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Optics > lecture 31 - colour vision assessment I > Flashcards

lecture 31 - colour vision assessment I Flashcards

1
Q

what is normal trichromatic colour vision?

A

. 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

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2
Q

what results in differences in colour vision?

A

. genetic mutations in L and M cones genes that can cause shift in peak sensitivity that results in differences in colour vision

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3
Q

what are things that can explain differences in colour matching experiments ?

A
  • 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
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4
Q

what are the classification of anomalies ?

A
  1. congenital
    - shift in cone peak sensitivity
    - no cone present
  2. 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
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5
Q

what are the characteristics of congenital anomalies ?

A
  • 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
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6
Q

how are congenital anomalies classified?

A
  • 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
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7
Q

what are anomalous trichromat ?

A
  • 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
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8
Q

what is a dichromat ?

A
  • when a specific cone is missing
  • tritanope - S cone missing
  • deuteranope - M cone missing
  • protanope - L cone missing
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9
Q

what is a cone monochromat ?

A
  • only have one type of cone

- colour blind

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10
Q

what is rod monochromat ?

A
  • only have one rod

- colour blind

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11
Q

what are the most typical types of colour deficiency ?

A
  • deuteranomalous
  • deuteranope
  • protanomalous
  • protanope
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12
Q

what is dichromatism ?

A
  • normal trichromat - has three cones ( S , M , L )
  • tritanope - missing S cone
  • deuternaope - missing M cone
  • protanope - missing L cone
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13
Q

what is the prevalence of congenital deficiencies ?

A
  • RG are most common (8% of men and 0.4% women)

- YB deficiencies are very rare

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14
Q

why do men have congenital deficiency more than women ?

A
  • 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
  1. male with normal colour vision
  2. female missing M cone on x chromosome ( potential carriers )
  3. 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
  1. colour normal male
  2. colour deficient female
  3. colour normal female

. as long as a female has a whole set of photoreceptors on one X chromosome , females will have normal colour vision

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15
Q

how does luminance discrimination can vary amongst normals and colour deficient observers ?

A
  • 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 )

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16
Q

how is the luminance efficiency function in deuternaopia?

A
  • in deuternaopia we have a missing M cone

- the relative luminous efficiency is virtually normal ( show reduced colour discrimination)

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17
Q

how is the luminance efficiency function in protanopia?

A
  • 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 )
18
Q

how is the luminance efficiency function in protanomaly?

A
  • 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 )
19
Q

what are the characteristics for acquired anomalies ?

A
  • 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
20
Q

what is the importance of having normal colour vision?

A
  • 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
21
Q

how have the majority of colour vision test been created?

A
  • 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
22
Q

what do colour vision tests need to do?

A
  • identify, measure severity and differentiate congenital and acquired colour deficiency
23
Q

what are the different functions of colour vision tests ?

A
  • 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:
24
Q

what is the ishihara pseudoisochromatic plates?

A
  • 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
25
Q

what are isochromatic confusion lines ?

A
  • 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
26
Q

what is the American optical - HRR?

A
  • 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
27
Q

what is the Farnsworth D15 test?

A
  • 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
28
Q

what is the city university test ?

A
  • 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
29
Q

what is the Nagel anomaloscope test ?

A
  • origin Germany
  • the standard reference test for the identification and diagnosis of types of RG colour deficiency
  • Maxwellian view spectroscope
  • Rayleigh colour matching
30
Q

what is the examination procedure of the Nagel anomaloscope?

A
  • 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
  1. subject makes several colour matches by adjusting both RG mixture ration and the luminance of the Y test field
  2. the examiner sets the RG mixture ration and subjects ascertains whether an exact match can be made by altering the Y test field
31
Q

explain the Nagel anomaloscope results ?

A
  • 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
32
Q

explain the nagel anomaloscope results graph ?

A
  • 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
33
Q

what is CAD test?

A
  • 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
34
Q

how can we show the effectiveness of the LC masking technique ?

A
  • 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
35
Q

how has the CAD test been used ?

A
  • 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
36
Q

explain the CAD measure of chromatic sensitivity ?

A
  • 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
37
Q

how do acquired deficiencies affect colour vision?

A
  • 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
38
Q

how is colour vision in subjects with diabetes?

A

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
39
Q

what happens in acquired deficiency if we increase the size of the stimulus?

A

in acquired deficiency if we increase the size of the stimulus, you can decrease colour threshold

40
Q

what is an efficient assessment of chromatic sensitivity ?

A
  • 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

Optics (34 decks)

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