Contrast - Visual Neurophysiology and Perception Spring 2026

VA is typically measured in the clinic using high contrast targets like black letters on a white background

With what contrast in VA typically measured in clinic?

reduce

Reducing the contrast, such as making the letters lighter gray instead of black, will (reduce/increase) VA measurements

to assess visual function

What is measuring the ability of your patients to detect lower contrast targets important for?

mobility

In our everyday life, there are varying levels of contrast between objects of interest and the background. Contrast sensitivity is very important for what activity?

Letter Charts for Measuring VA and Contrast Sensitivity (Pic)

Letter Charts for Measuring VA and Contrast Sensitivity (Pic)

-100% contrast

-10% contrast

What are the different forms of the Bailey-Lovie Acuity Chart?

Letters all the same size, decreases in contrast sensitivity as decreases down the chart

What is the Pelli Robinson Contrast Chart?

how much light varies across the stimulus

What is the definition on contrast?

Weber

For an aperiodic stimuli, the contrast will be equal to the ____ fraction

ΔI / Ib

ΔI = change in luminance of the spot or =Imax - Imin

Ib = luminance of the background (surrounding)

What is the Weber fraction?

decrease

For a fixed white background luminance, as the luminance of a letter increases, the contrast of the image will (increase/decrease)

high

High contrast between object and background will result in a ΔI that is (high/low)

low

Low contrast between object and background will result in a ΔI that is (high/low)

No

At 0 contrast, can we differentiate a letter from the background surrounding the letter

-ΔI / Ib

-ΔI = 0.4-0.2

-Ib = 0.4

-0.2/0.4 = 0.50

What is the Weber Fraction of the following scenario?

Greyscale image with letter luminance Imin = 0.20; Background Luminance = 0.40.

-ΔI / Ib

-ΔI = 0.8-0.4

-Ib = 0.8

-0.4/0.8 = 0.50

What is the Weber Fraction of the following scenario?

Greyscale image with letter luminance Imin = 040; Background Luminance = 0.80.

true

True or False:

The Weber fraction for anything printed is the same under bright and dim illumination

No

Does illumination typically affect contrast on printed material?

Yes

Does illumination affect acuity on its own?

No -- it will reduce the contrast of that stimulus

Will shining a light on a self-luminous stimulus increase contrast?

luminance changes gradually from low to high and then back to low (described mathematically by a sine function)

REVIEW: Describe the luminance of a sine-wave grating

sharp luminance edges

REVIEW: Describe the luminance of a sqaure-wave grating

contrast

A sine-wave grating has a ____ value

of how bright the bright parts are compared to the dark parts of the sine-wave grating

What is the contrast value of a sine-wave grating a measure of?

100% Contrast Sine-Wave Grating v Low Contrast Sine-Wave Grating (Pic)

100% Contrast Sine-Wave Grating v Low Contrast Sine-Wave Grating (Pic)

No -- we will use the Contrast Formula (Michelson)

For a grating (such as a sine-wave grating), do we typically use the Weber fraction to calculate the contrast?

(Imax - Imin) / (Imax + Imin)

Imax = brightest luminance

Imin = dimmest luminance

What is the Grading Contrast Formula?

-Imin = 0

-Grading Contrast Formula = 1

-100% contrast

If the black bars in a grading have 0 contrast, what is the contrast of the sine-wave grading?

No! -- the contrast will always be 100%

If the Imin = 0, does it matter what the Imax is?

The contrast will approach 100%

(Imax - Imin) / (Imax + Imin)

(90-1) / (99-1) = 0.91

Value will be somewhere between 0-1

If there is a big difference between the Imin and Imax but the Imin is very low, what will the contrast of the grating be?

depend on the actual value of Imax and Imin

If there is a medium difference between the Imin and Imax, what will the grading contrast depend on?

depend on the actual value of Imax and Imin

If there is a small difference between the Imin and Imax, what will the grading contrast depend on?

