VOP - Contrast Sensitivity - Topic 4 Flashcards
What are the axis of a contrast sensitivity function.
- x-axis is spatial frequency in c/deg
- y-axis is contrast sensitivity in cd/m2
What does the Spatial Contrast Sensitivity Function (CSF) show?
It shows CSF shows human sensitivity to sine waves of different frequency.
What is the shape of the CSF?
•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)
What is an octave?
Its a doubling
What is the SP cutoff for the CSF of adults?
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.
What is the peak sensitivity for the CSF?
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)
Under what conditions do we get the normal adult CSF?
Photopic Conditions, bright
What are Luminance gratings?
They’re stimuli for measurement of CSF
What are the 4 defining parameters of Gratings?
SF
Phase
Orientation
Contrast
Why are gratings useful targets for assessment of vision?
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
How do you calculate the Michelson Contrast?
Contrast (Michelson) = Lmax – Lmin/ Lmax + Lmin
Range: 0-1 or 100%
Lmax= Peak of wave Lmin= Trough of wave
Can the shape of the CSF be changed psychophysically(behaviourally) and why ?
Yes, because we have neurons in the brain that respond to different frequencies. So we can selectively impair a group of neurones.
What is the Michelson Contrast used for?
Its used to define a curve of a repetitive sine wave grating.
What are the units for spatial frequency?
Cycles/deg
Define Spatial Frequency(SF) in vision?
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.
What size of an image is said to subtend one degree of visual angle on the retina?
One-third of a millimeter of an image
What does a higher spatial frequency mean?
It means that there’s a higher cycles/deg, so higher contrast.
Define contrast sensitivity.
is the ability to detect subtle differences in shading and patterns.
How do you interpret contrast sensitivity?
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.
Why is the visual system less sensitive to high sfs?
-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
Gratings higher than 60 c/deg do not get through eye’s
optics even at 100% contrast. Why not?
At high SFs,
peaks and troughs are blurred together so that the
effective contrast of high SF gratings is reduced.
What is grating frequency?
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.
How does a grating work?
grating is an optical element, which separates light into wavelengths(colours)
Why is the visual system less sensitive to low sfs?
Neural Factors- there are fewer neurons in brain “tuned” to lower
spatial frequencies. Also lateral inhibition
