Measurement Colour Vision

The essential elements and techniques of assessing colour vision.

Colour Vision Function

• Colour is set of perceptions elicited from the spectral distribution of light (wavelengths)

• Found in many species

• Evolved in primates

• Chromatic contrast allows us to see objects that would be otherwise indistinguishable from the background – helps distinguish borders

Colour is set of perceptions elicited from the spectral distribution of light (wavelengths). True or False?

True

___________ is visible to humans due to human physiology!

Visible Spectrum

Visible light encompasses wavelengths roughly from ______ to _____ nanometers.

380nm; 780nm

Snakes have __________ and birds and insects have ________.

Infrared Sensitivity; Ultraviolet Sensitivity

The Colour Stimulus:

• Isaac Newton (1704) - stimuli of different wavelengths within the visible range produce different colour sensations

• Prism split into “rainbow of colours” - index of refraction inversely proportional to wavelength (diffraction)

• ROY G BIV

Issac Newton figured of the colour stimulus in _______.

1704

Colour Perception:

• Colours are generated by the visual system

• Color is not a physical property but a psychophysical property

• There is no red in a 700 nm light! … its what we perceive

• The colour of an object is due to:

  • The wavelength reflected to the eye

  • The reaction of the visual pigments to those wavelengths

  • Neural processing of those signals

Colour is _______ and is only due to how wavelengths reflect to the eye, reaction of the visual pigments to those wavelengths and neural processing.

Psychophysical

Trichromatic Theory

• Proposed by Thomas Young (1802) and Hermann von Helmholtz (1866)

  • Theory of color vision that says eye has three kinds of receptors that are “tuned” to be sensitive to red, green and blue light

  • All the colors we see are a combination of these three colors

  • Relative activities of these different receptors encode colour

• James Maxwell (1831 – 1879) developed a colour-matching technique that is still used today (Nagel Anomaloscope)

The eye has _____ kinds of receptors that are tuned to red, green and blue lgiht.

3

Trichromacy

• Mixing Short, Medium and Long Wavelength Light can match (nearly) any colour

• A person with trichromatic vision can match lights with 3 lights on one side of matching field

• The two matched fields of light are known as metamers

Two matched fields of light are known as ________. Colors that appear identical but are physically different (they emit or reflect physically different light wavelengths) are ________.

Metamers; Metamers

Evidence of Trichromacy:

• Neurophysiological experiments confirmed rods and three different types of cones

  • Micro spectrophotometry

  • Suction electrode technique

• The three classes of cones are termed Short (S), Medium (M) and long (L) wavelength and also referred to as Blue, Green and Red

There are three different types of ________.

Cones

Distribution of Cones:

• few “S” cones at fovea and only about 10% of cones “S” – centre of fovea

• Trichromatic colour vision – extends 20-30 deg from fovea

• S-pigment – response to short wavelengths (BLUE)

• M-pigment – response to middle wavelengths (GREEN)

• L-pigment – response to long wavelengths (RED)

Only _____% of cones at the centre of the fovea

10

Cones:

• Conical in shape

  • Look like rods at the fovea

• Colour Vision

  • Visual pigment

  • Three types Sensitive to three different wavelengths of light

• Photopic vision

  • High levels of illumination

• High levels of visual acuity

  • Tightly packed cones at the fovea

  • Only green and red at fovea

______ are tightly packed at the fovea and have only _____ and _____ vision at the fovea.

Cones; Red; Green

A single ____ shows different responses to lights of different wavelengths but the same intensity.

Photoreceptor

Just one ________ cannot discriminate colour.

Photoreceptor

Principle of Univariance:

• When a photopigment molecule absorbs a quantum of light, it can only signal information about how much light is absorbed and not what the wavelength of the light is

• A single cone cannot give any indication of the colour of an object because its response can only vary in one dimension

A ________ molecule can only signal information about how much light is absobred not the wavelength of light

Photopigment

An individual with only one photopigment is know as a ________. Monochromats are unable to make wavelength based discrimination – two patches of light can always be made to appear identical by adjusting their intensities so number of quanta absorbed is equal – do not see colours!

Monochromat

The two wavelengths that produce the same response from one type of _____produce different patterns of responses across the three types of cones (S, M, and L).