0% contrast

(Imax - Imin) / (Imax + Imin)

(90-90) / (99-99) = 0

If there is no difference between the Imin and Imax, what will the grading contrast be?

Human Contrast Sensitivity Function (Pic)

Human Contrast Sensitivity Function (Pic)

a summary of how well the patient sees sine-wave gratings of different spatial frequencies

What is the human contrast sensitivity function?

medium spatial frequencies

What spatial frequency do humans see the best?

high spatial frequencies -- there is a large drop off

What spatial frequency do humans see the least?

Have a subject look at the sine-wave gratings of different spatial frequencies. Then, change the contrast of gratings and measure the threshold by the method of adjustment.

What is one way to measure contrast sensitivity in humans?

No -- because of center-surround antagonism

Are very low frequencies seen well by humans? Why?

Yes -- this is the low frequency roll off depicted in a contrast sensitivity function

Does a patients spatial frequency decline at low frequencies?

the optics of the eye

What is the loss of contrast sensitivity on the high spatial frequency side d/t?

2

REVIEW: For a spatial frequency of 30 cycles/min, each cycle is how many min of arc?

1

REVIEW: For a spatial frequency of 30 cycles/min, each stripe is _____ min of arc wide

d/t the optics of the eye

For the normal patient, the linear decline at high spatial frequencies on a log sensitivity v spatial frequency graph is d/t what?

the highest frequency the patient can see at 100% contrast

What is the grating VA on a log sensitivity v spatial frequency graph?

When log contrast sensitivity is shown as a function of log spatial frequency

log sensitivity v spatial frequency graph is curved when?

-diffraction at the pupil edge

-aberrated optics

-defocus

What is the high spatial frequency drop off d/t in a log sensitivity v log spatial frequency graph?

large

Only (large/small) receptive fields respond to very LOW-spatial frequency sine waves

All sizes of receptive fields -- because of the sharp edges of a square-wave

What receptive fields will respond to square-wave stripes? Why?

No -- only sine waves are selective at low spatial frequencies

Is there a low frequency roll off with a square-wave grating?

tiny -- regardless of sine or square wave

Fine stripes are good for _____ receptive fields and visual acuity

high

Visual acuity ONLY provides information to the clinician about _____-frequency resolution

defocus d/t refractive error

What is VA good at determining?

Yes -- with only a minor effect on the high-frequencies possible

Can a patient have a reduction in the small or moderate spatial frequencies?

Yes -- it is important to be able to see big things in order to have good mobility

Would a reduction in the small and moderate spatial frequencies be seriously handicapping?

normal contrast sensitivity function curve

Clinical Changes in the Contrast Sensitivity Function

What does A line depict?

reduction to low and medium spatial frequencies

Clinical Changes in the Contrast Sensitivity Function

What does B line depict?

no effect on VA

Clinical Changes in the Contrast Sensitivity Function

What is the effect of VA with line B?

reduction in high spatial frequencies

Clinical Changes in the Contrast Sensitivity Function

What does C line depict?

reduction in VA

Clinical Changes in the Contrast Sensitivity Function

What is the effect of VA with line C?

reduction in all spatial frequencies

Clinical Changes in the Contrast Sensitivity Function

What does D line depict?

reduction in VA

Clinical Changes in the Contrast Sensitivity Function

What is the effect of VA with line D?

overall contrast sensitivity loss by reducing contrast

What does glare lead to? How?

LASIK -- d/t flattening and increased aberrations in the cornea

____ can make glare worse (refractive sx)

reduced

Contrast sensitivity is (increased/reduced) when there is glare

-anterior segment pathology

-after refractive sx

When does glare commonly occur?

high

Blur will lead to selective loss of ____ spatial frequencies by reducing their contrast

blur

Normal vision is limited by _____

down and to the left

Increasing amounts of blur will shift the log contrast sensitivity v linear spatial frequency graph lines where?

steeper

Contrast sensitivity function is ____ with more blur in a contrast sensitivity v linear spatial frequency graph?

