amongst normals and colour deficient observers ?
- the luminance efficiency function is weighted by 2L+M
- the luminance efficiency function for a normal trichromat is the dotted curve (closer to L cone )
how is the luminance efficiency function in deuternaopia?
- in deuternaopia we have a missing M cone
- the relative luminous efficiency is virtually normal ( show reduced colour discrimination)
how is the luminance efficiency function in protanopia?
- in protanopia, the L cone is missing
- luminous efficiency function follows the M cone
- reduced luminous efficiency for long wavelengths
- this means that for this type of deficiency ( red looks dim )
how is the luminance efficiency function in protanomaly?
- in protanomaly , there still the three types of cones but L cone is slightly shifted towards M cone
- the relative luminous efficiency function follows the anomalous pigment L cone ( red still look dim but less dim than a protanope )
what are the characteristics for acquired anomalies ?
- frequently associated with other visual function loss such as acuity, visual field
- onset after birth ( colour vision previously normal )
- type and severity changes with time
- monocular differences in severity frequently occur
- monocular testing required
- equal prevalence in male s and females
- affects both RG and YB colour channels
- clinical tests are not designed to detect or monitor acquired deficiencies
what is the importance of having normal colour vision?
- good indicator of normal functioning of the retina
- chromatic sensitivity is affected most and earliest in a number of diseases of the retina and optic nerve
- colour vision testing provides a means of early detection/diagnosis and to follow-up progressive changes with disease
how have the majority of colour vision test been created?
- the majority of vision test have been designed to discover congenital colour deficiency
- the anomaloscope classifies the type of deficiency
- visual stimuli tend to produce signals in several vision information channels, it is important that colour vision test employ stimulus conditions that isolate the use of colour signals , most test employ some form of luminance contrast masking to reduce of detection of luminance cues in coloured targets
what do colour vision tests need to do?
- identify, measure severity and differentiate congenital and acquired colour deficiency
what are the different functions of colour vision tests ?
- screening test: identify people with normal or abnormal colour vision
- classify: protan , deutan and tritan
- grading severity of colour deficiency
- diagnosis: dichromat / anomals trichromat
- occupational suitability:
what is the ishihara pseudoisochromatic plates?
- origin Japan
- used worldwide due to ease of use
- viewing at 60-70 cm distance , need good illumination , 4 secs for each plate
- screening/identifying for red-green colour deficiency
- classifying only protan and deutan defects
- in full version of 38 plates , has 4 classification plates which allows it to identify between protan and deutan defects
- exploit isochromatic colour confusions with colour camouflage mainly YB and luminance masking
- full version 38 plates, 25 numerals and 13 pathways
what are isochromatic confusion lines ?
- typical normal trichromat will see all the colours the same within the ellipse
- protan type deficiency will see a colours on opposite sides the same - this is the protan isochromatic confusion loci
- deutan type deficiency will see colours on opposite sides of the line the same - deutan isochromatic confusion loci
what is the American optical - HRR?
- origin USA
- testing procedure? similar to Ishihara, need good illumination
- good at classifying protan, deutan defects, grading RG colour deficiency and identifying moderate tritans
- 24 plates with vanishing designs containing geometric shapes ( circles , crosses , triangles )
- shapes are printed on neutral colours on a background of a matrix of grey dots ( the saturation of the neutral colours increases in successive plates to produces designs with progressively larger colour different steps
- 4 introductory plates , 6 plates for screening ( 4 RG and 2 YB) , 10 plates for grading the severity of the severity of protan and deutan defects and 4 plates for tritan defects
- given in reverse order, from an east to a more difficult level
what is the Farnsworth D15 test?
- USA and worldwide
- intended for vocational guidance
- grading and classifying protan, deutan and tritan deficiency
- people with slight ( even moderate ) colour deficiency pass this test
- hue discrimination
- based on colour arrangement so you have 15 caps , the first one is the pilot cap and then the subject has to arrange the colours in what it is a natural sequence with a minimum colour step between one cap and the next
- colour arrangement made by the subject is drawn on a circular diagram representing hues
- more crossing of hue circles indicate a more severe colour deficiency
what is the city university test ?
- origin UK
- 10 plates: central colour and 4 peripheral colour
- subjects have to select the peripheral colour which looks most like the one in the centre
- three peripheral colour are typical isochromatic confusions with the central colour and the fourth colour is an adjacent colour in the D 15 sequence and is intended for normal preference
- identifies moderate and severe colour deficiency
( not intended for screening ) - > 60% of people who fail obtain mixed protan/deutan classification
what is the Nagel anomaloscope test ?
