Contrast Sensitivity & Visual Acuities

Question 1

Describe the psychophysical determination of Contrast Sensitivity Function?

Answer 1

1. Select a spatial frequency of the grating to be tested, for e.g., 1 cycle per degree (c/d)
   o To completely characterise a CSF, typically 5-10 different cycles per degree sampled in a log step (e.g. 0.5 c/d, 1 c/d, 2 c/d, 4 c/d, 8 c/d, 16 c/d)
2. Determine the contrast threshold using one of psychophysical methods (e.g., staircase)
3. Compute the inverse of contrast threshold to get the contrast sensitivity (1/threshold = sensitivity & vice versa)
4. Repeat for other spatial frequencies
5. Plot sensitivity (y-axis) as a function of spatial frequency (x-axis) in log-log scale – contrast sensitivity function (CSF)

Question 2

How do you calcualte contrast threshold from a staircase?

Answer 2

• To calculate contrast threshold from each staircase, have to find contrast at reversal points – reversal is where direction of contrast presentation is changing
• Discard 1st 2 reversals & take average of last 4 (no right or wrong answer in how many reversals need to discard as long as mention how many you discarded)
• 0.125 + 0.25 + 0.25 + 0.0625 = 0.6875  0.6875/4 = 0.172

Question 3

Describe threshold vs sensitivity?

Answer 3

• Sensitivity measurements in psychophysics are reciprocal to threshold measurements
• Sensitivity = 1/threshold (i.e. CS = contrast threshold-1)
• Lower the threshold, higher the sensitivity
• Lower the sensitivity, higher the threshold
• Often plotted in log-log coordinates
  Contrast Sensitivity is unitless

Question 4

What are the characteristics of a normal contrast sensitivity function?

Answer 4

• Curve itself borders between seen & non-seen regions
• Under curve represents area where observer can see  the resolved area
  o Any contrast below curve will be resolved regardless of spatial freq
  o Beyond curve is unresolved region
• CSF of normal observer peaks ~ between 2 & 4 cycles per degree
• From here, CS quickly drops off to no sensitivity ~40-60 cycles per degree
  o Known as high spatial cutoff frequency of human vision (spatial resolution limit)  finest detail we can resolve w/ 100% contrast
• Property of the optics: no loss in low spatial frequencies but unlike MTF, have a drop off in low spatial freq region -> lose sensitivity to contrast in low spatial freq

Question 5

Describe Modulation Transfer Function vs Constrast Sensitivty Function?

Answer 5

• When 100% contrast image passes through optics of visual system –> virtually no loss in contrast at lowest spatial frequencies –> contrast loss is greater at higher spatial frequencies
• Image, after eye, is subject to further neural processing & final output of visual system is CSF

Question 6

What is high frequency cut-off?

Answer 6

• Routine VA measurement is measuring smallest high contrast letter px can resolve  essentially measures only one single sensitivity on contrast sensitivity function at cut-off spatial freq
• High freq cut-off = visual acuity
• Factors affecting high freq cut-off:
  o Optical limitation such as refractive errors
  o Packing density of retinal photoreceptors
• A – high spatial freq grating projected on a coarse photoreceptor matrix  circle represents photoreceptors. Multiple bars fall on a single photoreceptor column  effective contrast ↓ & more difficult to resolve grating
• B – same grating w/ same spatial freq grating falling on finer photoreceptor matrix -> finer matrix able to resolve grating because width of bars of grating corresponds to column width of photoreceptor matrix so that it can maximally respond to contrast of grating
• Foveal cones subtend ~ ½ arcmin in diameter  if grating falls on retina like B then a cycle will subtend one arcmin
  o Such a grating has a spatial freq of ~60 cycles per degree  approx. matches high spatial freq cutoff of human CSF

Question 7

What is the clinical determination of Contrast Sensitivity Function?