-cataracts

-Corneal conditions (LASIK, keratoconus)

What are the optical disorders that can lead to a mixture of low/medium/high spatial frequency loss?

Cataract Effects on Contrast Sensitivity (Pic)

Cataract Effects on Contrast Sensitivity (Pic)

down and to the left

Increasing cataract severity will shift the log contrast sensitivity v linear spatial frequency graph lines where?

-glaucoma

-ARMD

-Retinitis pigmentosa

What are some examples of RETINAL disorders that can lead to reduced contrast sensitivity?

-MS

-Parkinson's Disease

What are some examples of NEURO disorders that can lead to reduced contrast sensitivity?

Log Contrast Sensitivity v Log Spatial Frequency Graph in Patients with Glaucoma (Pic)

Log Contrast Sensitivity v Log Spatial Frequency Graph in Patients with Glaucoma (Pic)

**Can have deficits in low/med/high spatial frequencies

VA will be normal

With a low/med spatial frequency deficit d/t glaucoma, what will happen to VA?

reduction in VA

With a med/high spatial frequency deficit d/t glaucoma, what will happen to VA?

reduction in VA

With a low/med/high spatial frequency deficit d/t glaucoma, what will happen to VA?

Letter

______ contrast sensitivity can be seriously impaired among glaucoma patients, even in those with near-normal VA (See red arrow)

decrease

VA will (increase/decrease) as you get further from the fovea

decrease -- peripheral retina has a lower CS than the fovea

Contrast sensitivity (increase/decrease) as you get further from the fovea

Contrast Sensitivity Functions in Normal and Patients with ARMD (Pic)

Contrast Sensitivity Functions in Normal and Patients with ARMD (Pic)

**shifted down and to the left

middle

There are examples cases in which contrast sensitivity to ____ spatial frequencies are more reduced that the others in patients with ARMD

high

Retinitis pigmentosa patients progressively lose peripheral vision. (High/med/low) spatial frequencies are lost first

Contrast Sensitivity Functions in Patients with Early, Moderate, and Late Retinitis Pigmentosa (Pic)

Contrast Sensitivity Functions in Patients with Early, Moderate, and Late Retinitis Pigmentosa (Pic)

**loss of high spatial frequencies first

middle

With MS, there will be very reduced ____ spatial frequencies

Notches in the spatial frequency function d/t decreases in dopaminergic neurons. These notches are unique to Parkinson's

With Parkinson's, what will the contrast sensitivity v spatial frequency curve look like?

True

SUMMARY:

True or False: Many vision disorders produce losses in both contrast sensitivity and visual acuity

sine-wave contrast sensitivity function

SUMMARY: The _______ is the "gold standard" for understanding the effects of disorders, because it describes the full contrast sensitivity function

Letter contrast sensitivity testing w/ Pelli Robinson Chart & letter VA testing

SUMMARY: What tests give a quick summary of a patients vision loss?

Letter Charts for Measuring VA and Contrast Sensitivity (Pic)

Letter Charts for Measuring VA and Contrast Sensitivity (Pic)

maximum contrast sensitivity

The Pelli-Robinson chart estimates what?

all are about 1 logMAR when tested at 1m

Letters on the Pelli-Robinson chart are what size?

Read down the chart until 2 letters are missed. Mark Contrast sensitivity on a score sheet

How to perform the Pelli-Robinson chart contrast sensitivity test?

Pelli-Robinson Score Sheet vs Contrast Sensitivity Chart (Pic)

**in groups of 3 letters for scoring

Pelli-Robinson Score Sheet vs Contrast Sensitivity Chart (Pic)

**in groups of 3 letters for scoring

subset -- tests middle spatial frequencies

Letter charts test a _____ of the overall contrast sensitivity function

Yes -- not in direct correlation with each other

If contrast sensitivity is reduced, is VA likely to be reduced? Are these correlated?

lower quality of life survey scores in patients

Reduced contrast sensitivity corresponds better to what compared to VA?

mobility

Contrast to low/middle spatial frequencies is very important for _______