W11: Abnormal Colour Vision

Abnormal Colour Vision

• Normal Trichromat

• Protanope

• Deuteranope - losses in red-green pathway - no M cone

• Tritanope

Colour Vision Learning Outcomes

• Characteristics of Human Colour Vision

  • Visual Function (e.g., hue and saturation discrimination)

• Abnormal Colour Vision

  • Monochromacy and Dichromacy

  • Types of colour vision deficiencies (congenital, acquired)

  • Characteristics of abnormal human colour vision

    • Colour perception of people with CVD’s

    • Colour confusions

    • Acquired colour vision deficiencies

  • Clinical testing for abnormal colour vision (pseudoisochromatic plates, arrangement tests, Rayleigh colour matching)

Characteristics of Human Colour Vision: Hue and Saturation Discrimination

Hue (chromatic) discrimination ability

• Moving parallel to spectral locus changes hue

• Hue/wavelength discrimination: the smallest change in wavelength (hue) which can be distinguished as a change in colour $\Delta hue$

  

• To measure hue discrimination, determine how much a wavelength can be changed before a colour difference is perceived.

• Discrimination ability varies with wavelength

• Trichromat has 2 regions of best discrimination (minima) at 490nm & 590nm; can detect 1-2nm (3rd region at ~440nm)

• poorer discrimination in-between & at spectral extremes (maxima at ~460nm & 530nm)

• short and long wavelengths have reduced discrimination

• To discriminate between different wavelengths, the ratio of each cone photoreceptor output to the light is compared

• Discrimination is best when the difference in photoreceptor output is highest

• when outputs are close together, thats when you start to require greater spectral differences to notice.

Saturation Discrimination

• Calculate $\Delta p = L\lambda/(L\lambda + Lw)$

• Moving out from white changes saturation

• Saturation discrimination: Amount of colour added to a white stimulus so that the stimulus first appears coloured ( $\Delta saturation$ )

• Peak at 570-580 nm (yellow): Yellow is the “least saturated” spectral hue

• Saturation Discrimination is best (minimum) near 400 nm and intermediate at mid spectrum & long $\lambda$

• To measure saturation: Calculate $\Delta p = L\lambda/(L\lambda + Lw)$ (or similar statistic) “colorimetric purity, Pc”

• best saturation discrimination is at shorter wavelengths and longer wavelengths.

• worst saturation discrimination is in the intermediate wavelengths near yellow.

MacAdam Ellipses

• Precision of colour matching limited by sensitivity to small colour differences

• Within these ellipses all samples appear identical to observer

  • in green region need a larger change to detect discrimination (i.e., worse discrimination)

  • allows more errors in green region than in blues in manufacturing industries.

• Vary in size & orientation in different areas of colour space

Abnormal colour vision

Colour Blindness Fatal Consequences

• Lagerlunda (Sweden) collision of November 1875; 9 people died in the accident

• Driver did not see red stop lamp and thought the line was clear…

• Beginning of campaign for colour vision screening of railway employees and the development of Lantern colour vision test.

How CV depends on the number of cone photoreceptors?

• A photoreceptor only counts the number of photons it absorbs: The Principle of univariance

• Photoreceptors cannot discriminate wavelength, therefore a person with a single cone class is completely “colour blind”

• The implication is that 1 photoreceptor can match any test $\lambda$ with any other $\lambda$ by simply adjusting the irradiance

• This is a UNIVARIANT (colour) vision system called monochromacy (i.e., rods only), but it is very rare (~1 in 30,000)

2 cone photoreceptor systems

• In general, can match any test $\lambda$ by adjusting the irradiances of TWO other $\lambda$ ‘s.

• This is a BIVARIANT colour vision system

• Bivariant systems code differences in intensity & 1 dimension of colour: these systems are dichromatic

• There are many people with such visual systems….

Colour Vision Deficiencies - Dalton Eye’s

• 1794 John Dalton (developed atomic theory) analysed his own "strange" colour vision

• Saw two main hues only in rainbow spectrum: ROYG - BIV

• Brother had same CVD

• Thought that all CVD was the same

• Proposed CVD was due to blue tinted vitreous (absorbing all red light)

• Instructed that his eyes be dissected after his death…

• Dalton died age 78 (27 July 1844)

• His medical attendant, Joseph Ransome did an autopsy the next day

  • Collected humours of one eye into a watch glass

  • Found to be "perfectly pellucid” (i.e. clear)

  • Lens was normal yellow for age

• Other eye left basically intact

  • Sliced off posterior pole to view through eye

  • He noted that red and green objects were not distorted

• Pity Dalton was not around to believe his own eyes

  • Ransome found no support for CVD being due to a pre-retinal filtering

  • Did not discard the eyes, but kept them in air, and they are still intact (more or less) today

• Small fragments were taken (1 mm3) for DNA analysis

• Confirmed Dalton had DNA to code for only 2 cone types

• Plotted are flowers that looked blue to Dalton (e.g. red campion, ragged robin).