- origin Germany
- the standard reference test for the identification and diagnosis of types of RG colour deficiency
- Maxwellian view spectroscope
- Rayleigh colour matching
what is the examination procedure of the Nagel anomaloscope?
- when you look down the Nagel anomaloscope yous see a test field at that is circular and split into a half
- the bottom half is a mix of yellow which goes from low to high luminance
- top half we have a mix between red and green and colour between
- subject makes several colour matches by adjusting both RG mixture ration and the luminance of the Y test field
- the examiner sets the RG mixture ration and subjects ascertains whether an exact match can be made by altering the Y test field
explain the Nagel anomaloscope results ?
- if you are a protanope or a deuteranope you can match pure green to yellow and pure red to yellow - protanope uses a high luminance on yellow scale for protanope to match to green and low luminance at red
deuteranope uses equal amounts of yellow at both extremes - deuternaomalous and protanomalous observers are outside normal match- they form two distributions
- protanomalous require more red light in colour mixture
- deuteranomalous require more green light
- the extent of matching range gives an indication of the severity of the discrimination of the colour deficient observer
explain the nagel anomaloscope results graph ?
- at the top you have deuternaopes - match 0 and 73
- then you have extreme deutans which match across the normal range ( around 40)
- deuteranomalous observes that require more green in the match
- the population of normal trichromat which match around 40
- protanomalous observers which use more red in the mixture
- extreme protanomalous which are subjects that don’t match the entire range
- protanopes which match from 0 to 73
what is CAD test?
- the colour stimulus is buried in dynamic luminance contrast noise which isolates the use of colour signals
- the varying luminance prevents the subject from using any luminous cues to detect the moving target
- the subject indicates the direction of movement of the colour stimulus by pressing one of the four buttons
- it is a four alternative choice procedure test
- measures 16 different directions aligned at the two chromatic channels RG and YB
- 8 for RG and 8 YB - this provides enough information to measure accurately chromatic discrimination and distinguish between normal and effective colour vision
how can we show the effectiveness of the LC masking technique ?
- the chromatic threshold for deuteranopic observer increase with a level of luminance contrast noise until no further cues can be used
- if you use 12% or 25% LC noise for a deuteranopic observer you prevent the subject from using illuminance cues to detect the coloured moving stimulus
how has the CAD test been used ?
- the CAD test has been used to measure colour threshold in a large number of colour deficient observers and normals
- the extent of colour vision deficiency varies on a continuous scale from normal to 25% worse than average normal
- the units of CAD test are one , which indicates the median value for the population of normals ( one RG and one YB) which indicate the average normal observer on the CAD test - this value allows us to measure how bad or how good someone’ s colour vision compared to average normal
explain the CAD measure of chromatic sensitivity ?
- the grey shaded area shows the variability among subjects with normal colour vision
- this subject has a RG green threshold of 0.68 and yellow blue threshold of 0.71 - because these numbers are less than 1 , this subjects colour vision is actually better than the average normal
- deutan type deficiency - has a RG threshold of 2.79 times the signal strength of the median normal trichromat to detect RG colour signals and has a YB threshold of 1.15 , so the YB is basically normal
- more severe colour deficiency has a RG threshold of 22.44 , this subject has a severe loss of colour vision because it’s 22 times worse than normal trichromat
YB =1.03 - subject with RG=12.67 and YB=0.93 has a moderate loss of colour vision with 12.67 times worse than median normal trichromat and has normal YB
how do acquired deficiencies affect colour vision?
- left eye : wet AMD - the graph shows that this subject has a RG threshold of 4.5 and YB threshold of 11.58, this means that the loss of YB is greater than the loss of RG but both are abnormal
- Right eye : shows less loss
so RG is 3.8 and the YB is 7.16
how is colour vision in subjects with diabetes?
2007 - LE - RG = 5.13 YB = 4.13
loss of colour vision
2008- RG=1.81 YB=2.51
2007 - RE - RG =2.28 YB=2.78
loss of colour vision
2008 - RE - RG=1.6 YB = 2.71
- theses results show that the measurement of colour vision loss could be used to monitor diabetic control and management compliance
what happens in acquired deficiency if we increase the size of the stimulus?
in acquired deficiency if we increase the size of the stimulus, you can decrease colour threshold
what is an efficient assessment of chromatic sensitivity ?
- true isolation of colour signals is obtained
- based on data that describe the variance in colour discrimination amongst normal trichromats
- has adequate sensitivity and specificity to detect minimal deficiencies and to classify them
- can be used to detect and monitor significant changes in colour discrimination over time
