Vision Science I - Exam 2

Spatial vision

this refers to the study of visual mechanisms used for the detection and discrimination of objects

spatial vision

This is how we perceive objects and how we use this information for the assessment of a visual system

visual acuity

a visual-based measure of the threshold to detect or discriminate the details comprising a specific target

magnitude

Visual acuity is a metric that can quantify the ___________ of visual quality (sharpness or clarity)

visual debility

Visual acuity is a metric that can detect the presence of a ___________________________

visual disability

Visual acuity is a metric that can classify the level of a ______________________________

visual quality

Visual acuity is a metric that can track the stability, decline, or improvement in ________________________

Visual acuity

This is the quintessential test of quantifying visual ability

threshold

Taking visual acuity involves various methods of quantification, but all attempt to measure a __________________

visual ability

Not all tests of visual acuity are equivalent in their capacity of quantifying what?

- Recognition acuity

- Resolution acuity

- Localization acuity

- Detection acuity

What are the four levels of acuity measures?

Recognition acuity

What is the highest level of acuity measurement?

Detection acuity

What is the lowest level of acuity measurement?

Recognition acuity

This level of acuity measurement infers resolution capacity by requiring identification of targets

resolution acuity

This level of acuity measurement requires resolving the minimum separation between gratings, two lines, two spots, etc.

- optical quality of the imaging system

- sampling grain of the retinal surface

The ability to resolve between two points or two lines is dependent on what two aspects of the human eye?

sampling grain

The ____________ of the retinal surface refers to the size of the retinal fields and how tightly packed they are

False (little variation)

True or False: The sampling grain of the retinal surface varies widely between individuals

30-40 arc seconds

What is the approximate size of a foveal cone photoreceptor

quality of the retinal image produced by the optics of the human eye

What is the major limiting factor in a two point resolution task, specifically for foveal vision?

localization acuity

This level of acuity measurement requires detecting the minimum change in location or relative position of two or more targets

- vernier acuity

- spatial interval acuity

- bisection acuity

What are three types of localization acuity?

detection acuity

This level of acuity measurement detects the minimum luminance or contrast required to see a stimulus

high level recognition processes

Recognition acuity requires what of the individual being tested?

Hermann Snellen

In 1862 this person introduced standardized charts by which to measure visual acuity

optotypes

visual acuity charts are comprised of specially designed symbols called what?

"normal"

Hermann Snellen introduced the concept of a standard _______________ observer

normal observer

A ____________ is an individual that can correctly identify an optotype that subtends 5 arc minutes comprising details separated by 1 arc minute

Robert Hooke

The criterion of Hermann Snellen's 5 arc minutes was inspired by who?

~ 1 arc minute

Robert Hooke reported that the human eye can resolve two stars separated by what?

1.75mm

The details of a standard 20/20 optotype, if viewed from 6m or 20ft, are what size in mm?

nodal point

Visual acuity cannot be solely based on the linear distance because what changes?

doubles

Halving the linear distance in regards to an optotype does what to the angle that it subtends?

5x5

All snellen optotypes can be constructed from what type of grid?

1 arc minute

To construct a snellen letter that subtends 5 arc minutes, each square within the 5x5 grid must subtend how much?

Serif

The original snellen letters are serif or sans serif?

Sans Serif

The current snellen letters are serif or sans serif?

Landolt C

What type of optotype is considered the "gold standard"?

True

True or False: For the same magnitude of defocus some optotypes are easier to discriminate than others

- sloan letters

- british standard letters

What are the two standardized groups of optotypes?

5x4

British standard optotypes use what type of grid?

one

The Landolt C is considered the "gold standard" optotype because it varies along how many dimensions?

4

The Landolt C is sensitive to astigmatic blur especially if at least how many orientations are used?

- trouble for examinee to keep track of optotype

- more instruction required to train the patient

- requires patient to have well developed sense of laterality

What are three limitations of testing visual acuity with the Landolt C?

~2

Sloan letters are about how much lower (better) than the Landolt C

~0.25

British standard letters are about how much lower (better) than the Landolt C

Sloan Letters

Which letters have a similar test-retest variability relative to the Landolt C?

British Standard Letters

Which letters have a marginally better test-retest variability, or are slightly more repeatable, relative to the Landolt C?

Sloan

Which letters, British or Sloan, generally produce slightly better acuities?

Confusion Letters

visual acuity chart letter groups that observers often mistake for one another

test distance

Within the Snellen Fraction, the numerator represents what?

critical distance

Within the Snellen Fraction, the denominator represents what?