• Dalton also judged red sealing wax & the upper side of a laurel leaf be similar.

• The protan and deutan white confusion lines are shown.

• What type of CVD might Dalton have had? Dalton was missing a M cone -= deutanope

• Deutan confusion - spectral sensitivity of the M cone

• Protan confusion - spectral sensitivity of the M cone ????????

Types of Colour Vision Deficiencies

Genetics of Colour Vision

• Photopigments are opsins (proteins) coded by genes

• In trichromats there are 3 cone opsins (L, M and S) plus rhodopsin & melanopsin

• Rod photopigment gene: Chromosome 3

• Melanopsin photopigment gene: Chromosome 10

• S photopigment gene: Chromosome 7

• L and M photopigment genes on X chromosome (one after the other)

• Most common CVD are congenital L- & M-cone defects

• Sex linked recessive means the abnormal gene is carried on the X chromosome & is recessive to the normal gene

• CVD’s can arise due to a recombination of the opsin genes that lead to abnormal genes that are non-functional or compromised because of:

  • Point mutations (hybrid gene)

  • Sequence deletions

  • Gene duplication

Red-green CVD inheritance

Congenital CV Defects: Degree

• People with congenital CVD’s have one or more cone pigments that are either missing or altered; this determines the degree of their colour vision deficiency

• Two types of monochromats:

  • Rod monochromat: no functioning cone pigments, low VA, photophobia, nystagmus

  • Cone monochromat: two cones totally malfunction, S- only?, reduced VA, photophobia? Nystagmus? rare

Dichromatism

• One cone type absent

• Protanopia: no LWS, L-cone opsin (erythrolabe)

• Deuteranopia: no MWS, M-cone opsin (chlorolabe)

• Tritanopia: no SWS, S-cone opsin (cyanolabe)

• With 2 cone types, can match all colours with 2 primaries

Anomalous Trichromatism

• most common type of CVD

• 3 cone types, but 1 has a shifted (peak) spectral sensitivity

• Range in severity (depending on the shift in spectral sensitivity)

• Protanomalous: LWS shifted to MWS (10nm peak difference)*

• Deuteranomalous: MWS shifted to LWS (6nm peak difference) *

• Tritanomalous: Abnormal SWS, reduced blue

• 3 primaries to make a match; matches differ from trichromats

• *DeMarco, Pokorny & Smith (1992): Note that the peak difference in LMS and MWS opsins is ~23nm in trichromats

Inheritance and incidence of colour vision deficiencies

• Rod, cone and atypical monochromacies are very rare

(Female) Carriers of Colour Vision Deficiencies

• ~15% of women are heterozygous carriers of X-linked red-green CVD

• ~4.2% are protan carriers; less sensitive to red light: Schmidt’s sign (Schmidt, 1934)

• ~11.7% are deutan carriers; more sensitive to red light: deVries sign (deVries, 1948)

• (Note: deVries sign is more difficult to demonstrate than Schmidt’s sign)

• Many carriers present with slight or moderate reductions in colour vision as indicated by:

  • Increased errors on the Ishihara test

  • Slight shift in the Nagel match midpoint & enlarged Nagel matching range on Raleigh anomaloscopy

Colour Perception of Persons with Colour Vision Deficiencies

• Persons with colour vision deficiencies have:

  • Confusion of colours which appear very different to a person with trichromatic colour vision

  • A reduction in the number of separate colours which may be seen

  • Colour matching in CVD does not look like a match to a person with trichromatic colour vision

    • However, at mesopic light levels, the rods act as a third photopigment and dichromatic colour matches are comparable to trichromats (Smith & Pokorny, 1977)

    • The presence of a 3rd, poorly represented cone type, may be involved in this categorisation (Montag & Boyton 1987)

Relative Luminous Efficiency

• trichromatic: peak at 555nm

• Protanope: Peak shifts to shorter wavelengths (protanope: 530 nm), marked reduction in long wavelength sensitivity

• Deuteranope: Peak shifts to longer wavelengths (deuteranope: 565 nm)

• Tritanope: Almost normal curve, slight reduction at short wavelengths (<540nm)

