Spatial Vision - Contrast Sensitivity

contrast (luminance contrast)

the luminance difference between area

c=(Lmax-Lmin)/(Lmax+Lmin)

What is the michelson equation?

k = Delta I/Ib

What is weber's contrast equation?

contrast is everywhere in life; our goal is to ensure that our patients are able to comfortable live

What do we use tests to measure contrast in clinic?

teller acuity card, snellen charts (above 90%), color vision (chromatic)

What are examples of where contrast is used in clinic?

90%

What is the average contrast acheived with using snellen charts in clinic?

False (only should measure color contrast if measured properly, no luminance!)

True or False: When measuring color vision, both luminance and color contrast are measured.

helps perceive a boundary, object, or motion

What are the three reasons why contrast is so important for our vision?

spatial, temporal

Perceiving motion is related to __________ and __________ contrast.

False (contrast is more important)

True or False: Luminance is more important for our spatial vision

lightness/brightness constancy

phenomenon that shows how the same object under different illumination conditions is still perceived as the same object

simultaneous contrast

the phenomenon where the center square on the left appears brighter, even though the two center squares are the same color

darkness surround the left center square causes lateral inhibition, leading to the center square being perceived as brighter (basically contrast)

How does simultaneous contrast work?

assimilation

how our visual system processes contrast to build a more complex contrast relation from area to area

two, three

With simultaneous contrast, there are two/three areas of contrast within each box, whereas with assimilation, there are two/three areas of contrast within each box.

tells us how complex contrast processing is

Why do we compare simultaneous contrast to assimilation?

contrast threshold

the minimum luminance difference (in %) required to tell apart between areas

contrast sensitivity

reciprocal of contrast threshold; measures the ability to differentiate fine increments or decrements of light luminance

Yes, common in older patients

Stairs are an everyday contrast example. Is it possible for someone to have enough trouble differentiating contrast to misjudge steps?

increases, decreases

Stairs are an everyday contrast example. Older patients often have trouble differentiating contrast and misjudge steps. This means that over time our contrast threshold decreases/increase and our contrast sensitivity decreases/increases.

contrast sensitivity function

depicts the contrast sensitivity for different spatial frequency; basically ability to process contrast for different sized objects

spatial (achromatic) luminance contrast, spatial chromatic contrast

What are the two types of contrast we are concerned with?

tells how well our visual system is working and is a good predictor of daily visual performance

Why is using a contrast sensitivity function important (basically visual performance vs visual acuity)?

amblyopia, glaucoma, diabetic retinopathy

Contrast sensitivity deficits have high correlation with the presence of many ophthalmologic diseases. What are three examples of these diseases?

spatial modulation transfer function (SMTF)

used to quantify how well and optic lens transfers contrast information for stimulus of various spatial frequencies

function used to quantify how well a lens system in the eye can keep a similar contrast of an object from one side to another side

Summarize the idea of spatial modulation transfer function in your own words (i'll put mine down!)

less, slow

Review: Low spatial frequency refers to images with less/more detail and slow/quick changes in an image.

more, quick

Review: High spatial frequency refers to images with less/more detail and slow/quick changes in an image.

large

Review: Low spatial frequency correlates to small/large object.

small

Review: high spatial frequency correlates to small/large object.

high (able to see the detail of the small stem in comparison of the larger apple)

The stem of the apple represents and area of low/high spatial frequency.

low (relatively flat area with no change)

The body of the apple represents and area of low/high spatial frequency.

left

Look at the following picture. Do you think the left or right side of the picture is easier to process contrast?

Co>Ci

A spatial grating with a specified spatial frequency (Co) and contrast is focused by an optical lens. The image contrast Ci is then measured. How would Ci and Co compare in terms of contrast?

as light bounces off Co and travels through the lens, contrast information is lost

A spatial grating with a specified spatial frequency (Co) and contrast is focused by an optical lens. The image contrast Ci is then measured. As previously discussed, Co would be perceived to have a higher contrast the Ci. Why?

Ratio = Ci/Co (0-1)

What formula is used to describe the contrast difference between Co and Ci?

how well the lens transfers information like contrast contained in an object.

