VOP - Contrast Sensitivity - Topic 4

Question 1

What are the axis of a contrast sensitivity function.

Answer 1

• x-axis is spatial frequency in c/deg

- y-axis is contrast sensitivity in cd/m2

Question 2

What does the Spatial Contrast Sensitivity Function (CSF) show?

Answer 2

It shows CSF shows human sensitivity to sine waves of different frequency.

Question 3

What is the shape of the CSF?

Answer 3

•The function has a characteristic inverted U shape,
with the peak somewhere around 4 c/deg

• On either side of this peak there is a fall in sensitivity

• The whole function spans about 7 octaves (0.5 c/deg to
40-60 c/deg)

Question 4

What is an octave?

Answer 4

Its a doubling

Question 5

What is the SP cutoff for the CSF of adults?

Answer 5

The high frequency cut-off is at about 60 c/deg, though
more often is lower at around 40 c/deg. This is equivalent
to the resolution acuity limit of vision.

Question 6

What is the peak sensitivity for the CSF?

Answer 6

The peak sensitivity is around 200, i.e., the measured

contrast threshold is about 0.5% (you only need 0.5% of contrast to see 1 c/deg)

Question 7

Under what conditions do we get the normal adult CSF?

Answer 7

Photopic Conditions, bright

Question 8

What are Luminance gratings?

Answer 8

They’re stimuli for measurement of CSF

Question 9

What are the 4 defining parameters of Gratings?

Answer 9

SF
Phase
Orientation
Contrast

Question 10

Why are gratings useful targets for assessment of vision?

Answer 10

They only have a single spatial frequency in them, therefore we can target specific neurones in brain.

-We normally use letters, they have all Spatial frequencies in them, so we CANT target specific neurones in brain

Question 11

How do you calculate the Michelson Contrast?

Answer 11

Contrast (Michelson) = Lmax – Lmin/ Lmax + Lmin
Range: 0-1 or 100%

Lmax= Peak of wave
Lmin= Trough of wave

Question 12

Can the shape of the CSF be changed psychophysically(behaviourally) and why ?

Answer 12

Yes, because we have neurons in the brain that respond to different frequencies. So we can selectively impair a group of neurones.

Question 13

What is the Michelson Contrast used for?

Answer 13

Its used to define a curve of a repetitive sine wave grating.

Question 14

What are the units for spatial frequency?

Answer 14

Cycles/deg

Question 15

Define Spatial Frequency(SF) in vision?

Answer 15

refers to the number of pairs of bars imaged within a given distance on the retina.
- So more bars viewable in 1 degree means higher S.F, so higher cycles/deg, so higher contrast.

Question 16

What size of an image is said to subtend one degree of visual angle on the retina?

Answer 16

One-third of a millimeter of an image

Question 17

What does a higher spatial frequency mean?

Answer 17

It means that there’s a higher cycles/deg, so higher contrast.

Question 18

Define contrast sensitivity.

Answer 18

is the ability to detect subtle differences in shading and patterns.

Question 19

How do you interpret contrast sensitivity?

Answer 19

The symbols of the visual acuity charts are close to the maximum contrast.

If the lowest contrast perceived is 5%, contrast sensitivity is 100/5=20. If the lowest contrast perceived by a person is 0.6%, contrast sensitivity is 100/0.6=170.

Question 20

Why is the visual system less sensitive to high sfs?

Answer 20

-Optical factors- at high SFs, peaks and troughs are blurred together so that the
effective contrast of high SF gratings is reduced

-Neural Factors- Cone spacing is limiting factor on SF at 100% contrast

Question 21

Gratings higher than 60 c/deg do not get through eye’s

optics even at 100% contrast. Why not?

Answer 21

At high SFs,
peaks and troughs are blurred together so that the
effective contrast of high SF gratings is reduced.

Question 22

What is grating frequency?

Answer 22

A grating of high spatial frequency – many cycles within each degree of visual angle – contains narrow bars. A grating of low spatial frequency – few cycles within each degree of visual angle – contains wide bars.

Question 23

How does a grating work?

Answer 23

grating is an optical element, which separates light into wavelengths(colours)

Question 24

Why is the visual system less sensitive to low sfs?

Answer 24

Neural Factors- there are fewer neurons in brain “tuned” to lower
spatial frequencies. Also lateral inhibition

Question 25

For the CSF what are the effects of Luminance?

Answer 25

1) Overall human sensitivity decreases as light levels
decrease.
2) Peak sensitivity shifts to left (towards lower SFs).
3) The high frequency cut-off shifts to left (towards lower
SFs)

Question 26

How does the CSF change with Age?

Answer 26

• Lower SF peak (c/deg)

- Lower Contrast Sensitivity peak (1/threshold Contrast)

Question 27

What factors affect CSF?

Answer 27

• Luminance
• Age
• Disease

Question 28

Examples of diseases that affect CSF and how they comapre to normal?

Answer 28

1) Multiple Sclerosis(MS)- SF peak (x-axis) is same, Contrast Sensitivity peak (y-axis) is lower, Cut-off for SF is lower
2) Cataract- SF peak is lower, Contrast Sensitivity peak is lower, Cut-off for SF is lower
3) Refractive blur or Amblyopia- SF peak is same, Contrast Sens is same, Cut-off for SF is lower