Answer 7

Chart based:
VISTECH: gratings w/ differing contrast & spatial frequencies displayed on printed chart
Low contrast logMAR chart – same shape as logMAR, letters just lower contrast
Determines contrast sensitivity somewhere in between high spatial freq cut-off & peak frequency
Pelli-Robson Chart: optotypes w/ differing contrast of same size printed on a chart  composed of 8x6 letters, all Sloan letters of same size, they are ½ a degree visual angle at 3metres (~6/18) –> Depending upon spatial freq component of each letter, chart tests ~3-5 cycles per degree (peak sensitivity)
Each row contains 2 triplets of same contrast & contrast ↓ by 1/√2 from top left to bottom right
Trying to determine contrast sensitivity at most sensitive spatial freq
Tablet based:
qCSF

Question 8

What the clinical implications of Contrast Sensitivty Function?

Answer 8

• VA only measures ability to see highest spatial frequencies providing little info about rest of CSF
• Majority of complaints for visiting an optometrist is uncorrected refractive error – measuring VA is still a good practice for clinical refraction
• Some disease may affect low SF more than high SF – CS is better than VA

Question 9

Describe letter acuity and cut-off sensitivity function - give values?

Answer 9

• Resolution acuity: smallest separation between 2 features that can be resolved
  o This is measured in most optometry practices
• Minimum Angle of Resolution (MAR)
  o Equal to width of one stroke of letter
   Size of 6/6 letter is scaled so angular size of one limb of letters actually subtends one arc minute at 6m
  o Resolution acuity limit
  o Angular size of minimum separation
  o 1 arcmin, 6/6 vision, or 30 c/deg
• Letter acuity is closely related to high-spatial cut-off frequency of CSF
• Letter acuity is a kind of resolution acuity as should be able to resolve the gaps between the strokes of the letter but at same time must also identify what the letter is among other letters  RECOGNITION
• Letter acuity tests both resolution & recognition
• Letter acuity is most commonly utilised clinical measure primarily because of utility in diagnosis of foveal disease & detection & correction of refractive errors

Question 10

Describe letter, MAR, Snellen and Cut-off Sensitivity Function?

Answer 10

• Assuming letter is a grating consisting of dark & bright bars then a pair of one bright & one dark bar comprises a cycle
• At 6m, by definition, a bar subtends one arc minute
• A cycle is 2 arc min
• If px has 6/6 vision then expected cutoff spatial frequency will be 30 cycles per degree

Question 11

What are the factors affecting letter acuity?

Answer 11

1. Refractive error:
   o Will cause defocus at retina – makes letters blurry – losing fine details around edges
2. Choice of letters:
   o Relative legibility is different letter to letter e.g. C & O more difficult to identify than L & T
3. Letter spacing:
   o Affects letter acuity – identification of letters depends on proximity of neighbouring letters
   o Maximum degradation in recognition of target letter occurs at half a letter width between target & surrounding letters  known as crowding or contour interaction
4. Retinal illumination:
   o Vas are usually measured using standard chart under bright light condition – mediated by cones
5. Contrast:
   o Letter acuity falls as contrast is lowered
6. Eccentricity:
   o VA falls exponentially w/ increasing eccentricity
   o Letter acuity is almost worsened at twice the foveal value at 2° of eccentricity

Question 12

Describe the VA: Vernier Acuity (Hyperacuity)?

Answer 12

• Measures minimum displacement can detect between 2 lines
  o V slight vertical displacement between line A & B (B slightly above A)
• VA thought to be limited by cone size
• Minimum offset of 2 lines that can be detected
• Thresholds ~2-10 arcsec (much smaller than single cone diameter & spacing in the retina)
• This acuity is far better than you would expect based on cone spacing, Vernier acuity is sometimes called a hyperacuity

Question 13

Describe VA: Minimum (Detectable) VA?

Answer 13

• Smallest angular size of an object that eye can detect
• Finding minimum angular size of a line that can be detected against a uniform background
  o Task is equivalent to contrast increment detection against background
• Threshold is v small (1/6th of an arcmin) ~1 arcsec, much smaller than diameter of a single cone (~0.4 arcmin)
• When contrast of an image exceeds observer’s contrast threshold, object will be visible