• Deuteranomaly: M-cone spectrum displaced toward long wavelengths

• Protanomaly: L-cone spectrum displaced toward short wavelengths

Solid curves show the normal cone absorption spectra, dashed curves the locations of the displaced spectra

Colour Naming in Dichromats

• Dichromats can name the appearance of 4°diameter colour samples (from the OSA Uniform Colour Scale) in fair agreement with colour normal observers, including along the red-green dimension (plus the lightness and blue-yellow dimensions)

• When stimuli are limited to the central fovea, or when rods are bleached, dichromats cannot categorize colours along the red-green dimension

• At scotopic illuminations, dichromats do not assign colour names based on the scotopic lightness or stimulus spectral composition (as do trichromats); their lifetime experience with a reduced photopic colour gamut may contribute to this.

Appearance of the colour spectrum

• Normal CV: Ends of spectrum are dim compared to middle two areas. Most white appearing area (desaturated) is in the yellow portion of the spectrum

• CV defective: Observations differ depending on defect

  

• Density of vertical lines reflects wavelength discrimination.

• Spectrum for dichromats (protanopes/ deuteranopes) divided into a blue and “variable” region separated by the white neutral point (W).

• Variable: observers label these wavelengths based on brightness & context cues

Hue / Wavelength Discrimination

• Protanopes/Deutanopes: Single minima at ~495nm, unable to detect differences >520-530nm

• Protanomalous/Deutanomalous: Range between the normal & dichromat, minima ~495 & 610, no short wavelength minima

• Tritanopes: 2 minima ~430 & 580nm, unable to detect differences between 450 & 480nm

Colour Confusions

• Dichromats accept trichromatic colour matches whereas dichromatic colour confusions are easily distinguished by the trichromat

• People with CVDs will confuse colours located on the same confusion lines (i.e., are unable to discriminate between equal-luminant colours)

• Confusion line passing through white that intersects the spectral locus defines the point in the spectrum which appears achromatic (neutral point)

• These neutral points (perceived as grey/achromatic) occur at

  • 492 nm (protanope)

  • 498 nm (deuteranope)

  • 569 nm (tritanope)

• Confusion lines converge at a single point, the copunctal point (cp), which is the spectral response of the missing photoreceptor; people with CVDs confuse colours located on the same confusion lines

• Neutral Point (NP): part of the spectral locus that appears achromatic

  • draw from CP through white to locus point.

• protan and deutan will confuse red and green (same confusion line)

• tritan will confuse blue and green (same confusion line)

Dichromatic Colour Gamuts

• Trichromat: >2,000,000 colour combinations. Spectrum seen as R O Y G B I V

• Protanope: 17 distinct colours. Spectrum divided into yellow and blue-violet on either side of the neutral point

• Deuteranope: 27 distinct colours

• Tritanope: spectrum divided into green and red

Saturation Discrimination

• Normal observer: 570nm least saturated

• Protanope: function crosses x-axis at 492nm

• Deutanope: at 498nm

• Tritanope: at ~569nm

• When cross x-axis, these wavelengths appear white: Neutral Points

Acquired Colour Vision Deficiencies

Acquired CV Deficiencies

• Wald Marre Type = location effected

Distinctions between hereditary and acquired colour vision defects

Testing for Colour Vision Deficiencies

• Most tests rely on colour confusions in CVD

  1. Pseudoisochromatic (PIC) Plates (e.g., Ishihara)

  2. Hue discrimination (arrangement) tests: D15, Farnsworth-Munsell 100-Hue, Lanthony desaturated

  3. Colour matching (Nagel anomaloscope)

• The tests use colour confusions designed to detect & classify congenital defects, not acquired colour defects

Ishihara Pseudoisochromatic plates

• Pseudoisochromatic plates are examples of colour camouflage

• Colours within confusion zones can be substituted without a person with a CV defect noticing:

  • These are "isochromatic" (same colour) pairs for those with CVD

  • hence "pseudo-" (falsely) "-isochromatic" (same colour)

• Ishihara plates screen for red-green CV

• American Optical HRR (Hardy Rand Rittler) plates screen for tritan defects

• Embeds chromatic differences within spatially varying luminance-contrast noise (spots of different sizes)

• Noise ensures detection relies on chromatic discriminations & not luminance differences associated with the different colours (important because individual luminous efficiencies differ)

  • dichromate cannot use luminance differences to detect target.

    

    • top image: what number?