Cone (M)

Trichromatic Theory Phenomena

• Many visual phenomena that are not easily explained by the trichromatic theory - no greenish red or yellowish blue perceived

• Complementary colours – produce a grayscale colour

  • Red - green

  • Blue - yellow

• Complementary Afterimages

  • A red stimulus elicits a green after-image and a blue stimulus elicits a yellow afterimage (and vice versa)

Red and _____ are complimentary and ______ and yellow are complimentary.

Green; Blue

Colour Opponent Theory:

• Herring (1892) - trichromatic signals from the cones fed into subsequent neurons and processed by three “opposing” channels:

  • red vs green

  • blue vs yellow

  • black vs white

• Channels can signal only one of the two colours at any given time

There are ______ opposing channels.

Three

Electrophysiological Evidence:

• Single unit recording in retina and LGN found colour opponent cells*

• Short wavelength stimulus causes inhibition, while long wavelength stimulus produces excitation

• Colour opponent cells occur first at level of the bipolar retinal cells in primates

Short wavelengths can cause ________, while long wavelengths produce _______.

Inhibition; Excitation

Opponency Neurons - Channels

• Red-Green Channel - Compares output from L and M cones

• Blue-Yellow Channel - Coompares output from S cones with combined output of L and M cones.

• Black-White Channel - Luminance Channel adds the cone outputs

_________ refers to a receptive field structure in the visual system where the central area of a cell is excited by light, while the surrounding area is inhibited

Centre - Surround Organisation

Stages of Colour Vision:

• Trichromatic photoreception (1st Stage)

• Opponent - process neural coding (Retina and lateral geniculate nucleus (LGN)) (2nd Stage)

• Higher level cortical processing (3rd Stage)

There are ______ stages to colour vision.

Three

Parallel Processing of Colour Signals - Retina to LGN:

• R-G "P" ganglion cells → LGN parvocells, which show R-G opponency

• Y-B ganglion cells → LGN Konio cells, which show Y-B opponency

• Spectrally opponent LGN cells about 60% of LGB neurons

• Koniocellular (K) cells and parvocellular (P) cells are two distinct types of neurons found in the lateral geniculate nucleus (LGN) of the thalamus, which plays a crucial role in relaying visual information to the visual cortex.

P ganglion cells got to __________ in R-G opponency, meanwhile Y-B ganglion cells go to _______ in Y-B opponency.

LGN parvocells; LGN koniocells

Cortical Processing:

• Visual cortex (V1) – cortical area where most LGN axons synapse

  • Concentrically organised

  • double colour-opponent neurons

  • Parvo input – RG

• V1 to prestriate cortex V4

V1 is where______ LGN axons synapes are.

Most

Colour Constancy

• A blue shirt looks blue under vastly different lighting condition:

  • Indoors

  • Natural sunlight (e.g. day time, sunset)

  • Fluorescent light

• The apparent colour of a reflective surface remains constant even when changes in the illuminance alter the wavelengths reflected from it

• Adelson’s illusion - an optical illusion where two squares of the same color and brightness appear different due to the perceived presence of shadows.

• Visual system able to discount the illumination by “looking at lots of objects at once”

• If all reflecting long wavelengths, probably means objects are being illuminated by red light and system compensates for this…blue shirt appears blue as it is bluer than things around it!

Lighting affects ________.

Perceived Object Colour

Describing Color Perception:

• Hue - Lay person’s term for colour – the colour we see

• Saturation

  • Purity of colour - a desaturated colour appears as though it has been mixed with white

• Brightness

  • Perceived intensity of the coloured surface or source

• Primary colours – Red, Green and Blue lights – mixed together see white

• Complementary colours- two colours mixed produce white

• Color space: A three dimensional space that describes all colors.

• There are several possible color spaces

  • RGB color space: defined by the outputs of long, medium, and short wavelength lights (i.e., red, green, and blue).

  • Hue, Saturation, Brightness color space

_______ is a three dimensional space that describes all colours.

Color Space

CIE Colour System:

• This system was developed by the International Commission of Illumination (CIE – Commission Internationale de L’Eclairage)

• Standard graphical representation of the hue and saturation attributes of color, based on color matching data obtained from large groups of observers

• Important colour metric used internationally in industry for colour specification - defines colour experience in quantitative way

CIE stands for ____________.

