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assessment of vision 1 > Week 6: Contrast Sensitivity & Colour Vision > Flashcards

Week 6: Contrast Sensitivity & Colour Vision Flashcards

1
Q

Describe Contrast Sensitivity

A
  • Ability to detect differences in luminance between regions that are not separated by physical borders
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2
Q

List 5 Clinical Reasons to Test Contrast Sensitivity

A
  1. Ocular disease
  2. Low vision
  3. Contact lenses
  4. Dry eye
  5. Glare issues
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3
Q

Define Contrast (Luminance Contrast) and what is its formula?

A
  • Is the relationship between the luminance of a brighter area of interest & that of an adjacent darker area

Weber Contrast – most commonly useful one in the context of lighting:
C = (Lmax - Lmin) / Lmin

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4
Q

Define Simple Contrastand what is its formula?

A
  • Values are often used in photography, to specify the difference between bright & dark parts of the picture

Csimple = Lmax / Lmin

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5
Q

Define Peak to Peak Contrast (Michelson Contrast, Modulation) what is its formula?

A
  • Measures the relation between the spread and the sum of the two luminances

To determine the quality of a signal relative to its noise level:
Modulation = (Lmax - Lmin) / (Lmax + Lmin)

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6
Q

Define Fourier theory for Sine Wave Gratings

A
  • Any pattern in the visual field can be derived from the summation of a series of sinusoidal patterns of specific frequencies, amplitudes, orientations and phases
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7
Q

What is the formula for Spatial Period ?

A

Width of one cycle = wavelength

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8
Q

What is the formula for Spatial Frequency?

A

Cycles/degree

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9
Q

What is the formula for Mean Luminance

A

Average of maximum & minimum luminance = (Lmax + Lmin / 2)

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10
Q

What is the formula for Modulation Amplitude

A

Difference between maximum & mean luminance = (Lmax - Lmin) /(Lmax +Lmin)= contrast

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11
Q

What happens when low modulation occurs?

A
  • Low modulation = low contrast
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12
Q

What is the Non-repetitive backgrounds/background formula that remains constant?

A
  • Weber contrast: Brightness difference/background

CW = (Lmax – Lmin)/Lmin

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13
Q

What is the formula for repetitive/changing patterns?

A
  • Average brightness/brightness amplitude
  • Modulation CM= (Lmax – Lmin)/ (Lmax +Lmin)
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14
Q

What charts would suit variable size / variable contrast?

A
  • Vistech Chart
  • Functional Acuity Contrast Test (FACT)
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15
Q

What charts would suit variable size/fixed contrast?

A
  • Bailey-Lovie low contrast chart
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16
Q

What charts would suit fixed size / variable contrast

A
  • Pelli-Robson Chart
  • Melbourne Edge Test
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17
Q

Describe Vistech Chart

A
  • No literacy required
  • 5 rows with gratings (1-24 cycles/degree)
  • 9 contrast levels with an average step size of 0.25 log units
  • Patient indicates whether the patch is blank or which direction the grating is tilted
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18
Q

Describe Functional Acuity Contrast Test (FACT) Chart

A
  • Similar to Vistech but has smaller contrast steps of 0.15 log units
  • Size of the gratings are larger
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19
Q

List the Grating Method Testing

A
  1. Place chart at eye level 3 m from patient
  2. 60 to 120 cd/m2 even lighting illumination
  3. Demonstrate high contrast samples at bottom of chart to teach
  4. Test monocularly
  5. Ask patient to read from top & across each line left to right and identify orientation of lines
  6. Ask patient to guess at the orientation of the lines until they can no longer see the gratings
  7. Record the patch with the lowest contrast correctly identified & compare with normal range
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20
Q

What are the benefits of Grating Chart?