What does the ratio = Ci/Co help tell us?

image contrast is the same as object contrast (meaning as the light from the object travels through the lens, no contrast information is lost)

What does it mean if Ci/Co = 1?

the image appears as a homogenous area, ie no contrast perceive from object to image (all contrast lost as information travels through the lens)

What does it mean if Ci/Co = 0?

spatial frequency

What is the x-axis of a spatial modulation transfer function?

Ci/Co

What is the y-axis of a spatial modulation transfer function?

lens contrast processing ability in regards to spatial frequency (different sized objects)

What is the purpose of using a spatial modulation transfer function?

1 (due to ratio of Ci/Co)

What is the highest possible value on the y-axis of the spatial modulation transfer function?

a little

The following graph shows a SMTF for an optical lens that is in focus. This shows that with low and moderate spatial frequencies, the image is transferred with a little/a lot of image degradation.

a lot

The following graph shows a SMTF for an optical lens that is in focus. This shows that with higher spatial frequencies, the image is transferred with a little/a lot of image degradation.

due to aberrations in the optical system

The following graph shows a SMTF for an optical lens that is in focus. This shows that with higher spatial frequencies, the image is transferred with a lot of image degradation. Why does this occur with higher spatial frequencies?

Reduction

The following graph shows a SMTF for an optical lens that is out of focus. This shows the defocus results in a(n) reduction/increase in image quality more for high spatial frequencies, than for low and moderate frequencies.

reduction

The following graph shows a SMTF for a translucent lens. This shows that the translucent lens results in a general reduction/increase in image quality for all spatial frequencies

measure contrast threshold and contrast sensitivity function

How are we able to quantify the human visual system's ability to process contrast information?

1. What kind of visual stimuli can be used to measure contrast threshold?

2. What kind of psychophysical method should be used?

What are the two questions we should ask when wanting to measure contrast threshold/contrast sensitivity?

target with a background, AFC stimuli (like 4 ARF), grating

What are three examples of stimuli that can be used to measure contrast threshold?

michelson's contrast (Lmax-Lmin/Lmax+Lmin)

What formula can be used when finding the percent contrast for a given visual stimulus?

vistech chart

chart that measures contrast threshold by presenting various target stimuli with various contrast levels

3, 6, 12, 18

What are the four spatial frequencies tested in a vistech chart?

decreasing

With a vistech chart, 3, 6, 12, and 18 cpd are tested with contrast decreasing/increasing from left to right.

Pelli-Robson Chart

chart that measures contrast threshold by presenting three-letter set of visual stimuli with decreasing contrast levels

false (they are all the same size because the goal is to measure contrast sensitivity, not VA)

True or False: The letters in a Pelli-Robson Chart are different in size.

high, contrast

VA measures sensitivity to ______ spatial frequencies, whereas the Pelli-Robson data measures sensitivity to high spatial frequencies and ___________ sensitivity to lower spatial frequencies.

provides estimation of contrast sensitivities at certain spatial frequencies and easy to administrate at clinical settings

What are the advantages to the pelli-robson and vistech charts?

lack of reliability

What is the disadvantages to the pelli-robson and vistech charts?

true

True or False: Contrast threshold/sensitivity is different for stimuli with different spatial frequencies

true

True or False: Within high spatial frequencies it becomes extremely difficult for the lens to process contrast, regardless of the lens conditions.

MCS

When measuring contrast threshold, if using a computer, you can use a dot target stimulus that provides contrast against the computer screen. What psychophysical method can be used?

background

When measuring contrast threshold, if using a computer, you can use a dot target stimulus that provides contrast against the computer screen. MCS can be used. When using this visual stimuli, should the background or target stay at a fixed luminance (cd/m^2)?

as a psychometric function

When measuring contrast threshold, if using a computer, you can use a dot target stimulus that provides contrast against the computer screen. MCS can be used. When we get our results, how should we expect them to be plotted on a graph?

fixed contrast level of stimulus

When measuring contrast threshold, if using a computer, you can use a dot target stimulus that provides contrast against the computer screen. MCS can be used. What should the x-axis to be?

percent detection

When measuring contrast threshold, if using a computer, you can use a dot target stimulus that provides contrast against the computer screen. MCS can be used. What should the y-axis to be?

50% (NOT 75% because that would be forced choice :))

When measuring contrast threshold, if using a computer, you can use a dot target stimulus that provides contrast against the computer screen. MCS can be used. What percentage should we view as the threshold?

spatial frequency

What is the x-axis of a human contrast sensitivity function?

contrast sensitivity (threshold would be calculate as 1/sensitivity)

What is the y-axis of a human contrast sensitivity function?

