Color Constancy and Visual Acuity - Visual Neurophysiology and Perception Spring 2026

the constant appearance of surface color properties even through the illuminant varies widely

What is color constancy?

Yes

Are people pretty good at judging the spectral reflectance of the surface independently from the "light itself"?

yes

Is color constancy approximate?

how the visual system responds to patterns of light and dark across space

What is spatial vision?

sine-wave gratings

What are useful tools for probing properties of spatial vision?

an index of the function of center-surround units of various sizes

What do sine and square wave gratings provide?

Sine wave = luminance changes gradually from low to high and back to low (sine function)

Square wave = have sharp luminance edges

Describe sine-wave gratings compared to square wave gratings

a single spatial frequency

A sine-wave grating is composed of what?

the number of cycles per given unit of space (usually in cycles per degree of visual angle)

What is the frequency (definition) of a sine-wave grating?

Lower Spatial Frequency v High Spatial Frequency (Pic)

Lower Spatial Frequency v High Spatial Frequency (Pic)

Light on the center of the RGC and the surround is in darkness. Will increase the firing rate by depolarization & increasing action potential. Dark on the surround will also increase the firing rate.

This is an ON-center ganglion cell. What is the likely response? (See Pic)

Light is on the surround of the RGC and the center is in darkness completely. Will decrease the firing rate by hyperpolarization & decreasing action potential. Light on the surround will decrease firing rate.

This is an ON-center ganglion cell. What is the likely response? (See Pic)

Light on the center of the RGC and the surround is in darkness. Will decrease the firing rate by hyperpolarization & decreasing action potential. Dark on the surround will also reduce the firing rate.

This is an OFF-center ganglion cell. What is the likely response? (See Pic)

Light is on the surround of the RGC and the center is in darkness completely. Will increase the firing rate by depolarization & increasing action potential. Light on the surround will increase firing rate.

This is an OFF-center ganglion cell. What is the likely response? (See Pic)

Light completely fills the center of the RGC and this will increase firing rate of the RGC. The dark in the surround will increase the firing rate. The center will win out.

This is an ON-center ganglion cell. What is the likely response? (See Pic)

Dark completely fills the center of the RGC and this will decrease firing rate of the RGC. The light in the surround will decrease the firing rate. The center will win out.

This is an ON-center ganglion cell. What is the likely response? (See Pic)

Light completely fills the center of the RGC and this will decrease firing rate of the RGC. The dark in the surround will decrease the firing rate. The center will win out.

This is an OFF-center ganglion cell. What is the likely response? (See Pic)

Dark completely fills the center of the RGC and this will increase firing rate of the RGC. The light in the surround will increase the firing rate. The center will win out.

This is an OFF-center ganglion cell. What is the likely response? (See Pic)

true -- see pic

True or False:

Depending on where it falls relative to a receptive field, a stimulus with wide stripes can elicit zero response or a strong response

**These are ON-center RGCs

true -- see pic

True or False:

Depending on where it falls relative to a receptive field, a stimulus with medium-sized stripes can elicit zero response, a modest response, or a very strong response

**These are ON-center RGCs

False -- no matter where they fall, they do not elicit much of a response

True or False:

Depending on where it falls relative to a receptive field, a stimulus with narrow stripes can elicit zero response, a modest response, or a very strong response

**These are ON-center RGCs

No

Do uniform fields elicit much of a response on ON-center RGCs?

Stimuli that have a width that approx have the same width of the center receptive field

SUMMARY: A RGC is going to respond the best to what stimuli?

Yes

SUMMARY: Does the center receptive fields of RGCs correlate to the width of their dendritic spread?

wide

Parasol RGCs have ____ dendritic fields

wide

With wide dendritic fields, parasol RGCs have a ____ receptive field in their center

Large gratings, letters, etc

Parasol RGCs respond best to what stimulus?

narrow

Midget RGCs have ____ dendritic fields

small

With narrow dendritic fields, midget RGCs have a ____ receptive field in their center

small gratings, letters, etc

Midget RGCs respond best to what stimulus?

true

True or False:

You will be measuring VA in all (or close to it) of your patients, typically as your first assessment following case hx

the ability of the patient to see fine detail & their capacity to resolve differences in luminance across space

What are you assessing when determining VA?

recognition and detection

VA assessment can include target ____ and _____ tasks

-Landolt C

-Tumbling E

Detection VA Examples

Snellen (must be able to name the letters seen)

Recognition VA Examples

top

Typically, VA charts are organized with larger objects at the (bottom/top)

bottom

Typically, VA charts are organized with smaller objects at the (bottom/top)

Descending limits

The method of ______ is used to assess acuity

minimum angle of resolution

In the clinic, the ________ is usually determined using optotypes, typically letter recognition

For the Snellen E, what is the critical detail value?