International Commission of Illumination (CIE – Commission Internationale de L’Eclairage)

Anomalies of Colour Vision:

• Occur in around 8% of males and 0.4% of females

• Colour matching divides colour anomalies into:

  • Anomalous trichromat require three primaries to achieve metameric matches

  • Dichromats require only two primaries to achieve metameric matches

  • Monochromats require only one primary

• Classified according to the number of primary colours required to match all the spectral hues

  • Means the number of primary colours required to match to the number of photopigments present

• Opponent theory of excitation and inhibition allows us to make those assumptions according to the hue presented

_______ only require two primaries to achieve metameric matches.

Dichromats

Anomalous Trichromacy:

• Trichromatic vision as has three types of photopigment in their cones

• One of the pigments is shifted from the normal position

• The greater the displacement the more severe is the colour vision anomaly

• Require a different mixture of primaries to a normal in order to obtain a colour match

In ______ one of the pigments is shifted from the normal position.

Anomalous Trichromacy

Protanomalous Trichromacy:

• Mutation of L-cone photopigment gene

• L cone pigment (erythrolabe) spectrum is displaced toward shorter wavelengths –

• Hereditary – X-linked recessive trait; Approx. 1 % males

In ______ the L cone pigment spectrum is displaced towards _____ wavelengths.

Protanomalous Trichromacy; Shorter

Deuteranomalous Trichromacy

• M-cone photopigment gene replaced by hybrid L/M gene

• M cone pigment (Chlorolabe) - spectrum is displaced towards longer wavelengths.

• Most common form of congenital colour deficiency – about 5% population

• Hereditary – X-linked recessivecyn

In _________ the M cone pigment (chlorolabe) is displaced towards longer wavelengths.

Deuteranomalous Trichromacy

In ___________ the S cone pigment (cyanolabe) spectrum is displaced towards longer wavelengths (Rare).

Tritanomalous Trichromacy

The defect in dichromatic colour vision tends to be more severe than in ______ with Dichromats lacking ______ type of pigment.

Anomalous Trichromacy; One

________ is dichromacy with a missing L cone and reduced sensitivity to long-wavelength.

Protanopia

_______ is dichromacy with a missing M cone.

Deuteranopia

_______ is dichromacy with a missing S cone.

Tritanopia

Colour Labelling:

• Individuals with red-green dichromacy are essentially monochromatic for wavelengths beyond approximately 545nm, yet do surprisingly well at labelling colours, especially when other cues present such as

  • Brightness

  • Context

____________ dichromacy is essentially monochromatic for wavelengths beyond _____.

Red-Green; 545nm

______ classifications end with _______

‘-anomalous’; '-opia'

Achromatopsia

• Extremely rare condition presenting as monochromatic vision either as:

  • Cone monochromatic

    • Characterised by presence of only one primary colour and thus person is truly colour blind

    • Such patients usually have normal visual acuity

    • Defect in post receptor processing

  • Rod monochromatic

    • Autosomal recessive trait, very rare

    • Characterised by total colour blindness, poor visual acuity, nystagmus, photophobia

There are two types of monochromaticism _______ and ______.

Cone; Rod

Acquired Colour Vision Anomalies

• • Secondary to disease or toxicity and may be R-G or BY.

  • B-Y anomalies rarely inherited, assume acquired until proven otherwise

  • Acquired colour anomalies often unstable and may not produce clean test results

  • May be unilateral or asymmetric

  • Important to perform colour vision tests monocularly

B-Y anomalies are _______ and is assumed acquired until proven otherwise.

Rarely Inherited

Optic neuritis, Leber’s Optic Atrophy, Toxic Amblyopia or Leisions of Optic Nerve and Pathway are ______ colour vision defect.