1.5% (1/60x100)

When interpreting the circled point on the graph, it shows that at a frequency of 0.5, the subject was able to see the stimuli until their contrast sensitivity at 60. What is the contrast threshold calculated as a percentage?

was not

When interpreting the circled point on the graph, it shows that at a frequency of 0.5, the subject was/was not able to see a stimulus at a contrast sensitivity of 200

was (1/20x100 = 5%)

When interpreting the circled point on the graph, it shows that at a frequency of 0.5, the subject was/was not able to a stimulus we a contrast threshold of 5.0%?

true

True or False: Even though using human contrast sensitivity functions are important and reliable in measuring how a patient's visual system works, it is often not used in clinic because it is time consuming

2

Look at the circle point located on the human contrast sensitivity function. What spatial frequency was measured?

0.5% (1/200 x 100)

Look at the circle point located on the human contrast sensitivity function. What was the grating contrast threshold measured as a percentage?

would not, would

Look at the circle point located on the human contrast sensitivity function. The grating contrast threshold measured was 0.5%. If the patient was shown a grating contrast of 0.1% the patient would/would not see it and if the patient was shown a grating contrast of 1.0% the patient would/would not see it.

yes

When looking at the following graph of a human contrast sensitivity function, is the axes in log scale?

spatial frequency (cycles/degree)

When looking at the following graph of a human contrast sensitivity function, what is the x-axis?

contrast sensitivity

When looking at the following graph of a human contrast sensitivity function, what is the y-axis?

100%, in order to a spatial frequency that high, you would need 100% contrast or you wouldn't see the grating target's contrast

Look at the red dot on the graph. As the shown spatial frequency, what is the contrast threshold? What does this mean?

It means that the grating is easily distinguished

The following graph shows a human contrast sensitivity function. What does it mean on the graph when it says "resolved"?

The stimulus appears homogenous (despite being a grating pattern)

The following graph shows a human contrast sensitivity function. What does it mean on the graph when it says "unresolved"?

4 cpd (3-10 cpd)

Based on the following graph, at what spatial frequency does the visual system typically have maximum sensitivity to contrast?

true (means you have to infer what the spatial frequencies may be for below 0.5 cpd)

True or False: With very low spatial frequencies (like 0.5 cpd) it is harder to measure human contrast sensitivity functions.

high-frequency cutoff

Point where at 100% contrast, the grating can no longer be resolved

40-60 cpd (hard to determine from graph, typically this in young, health patients)

Where on the following graph is the high-frequency cutoff (estimate)?

band-pass filter

specific range of frequencies that our visual system either pass or reject a certain frequency (allows range of frequencies to pass through)

Low band-pass contrast sensitivity function

visual system is able to process low frequencies better than high frequencies

high band-pass contrast sensitivity function

visual system is able to process high frequencies better than low frequencies

low, high

The red line on the following graph demonstrates a low/high pass band filter, whereas the green line demonstrates a low/high pass band filter

red (so demonstrates a low sensitivity contrast function)

Which line demonstrates a lens contrast sensitivity function?

increases, increases

Based on the following graph, the contrast decreases/increases from top to bottom and spatial frequency decreases/increases going from left to light.

band pass filter

The CSF of other species all demonstrate what type of filter?

true

True or False: The following graph shows that different species are able to process different size and ranges of spatial frequency.

goldfish

Based on the following graph, the ________ has the highest contrast sensitivity for low spatial frequencies

falcon

Based on the following graph, the ________ has the highest contrast sensitivity for high spatial frequencies

large, worse

Based on the following graph, it is clear that a cat, as compared to a human (peak at 4 cpd), can see better in terms of contrast processing of small/large objects, which means that cats have a worse/better visual acuity than humans

large, worse

Based on the following graph, it is clear that a goldfish, as compared to a human (peak at 4 cpd), can see better in terms of contrast processing of small/large objects, which means that goldfish have a worse/better visual acuity than humans

small, better

Based on the following graph, it is clear that a falcon, as compared to a human (peak at 4 cpd), can see better in terms of contrast processing of small/large objects, which means that falcons have a worse/better visual acuity than humans