    • bottom image: trace path from left side to right side

4 pseudoisochromatic plate designs

1. Vanishing Design: Perceived by trichromats (e.g. #5), invisible with red-green CVD

1. Hidden Design: Figure is camouflaged for trichromats, visible to dichromats

   

2. Transformation Design: Combines hidden & vanishing concepts: Trichromats perceive one figure (e.g. #74), dichromats perceive another (e.g. # 21)

   

3. Classification design: Differentiates between protan & deutan: Different confusion loci depending on which cone system is defective

   • Protans confuse red and grey (perceive 6)

   • Deutans confuse red-purple and grey (perceive 2)

Hue Discrimination (arrangement) Tests

D-15

• 16 hues (one fixed reference hue): Munsell value = 5, chroma = 4

• Patients arrange colour in order

• Colours encircle illuminant C

• Identifies the CVD type and severity (moderate/severe)

L’Anthony Desaturated D-15

• Munsell hue same as D-15 with value = 8, chroma = 2 (desaurated)

• Used to analyse defect severity (and for screening acquired CVDs)

D-15 Principle

• Isochromatic confusions occur when colours from opposite sides of the hue circle are placed together

• Less severe CVDs make fewer confuse isochromatic confusions

  

Scoring

• Number of isochromatic crossings

• Sum of the colour differences b/w adjacent colours

The Farnsworth-Munsell 100 Hue Test

• Devised by Farnsworth (1943)

• 85 coloured samples with similar chroma & value encircle illuminant C

• Assesses colour discrimination

• Useful for moderate and severe congenital & acquired CVD

• Provides a colour aptitude assessment in normal trichromats

• Data indicate total error score (incorrect discriminations within a box) & axis

  

• Error Score for each cap (colour): sum of the difference b/w the preceding & following caps in the arrangement

• Total error score is the sum of the individual errors (AUC)

• Hue discrimination better when errors scores are lower

• retinitis pigmentosa - red / green defectsz

• optic neuropathy - yellow defects

The Rayleigh Match

• Spectral locus between 545 & 700nm approximates a straight line; any wavelength on this line can be matched be appropriate combination of 2 lights

• The dotted line formed by connecting Red, Yellow & Green primaries correspond to a colour confusion line shared by deuteranopes and protanopes

• Gold standard colour vision test for protanopia and deuteranopia

• Uses a Rayleigh (1881) match:

  • Vary proportion of Green 546nm & Red 670nm primaries (varying only in colour balance) to match the yellow 590nm primary (of variable luminance)

• The Moreland (1978) equation assesses S-cone function: Match a blue-green bicolour test field with 480 & 580nm to a mixture of 430 & 500nm

• number of quanta remains constant throughout.

• 2° field (Nagel, 1907), photopic (no rod intrusion)

• S-cones insensitive to the 3 primary lights

• Anomaloscope set to “deut” mode; spectral sensitivity of deuteranopes involve only L-cones for wavelengths >545 nm & so any R/G balance ratio (# quanta) will match the fixed yellow value (50% or 17 units)

  • for every combination of red and green, there will only be one amount of quanta in the yellow (~50%) that will match the other mixture field.

• Deuteranope: Matches mixture fields with a constant test field luminance

• Protanope: Matches all mixture fields; however, must adjust the test field luminance

• Trichromat: Average normal matches occurs at intersection of Protan/Deutan matches

• Age: Increased lens absorption causes Rayleigh matches to shift toward the green

Rayleigh matches with the Nagel Anomaloscope

• Adjusting the R/G balance changes the colour not number of quanta

• Rayleigh Match calibrated for an (L-cone) ('Deut' Mode)

• Trichromats accept only a small range of R/G ratios, near the middle (~45)

• Deutan:

  • Due to the “Deut’ calibration, all R/G ratios match 50% yellow whereas only one yellow luminance (i.e., 50%) matches the R/G ratio

  • Therefore test field (Y) flux is constant for all R/G ratio

• Protan:

  • Protanope (M-Cone) sensitivity to Y is reduced compared to Deuteranopes (compare middle and lower panels)

  • Because the R/G balance is calibrated to the irradiance of Y in 'Deut' mode, if R/G mixture ratio was set to all R (i.e., 70), then the mixture field would appear dimmer than the test

  • The Protan must therefore increase the irradiance of Y and compensate for a reduced sensitivity to R by adjusting the R/G balance toward G to make the match

Questions

• How does human saturation discrimination change between trichromats and dichromats?

• Discuss the limits of hue and saturation discrimination in humans

• Differentiate between hereditary and acquired colour vision defects

• How are colour vision tests designed to detect and classify congenital deficiencies in clinical practice?