R-G

• Colour psychophysical phenomenon … how we perceive a stimulus … normal colour vision = trichromatism

• Colour perception has 3 aspects: wavelength reflected to eye, visual pigments react, neural processing of information

• Colour contrast helps to distinguish objects (finding fruit …)

• At least two receptors needed to detect colour due to principle of univariance

• Trichromacy (Young & Helmholtz):

  • three primary colours can be mixed (different proportion) to achieve any colour to be perceived; two colours can be perceived identical despite different wavelength composition (i.e., metamer)

  • There are 3 different types of receptors sensitive to Short (S), Medium (M) and Long (L) wavelengths that encode the wavelength signal perceived

  • Does not explain complementary colours or complementary afterimages

• Colour Opponent theory (Herring): trichromatic signals from the cone receptors feed into subsequent neurons and are processed by opposing channels (R/G, B/Y, B/W)

• Colour vision anomalies/deficiencies can be congenital (x-linked recessive, rare: 8% males and 0.4% female) or acquired (monocularly or asymmetric)

  • anomalous trichromats, dichromats, monochromats

Normal colour vision = Trichomatism. True or False?

True

Perception of Colour

• Perception of colours depends on wavelength, amount of light absorbed, threshold stimulus + frequency of cone impulses

• Orange = 99% Red: 42% green: 0%

• Yellow = 50% red: 50% green: 0% blue

• Blue = 0% red: 0% green: 97% blue

Yellow = 50% red: 50% green: 0% blue. True or False?

True

Colour Opponets

• Opponents are processed within the same channel

  • Not possible to see both in a channel at the same time

  • When cones are excited their opposing colour is inhibited

    • No reddish green

    • No yellowy blue

• Activation of one channel inhibits the other

• Inhibition

  • Works within the channels and can result in fatigue of a cone’s excitatory ability

  • Fatigue of the channel’s processing ability

• The channels also compete for processing space

  • When the red and green cones are stimulated at the same time the blue-yellow channel is inhibited

  • We see yellow even though there is no yellow cone…

When cones are excited their opposing colour is _______.

Inhibited

Colour Vision

• Trichromatism

  • Normal colour vision

  • All three cone types work as they should

• Congenital Anomalies

  • Lack of functional cones

  • Cones all present, absorption characteristics abnormal

  • One or two photopigments rather than three

• Acquired Anomalies

  • Secondary to pathology

  • Something selectively causes the cones to not work

  • Something selectively causes the channel to not work

Trichromatism is ____________.

Normal Colour Vision

Colour Vision Deficiency

• Monochromats

  • Rod monochromatic

  • No functioning cones

  • Poor visual function and acuity

  • Nystagmus

  • Photophobia

• Cone monochromatic

  • Single cone response

  • Otherwise normal visual function

• Dichromats

  • Absent photopigment

  • Red

  • Green

  • Blue

• Anomalous Trichromats

  • Defective photopigment

  • Red

  • Green

  • Blue

Rod monochromats have no functioning _______.

Cones

Pro- means missing ______. Deutr- means missing _____. Tritan- means missing _____.

Red; Green; Blue

Protan and Deuteran defects have ___________ inheritance.

Sex-linked

• Colour Vision Deficiency Screening Tests

  • Ishihara

  • Standard Pseudoisochromatic Plates Part 2

  • Red Desaturation1

  • The City University Colour Vision Test Part 1

• Colour Vision Deficiency Grading Tests

  • The City University Colour Vision Test Part 2

  • Fansworth D-15 Test

The ________ can be used for screening and grading.

The City University Colour Vision Test

Take _______ measurement for testing congenital colour vision defect. Take ______ measurement if testing for acquired colour vision tests.

Binocular; Monocular

Ishihara Test

• Screening for red green colour deficiency at 75cm test distance

• 25 plates (abbreviated version) containing numerals and lines

  • Pseudoisochromatic

• First plate is an introductory plate

  • Demonstration of the task

  • Detecting malingerers

• Next twenty plates

  • Screening for protan/deuteran

• Four for classification

  • Protan/deuteran

• Screening for red green colour deficiency

• Transformation Plates (Confusion plates)

  • One number is seen by colour normals (8 and 5)

  • A different number is seen by those with colour deficiency (3 and 2)

• Vanishing number plates

  • A number is seen by colour normal

  • No number is seen by those with colour deficiency

• Classification of red green colour deficiency

  • Two numbers or lines presented

• Protan observers

  • See the right hand number (6)

  • bottom line

• Deuteran observers

  • See the left hand number (2)

  • top line

• When no number seen severe red-green defect

• When both numbers seen with significant errors previously ask if one of the numbers look clearer

Ishihara should be done at _______.