1/5 of the overall height

1 min of arc

**1/5 of 5

For a 5 min arc letter, MAR = ?

False -- square wave grating

True or False:

You can consider the "prongs" of the Snellen E as a sine wave grating

2.5 cycles / letter

How many square-wave cycles per Snellen E?

true

True or False:

There are usually 2.5 cycles / character for all the Sloan letters

0.5 cycles / min

**2.5 cycles / 5 min = 0.5 cycles / min

How many cycles / min in a 5 min of arc letter?

30 cycles / degree

*0.5 cycles / min 60

How many cycles / degree is normal VA?

ratio

Snellen Acuity is a _____

standard test distance (20 ft)

What does the numerator represent in the Snellen fraction?

the distance from which the patient's acuity-letter subtends 5 min or arc

What does the denominator represent in the Snellen fraction?

true

True or False:

The same VA letter will be different physical sizes, depending on the test distance

10

The standard 20/10 letter is 5 min of arc at ____ feet

5

The standard 20/20 letter is _____ min of arc at 20 ft

20/40

The standard ____ letter is 5 min of arc at 40 ft

minimum angle of resolution

MAR ?

The critical detail related to the acuity letter in minutes of visual angle

What is the MAR?

reciprocal

The MAR is the _____ of the Snellen fraction

decimal

MAR is expressed as a _____

1

Snellen Fraction --> Decimal Acuity

20/20 =

0.5

Snellen Fraction --> Decimal Acuity

20/40 =

0.1

Snellen Fraction --> Decimal Acuity

20/200 =

1

Snellen Fraction --> MAR

20/20

2

Snellen Fraction --> MAR

20/40

10

Snellen Fraction --> MAR

20/200

LogMAR = log10(MAR)

How to calculate LogMAR?

0

LogMAR(20/20) = ?

0.3

LogMAR(20/40) = ?

1

LogMAR(20/200) = ?

1) Equal number of letters / line (5)

2) Spacing between the lines and letters is based on letter size

3) Sizes of the letters progress systematically by 0.1LogMAR

4) Final score based on total of all letters read (add 0.2 LogMAR for each partial line)

What are the advantages to a LogMAR chart?

Yes

Are LogMAR charts available in different contrasts?

So that each letter has similar difficulty in being identified.

Optotypes are based on 10 specialty formed letters. Why?

20/20

**10^0 = 1

20/20^1 = 20/20

LogMAR --> Snellen

LogMAR = 0

20/40

**10^0.3 = 2

20/20^2 = 20/40

LogMAR --> Snellen

LogMAR = 0.3

20/200

**10^1 = 10

20/20^10 = 20/200

LogMAR --> Snellen

LogMAR = 1

30 cycles / degree

**20/20

Denominator = 20

Cycles / degree = 600 / 20 = 30

Snellen --> Cycle/Degree

Snellen 20/20

1 cycles / degree

**20/600

Denominator = 600

Cycles / degree = 600 / 600 = 1

Snellen --> Cycle/Degree

Snellen 20/600

2 cycles / degree

**20/300

Denominator = 300

Cycles / degree = 600 / 300 = 2

Snellen --> Cycle/Degree

Snellen 20/300

20/40

**15 cycles / degree

600/15 = 40

20/40

Cycles / Degree ---> Snellen

15 cycles / degree

20/200

**3 cycles / degree

600/3 = 200

20/200

Cycles / Degree ---> Snellen

3 cycles / degree

There are VA tests that can be used on young children that utilize gratings instead of optotypes (such as Teller Acuity Cards). Also, many clinical and research studies may use grating acuity tests instead of optotypes and so it can be useful in interpreting the results to understand how a treatment may affect your patient's Snellen acuity and allow you to counsel your patient as such.

Why would you want to convert from cycles/degree to Snellen and vice versa?

intellectually able, pre-literate children

When are the LEA symbols used for VA?

-optical blur (refractive error)

-illumination

-location on the retina

-contrast

-pupil size, eye movement, duration of stimulus and adaptive state of the retina

What are some factors that will affect VA?

Refractive error (sphere & cyl), other abberations, diffraction at the pupil can all cause defocus at the retina that blurs out fine detail, sharp edges, and affects the point/line spread function

What is optical blur?

true

True or False:

In young patients, the eye can accommodate to compensate for hyperopic defocus

Negative

(negative/positive) lenses will induce hyperopic blur. Young patients can accommodate through this to clear the image

better

With increasing illumination, VA will get (worse/better)

Cones can see smaller letters and thus have a better acuity limit

Which have a better acuity limit? Rods or cones

fovea

Visual Acuity is best at the _______

densest packing of cones

Why is acuity best at the fovea?

yes -- extra spatial summation in the periphery

Is VA expected to be worse in the peripheral retina?