Contrast Sensitivity

Question 1

What is contrast sensitivity?

Answer 1

CS is the ability to distinguish differences in luminance (e.g., ‘shades of grey’). Crudely speaking:

While visual acuity (VA) measures the smallest thing you can see [at 100% contrast]

CS measures the dimmest thing you can see

Question 2

How is contrast sensitivity measured?

Answer 2

CS is most often measured with Pelli-Robson letter charts. Though other tests exist.

Question 3

Why is conrast sensitivity important?

Answer 3

It is important because in real life need to be able to see large, faint objects (i.e., CS), as well as fine spatial detail at high contrast (i.e., acuity)

Question 4

Name some conditions that can reduce contrast senstivity.

Answer 4

Many conditions affecting eyes and visual pathways can reduce CS, independent of VA. E.g., amblyopia, cerebral visual impairment, cataract, optic neuritis, multiple sclerosis, diabetic macular oedema, glaucoma

Question 5

True or False - Contrast Sensitivity testing may also be helpful for screening for Parkinson’s or Alzheimer’s disease

Answer 5

True

Question 6

What is the formal definition of Contrast Sensitivity?

Answer 6

CS is the smallest difference in luminance between a target and a background that a person can detect reliably (e.g., on 90% of trials)

Question 7

What is perimetry often referred to as?

Answer 7

Visual field testing

Question 8

In perimetry what is contrast sensitivity refered to as?

Answer 8

Differential Light Sensitivity (DLS)

Question 9

True or False- Contrast is a relative measure

Answer 9

True - (It’s important to understand the slide)

Question 10

The contrast of a stimulus (stimulus contrast) can be expressed in mnay ways including but not limited to; the weber contrast, the micheson contrast and the RMS contrast. When would each of these contrasts be used?

Answer 10

The Weber Contrast would be used to express stimulus contrast for a uniform target

The Michelson contrast would be used to express stimulus contrast for 2 toned targets such as sinosuidal gradings.

The Rms contrast would be used to express stimulus contrast for complex images (targets) with different luminance levels

Question 11

True or False- Contrast sensitivity varies with temporal resolution/frequency

Answer 11

False - I literally made this up

Question 12

True or False - Contrast sensitivity varies with spatial frequency

Answer 12

True

Question 13

Why is contrast sensitivity regarded as a function (e.g. Contrast sensitivity function)?

Answer 13

Contrast sensitivity is dependant on spatial frequency thus you can’t just tell someone their contrast sensitivity is a certain value but rather you tell them their function ( in the sense that mathematically a function is an expression/line that is dependant on one or more variables - in this case spatial frequency.

Question 14

What unit is spatial frequency recorded in?

Answer 14

Cycles per degree / LogMAR or Snellen fraction ( depending on what the stimulus used was e.g. sinosuidal gratings, logmar or snellen letters respectively).

Question 15

What is the range for the peak of CSF in (healthy) adults?

Answer 15

Peak of CSF is generally 2-4 cpd in healthy adults

Question 16

What is plotted on the y axis of the CSF (Contrast Sensitivity Function)?

Answer 16

Contrast (c) or Contrast Sensitivity (1/c)

[It literally just depends on how you would like to format your graph]

Question 17

How can you work out someone’s visual acuity from their Contrast Sensitivity Function?

Answer 17

It is the furthest point (to the right hand side) of the x axis.

I.e. Smallest cycle you can differentiate.

Question 18

Why is the CSF an inverted ‘U’ shape and what is the evidence for this?

Answer 18

Question 19

Why is the idea that CSF is made up of different independent channels clinically significant?

Answer 19

The fact that the CSF represents multiple distinct channels is clinically important, because it implies that different regions of the CSF can be affected, independent of acuity, in different conditions.

Undertsanding this allows us to better understand how diseases may affect pxs and their quality of life.

Question 20

“Confusingly though, CS often is treated as if it is one number(!) – “their Pelli-Robson score was 1.7””

What does the output number from the pelli-robson test refer to?

Answer 20

The peak of the CSF

[Remember this is dependant on distance at whihc the test is conducted]

Question 21

At what distance is the Pelli-Robson test conducted?

Answer 21

3m away.

[Px sees 2.6-3 cycles per degree at 3m away -

Peak of the CSF is around 3 cpd in healthy adults, so at 3 m it gives a reasonable estimate of peak CS

And if you know the peak and acuity, you can have a fair guess at their whole CSF]

Question 22

How may the CSF be reduced to ‘one number’?

Answer 22

You may just take the peak as your ‘CSF’.

You may take the average by computing the area under the curve ( by doing this though you throw away key information about specific spatial frequencies)

Question 23

What are the different ways in which contrast sensitivity can be measured i.e. what are the different CSF tests available?

Answer 23

Question 24

How do all contrast sensitivity tests operate?

Answer 24

They measure contrast thresholds using a measure of limits. (i.e. find your limit of contrast sensitivity at each spatial frequency)

Be aware though not all tests test your entire contrast sensitivity function.

Question 25

Describe how a Pelli-Robson Contrast sensitivity test is carried out.

At what distance?

How does scoring work?

What score indicates ‘healthy vision’?

Answer 25

Question 26

What are limitations of the Pelli-Robson method?

Answer 26

Requires literacy

Somewhat bulky (re: school screening)

Cannot measure CS at different/specific spatial frequencies

Primarily aimed at screening. Perhaps not ideal for monitoring very small changes after time/treatment(?)

Question 27

What adult conditions are associated with a decrease in contrast sensitivity (in central vision)?

(Links are not confirmed)

Answer 27

• Cataract (this is better picked up by contrast sensitivity tests than visual acuity tests)
• AMD
• Diabetic Macular Oedema (DMO) - ( the CS score has shown to be associated with chnages in retinal thickness following anti-VEGF injections).
• Glaucoma (Glaucoma Pxs show lower foveal CS even though glaucoma affects peripheral vision)
• Multiple Sclerosis
• Parkinson’s ( infact it is believed that reduced CS could be a biomarker for parkinson’s)

Question 28

What childhood conditions are associated with reduced foveal Contrast Sensitivity?

Answer 28

• amblyopia (gives mixed results in differing subjects though this may depend on the form of amblyopia)
• optic neuritis
• congenital hypothyroidism
• retinitis pigmentosa
• cataract
• corneal edema
• cerebral lesions