False (will not change)

True or False: For a given chart used at a non-calibrated test distance the critical distance of the optotype will change with the test distance

True

True or False: For a given chart used at a non-calibrated test distance the actual snellen fraction will change depending on the actual test distance used

True

True or False: For a given chart used at a non-calibrated test distance the 20-ft equivalent snellen fraction will change according to the actual snellen fraction

Minimum angle of resolution (MAR)

This can be found by taking the reciprocal quotient of the Snellen fraction

True

True or False: MAR will remain invariant with fixation distance in a patient with optimal refractive correction and intact accommodation

True

True or False: MAR should be the same in both the actual and equivalent snellen

MAR

If you know this for a patient, then it is possible to predict the equivalent snellen acuity for that patient at any distance (if the patient is optimally corrected and has an intact accommodative system)

equal discriminability

The LogMAR scale is considered to be an _______________ visual acuity scale

0

A 20/20 letter has a LogMAR of what?

0.3

A 20/40 letter has a LogMAR of what?

-0.12

A 20/15 letter has a LogMAR of what?

negative

When the MAR is less than one the LogMAR is what?

negative

Any Snellen acuity better than 20/20 has a __________ logMAR value

angular subtense (MAR)

The logMAR notation is based on what of the optotype?

1.26x

An increase or decrease in the logMAR by 0.1 correlates to an increase or decrease of of MAR by a factor of what?

0.26

If patient has a VA of 20/40+2, what is the equivalent LogMAR acuity?

100-50(logMAR)

What is the equation used to convert logMAR to VAR?

0 VAR (20/2000 VA)

What VAR is classified by the World Health Organization as profound visual loss

20/2000

A VAR of 0 is equal to what VA?

Functional Acuity Score (FAS)

VAR can be used to calculate this for "Guides to the evaluation of permanent visual impairment"

(VARod + VARos + 3*VARou)/5

What is the equation for calculating the Functional Acuity Score (FAS)?

Visual Efficiency Scale (VE scale)

This was introduced by Snell and Sterling to quantify vision loss for legal and compensation purposes

M notation

What is the most common notation used to measure near visual acuities?

low

M notation is used frequently for patients that have ______ vision

Both represent the distance at which the optotype will subtend 5' arc (M notation is in meters, Snellen is in feet)

What is a major similarity between the denominators of the M notation fraction and the Snellen fraction?

0.4/2M

A patient can just about read the line of letters labeled 2M when holding her near card at 40cm. What is the M notation fraction that represents her VA?

20/100

A patient can just about read the line of letters labeled 2M when holding her near card at 40cm. What is this patient's equivalent Snellen Fraction?

20/50

A patient has an M notation fraction of 0.4/1M, what is the equivalent Snellen Fraction?

20/60

A patient has an M notation fraction of 1/3M, what is the equivalent Snellen Fraction?

20/250

A patient has an M notation fraction of 0.4/5M, what is the equivalent Snellen Fraction?

False (MAR - minimum angle of resolution)

True or False: The inverse of both the Snellen Fraction and the M notation fraction are equal to the VAR

ADD magnification

M notation is commonly adopted in low vision to calculate appropriate what?

meters (hence "M")

M notation is the distance in _____________ at which a near optotype will subtend 5 minutes of arc

J (Jaeger) Notation

This notation for measuring visual acuity has no mathematical meaning and is implemented inconsistently on different cards

- number of optotypes per acuity line

- legibility of optotypes

- spacing between optotypes

- progression of acuity sizes

List four design flaw of the Snellen Chart

False (not proportionally equivalent)

True or False: A one line loss/gain in Snellen acuity is proportionally equivalent between each line of the chart

- equal number of optotypes in each row

- letter spacing and row separation scales with letter size

- letter size follow a logarithmic progression

- letter legibility

- can be easily adapted for non-standard distances

What are five advantages of the Bailey-Lovie Chart?

MAR

When acuity is plotted as __________, the worse the acuity the larger the variability

LogMAR

When acuity is plotted as ___________, the variability of acuity is approximately equal at all levels of visual acuity

- diffraction

- optical defocus and aberrations

- receptive field size/sampling limit

- contrast sensitivity

Two point/ line resolution acuity is affected by what four things?

Airy disc

a disc containing a bright central peak surrounded by rings of light and dark caused by the effect of a circular aperture on the diffraction and retinal image formation

angle between peak and first minimum peak (in radians)

What does θ represent in the airy disc equation?

wavelength of incident light

What does λ represent in the airy disc formula?

pupil diameter

What does a represent in the airy disc formula?

Rayleigh criterion

This states that 2 points will be resolvable if the peak of the airy disc of one target coincides with the first minimum of the airy disc of the other target

resolving capacity

Rayleigh's criterion determines the ____________ of the eye

greater

The smaller the pupil diameter in Rayleigh's equation, the _________ the effects of diffraction

better

The smaller/shorter the wavelength in Rayleigh's equation, the _____________ the resolution