A
  • Assessing functional vision
  • Clinical research
  • Illiterate patients or children
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21
Q

Describe High & Low Contrast Bailey-Lovie Charts

A
  • Fixed number of letters per line
  • Logarithmic progression of letter sizes and spaces
  • Standardised letter set & scoring system
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22
Q

Describe Pelli-Robinson Chart

A
  • Uses letter targets
  • Letters composed of complex mixture of tests
  • Involves recognition task
  • The contrast of each successive group decreases by 0.15 log units
  • Score 0.05 log for every letter read correctly
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23
Q

Describe Melbourne Edge Test

A
  • Non-grating CS test
  • Test arranged in 4 rows each with 5 disks
  • Each disk contains an edge, which decreases in contrast from top to bottom of chart
  • Patient must indicate direction of edge in each disk
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24
Q

What is used due to glare sensitivity?

A
  • Measurement of effect of glare on patients’ visual function using CS chart and brightness acuity tester (BAT)
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25
Q

What are the types of glares? (hint 2)

A
  • Disability glare: causing reduction of visual performance
  • Discomfort glare: causing discomfort without any
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26
Q

Causes of Sensitivity to Glare? List 5

A
  • Patients with cataracts
  • Intraocular lenses
  • Contact lenses
  • Ocular disease
  • Older age
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27
Q

What is Brightness Acuity Tester (BAT)?

A

• Hand-held instrument with two functions
- Brightness acuity test
- Macular photostress test

28
Q

What are the three conditions Brightness Acuity Tester (BAT) can simulate?

A
  • High – direct overhead sunlight,
  • Medium/partly cloudy day
  • Low – bright overhead commercial lighting
29
Q

List the steps on how to use a Brightness Acuity Tester (BAT)

A
  1. Test monocularly
  2. Should be used at all 3 settings if possible
  3. Patient seated at standard distance from letter chart
  4. Undilated pupils in dark examination room
  5. Test and record visual acuity at each of the three settings
30
Q

Describe Berkeley Glare Acuity Tester (BGAT)

A
  • Reduced high and low contrast Bailey Lovie letter chart
  • Chart is front illuminated and glare source provided by transillumination of plexiglass panel
  • High and low contrast VA measured at 1 m with & without glare source
31
Q

What is colour?

A
  • A perceptual construct from retinal inputs to the primary visual cortex
32
Q

What is Colour Good For?

A
  1. Originally evolutionary advantage
  2. Segmenting the visual world
  3. Signalling
    - Traffic lights
    - Ripeness of fruits
    - Complex colour codes
33
Q

What is the spectrum measurement for visible colour?

A

380 – 780 nm

34
Q

What are the perceptual dimensions of colour?

A
  1. Hue
  2. Saturation/chroma
  3. Brightness/value
35
Q

Describe Hue

A

Colour (based on wavelength)

36
Q

Describe Saturation/Chroma

A
  • Intensity of hue
  • Quality of a colour in terms of purity/intensity/saturation
37
Q

Describe Brightness/Value

A
  • How much is reflected and/or emitted that enters the eye
  • Darker colours have a lower value
38
Q

Describe the young-Helmholtz Trichromatic Theory

A

• Humans have 3 cone types
- 3 different peak λ absorption
- different spectral sensitivities

• Cones are colour blind

39
Q

What are the limitation for Trichromatic Theory?

A
  • Fails to account for the four unique colours; red, green, yellow and blue
40
Q

Describe Hering’s Colour Opponent Theory

A
  • Colour is processed by bipolar hue channels – red/green or blue/yellow
  • Brightness is coded by a white/black channel
  • The theory explains how we see yellow even though there is no yellow cone
41
Q

What are the wavelengths for rods, S, M & L peak sensitivity?

A
  • Rods 493 nm
  • S peak sensitivity = 430 nm
  • M peak sensitivity = 530 nm
  • L peak sensitivity = 560 nm
42
Q

Describe trichromacy

A
  • Individual cones do not transmit wavelength information
  • Photoreceptors are blind & tend to have different sensitivity levels
  • These three colours can then be combined to form any visible colour in the spectrum
43
Q

Describe what is in photopigments?

A
  • Photopigment consists of a chromophore chemical and an opsin (protein)
44
Q

What are the different opsins?