75cm

Ishihara has transformation, _______, hidden digit and classification plates.

Vanishing

The City University Test

• Adjunct to a screening test

• Monitoring of acquired colour vision deficiency

• Tests for blue-yellow deficiency as well as red-green deficiency

• Passing the City University Test after failing the Ishihara Test is likely to indicate a mild colour deficiency which is unlikely to impact on occupational choices

• Part 2

  • Derived from the Farnsworth D-15

  • grading

  • 10 charts

  • Central colour with four surrounding colours

  • Patient is asked to select which of the surrounding colours best matches the central colour

  • Options displayed

    • Normal

    • Protan

    • Deuteran

    • Tritan

• Interpretation

  • Protan/Deuteran

    • Patients can select colours in both categories

    • Differentiating between protan and deuteran can be difficult

  • ‘Borderline’

    • One or two mistakes

  • ‘Fail’ (not clear if n/p/d/t)

    • More than two mistakes

  • Score out of 10

    • Number of mistakes in the normal column indicates the severity of the deficiency

Part two of the city university test is derived from ________ the test.

Farnsworth D-15

Farnsworth D15

• Developed to assist in vocational guidance

  • Farnsworth D-15

  • Farnsworth-Munsell 100 hue

• Farnsworth D-15

  • Indication significant defects

• The patient arranges the 15 colours in sequence as they see it

• This sequence is then plotted on a circular chart

• The shape of the pattern plotted indicates the type of defect

  • Protan

  • Deuteran

  • Tritan

• Number of errors indicates the severity of the defect

• Scoring

  • This sequence is then plotted on a circular chart

  • The shape of the pattern plotted indicates the type of defect

    • Protan

    • Deuteran

    • Tritan

• Number of errors indicates the severity of the defect

• Normal Colour vision

  • Trichromatism

• Protan

  • Severe (r)

  • Moderate (l)

  • Deficient with red

• Deuteran

  • Severe

  • Moderate

  • Deficient with green

• Tritan

  • Severe

  • Moderate

  • Deficient with blue

The two types of Farnsworth are the _________ and ______ tests.

Farnsworth D-15; Farnsworth-Munsell 100 Hue

Farnworth-Munsell 100 Hue

• Similar to D-15

  • More colour discrimination options

  • 85 Hues

  • Approximately equal steps

  • Four boxes with a reference hue at each end of each box

• Tests the hue discrimination in normal trichromatic

• Tests the hue discrimination in those with colour deficiencies

• Scoring on a circular graph

  • Error score created by comparing the numerical differences of adjacent colours

• Congenital defects are shown by errors in defined poles

  • Axis of confusion

• Degree of defect estimated from the total error score

• Good for acquired defects

  • Monitoring change over time

  • Fatigue effects

  • Learning effects

Farnsworth-Munsell 100 Hue colour vison test is good for __________ defects.

Acquired

Other Colour Vision Tests:

• Nagel Anomaloscope

  • Red-green defects

  • Normal or deficient (screening)

  • Dichromatism or anomalous trichromatism

  • Colour matching by altering luminance of a luminance control

• Colour Vision Lanterns

  • Red-green defects

  • Screening or grading

  • Patients identify the colour of the light emitted by the lantern

The ________ test is used for screening and the _______ test is used for screening or grading.

Nagel Anomaloscope; Colour Vision Lanterns

Colour Vision Tests for Occupation:

• Aviation - Ishihara

• Navi - Colour blindness does not restrict but limits progression, Ishihara or Holmes-Wright Lantern

• Fire Service - Farnsworth

• Train Drivers - Ishihara

• Police - Farnworth or 2nd Edition City

Fire and Police are ______, TAN is ________, and Police is _____ and Navi is ______.

Farnworth; Ishihara; City; Holmes-Wright Lantern

Red Desaturation

• Testing the sensitivity to the colour red

  • Acquired colour defect

  • Reduced VA

  • Pupil function

• Adjunct test for rapidly assessing the likelihood of neurological changes

  • optic nerve and visual pathway pathologies

• Comparison of how a red object is perceived with each eye separately

• Red colour appears washed out with the affected eye

The Red Desaturation test is used for ________ defects and should be tested ________.

Acquired; Monocularly