A
  • S-cone opsin
  • M-cone opsin
  • L-cone opsin
  • Rhodopsin
45
Q

Explain the CIE Chromaticity Diagram

A
  • Created in 1931, a map that shows what colours can be perceived by the human eye
  • Each colour has an x, y, z coordinate
  • CIE primaries are imaginary
46
Q

Briefly describe the statistics of colour vision anomalies

A
  • Approx 4.5% of people have colour vision anomalies
  • 8% males, 0.5% females
  • Most are inherited, some acquired
47
Q

Describe what is missing from Anomalous Trichromacy Cone Pigment Anomaly

A

Refer to week 5 table

48
Q

What are two categories of colour vision anomalies?

A
  1. Dichromacy (missing)
  2. Anomalous Trichromacy (displacement)
49
Q

Describe Anomalous trichromats

A
  • Have 3 photopigments, but their colour vision is abnormal
  • The greater the displacement of the photopigment the worse the colour vision problem
50
Q

What are the general features of Anomalous trichromats?

A
  1. Can see colours, just perceived differently
  2. Problem with mixture of red and green
  3. May be unaware of any colour deficiency
51
Q

Describe dichromatic

A

Have only two photopigments, 1 missing

52
Q

List the colour deficiencies for dichromatic

A
  1. Deuteranopia
  2. Protanopia
  3. Tritanopia
53
Q

Describe Deuteranopia

A
  • Red-Green deficiency
  • Chlorolabe is missing (Green cone opsin) – missing green pigment
  • No confusion with yellow and blue
  • Sees green objects black
54
Q

Describe Protanopia

A
  • Red-Green deficiency
  • Erythrolabe is missing – missing red pigment
  • No confusion with yellow and blue
55
Q

Describe what Protans & Deutans have in common

A
  • Red-green dichromats are monochromatic for wavelengths beyond 545 nm
  • However they can label colours very well (context of object or brightness of object)
56
Q

Describe Tritanopia

A
  • Blue-yellow deficiency (Cyanolabe) – s cone pigment
  • Very rare
  • Cannot distinguish blue and yellow
  • No issue with red or green
57
Q

Describe Monochromacy

A
  • Only one photopigment
  • Rod monochromacy or cone monochromacy
58
Q

Describe Strategies to Overcome Monochromacy

A
  • Labelling based on brightness
  • Intensity of image used to discriminate colour
59
Q

Describe achromatopsia

A
  • Rare condition
  • Patients will have monochromatic vision
  • Commonly Autosomal recessive
  • Complete achromatopsia – only rods present; see in Black and white
  • Incomplete achromatopsia – only Long or/& Medium cone function
60
Q

What are the Symptoms of Achromatopsia?

A
  • No/poor colour discrimination
  • Photophobia
  • Poor vision (6/60)
61
Q

Describe Treatment for Achromatopsia

A
  • Dark red lenses can be prescribed to minimise bleaching of rhodopsin & allow for rods to function in brighter lighting conditions
62
Q

Describe chromatopsia

A
  • Patients complain of distortion of colour vision with a coloured tinge/halo
  • Often can occur following removal of cataracts
63
Q

Inherited vs Acquired Colour Vision Defect

A
  • Inherited anomalies are bilateral and symmetric
  • Acquired anomalies can be unilateral or asymmetric
  • Acquired anomaly = if there is a colour vision difference between the pair of eyes
64
Q

Describe Kollner’s Rule

A
  • Outer retinal disease and changes in media causes blue-yellow colour vision anomalies
  • Red-green anomalies
  • Disease of inner retina, optic nerve, visual pathway & visual cortex causes
65
Q

List three colour vision tests

A
  1. Pseudoisochromatic tests (e.g. Farnsworth D-15)
  2. Hue discrimination (e.g. Farnworth Munsell)
  3. Colour matching
66
Q

What does the Ishihara detect?

A
  • Screening for protan and deutan defects only

assessment of vision 1 (10 decks)

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