limits of vision: visual acuity

Question 1

what is visual acuity?

Answer 1

1. the smallest detail that can be perceived
2. the smallest visible feature you can discern

. VA is the most important and common psychophysical measure of the visual system, both clinically and in research

Question 2

what is resolution acuity?

Answer 2

. smallest angular size at which observers can discriminate the separation between critical elements of stimulus pattern

1. the smallest separation of two points or lines that can be discriminated
2. the highest spatial frequency checkerboard ( smallest squares ) that the observer can see
3. the highest spatial frequency checkerboard ( smallest squares ) that the observer can see

Question 3

what is resolution grating acuity?

Answer 3

. the highest spatial frequency gratings ( thinnest grating lines ) that the observer can see

Question 4

how is resolution grating acuity measured ?

Answer 4

. generally expressed in terms of cycle/degree
. generally measured at high contrast, in which case resolution acuity forms the extreme right end of the contrast sensitivity function
. in healthy eye you can see 50 to 60 cycles/deg

Question 5

when is resolution grating acuity mostly used?

Answer 5

. acuity cards are an important paediatric measure for infants , children with learning difficulty and non-literate adults
e.g. Teller cards, Keeler cards
. mean grating luminance = background luminance
. the practitioner judges whether the patient saw the grating
. typically not a not necessarily a measure of resolution acuity

Question 6

how does the practitioner know if patient saw the gratings?

Answer 6

. by using a variety of non-defined cues including eye movements, head turns, facial expression and hand gestures

Question 7

what is detection acuity?

Answer 7

. the highest spatial frequency for which luminance gratings can be discriminated from a uniform field

Question 8

why does detection acuity differ from resolution acuity?

Answer 8

. if neural sampling is insufficient ( e.g. photoreceptor mosaic is too sparse ) then there may still be a coherent neural response but not a veridical one

Question 9

when are resolution acuity and detection acuity the same?

Answer 9

. they are the same when vision is optically limited ( i.e. foveal vision, in normally sighted people)

Question 10

when is detection acuity better?

Answer 10

. detection acuity is better than resolution acuity when vision is limited by neural-sampling ( e.g. peripheral vision )

Question 11

what happens in central vision?

Answer 11

. in central , optically limited vision: frequencies higher than the optical limit are blurred out

Question 12

what happens in peripheral vision?

Answer 12

. in peripheral , sampling limited vision: frequencies higher than the sampling limit are misperceived ( alias)

Question 13

what is vernier acuity?

Answer 13

. the ability to discern a disalignment among two objects

. also called dot-displacement hyperacuity

Question 14

what is the threshold for vernier acuity?

Answer 14

.10 arcseconds ( 0.78 um ) which is smaller than diameter of a foveal cone and some even report 2-6 arcseconds

Question 15

how can vernier acuity be so much better than recognition or resolution acuity?

Answer 15

. the answer involves pooling information across photoreceptors
. pooling information across multiple photoreceptors gives a way beating of sampling resolution

Question 16

what is the difference between vernier acuity and recognition/resolution acuity?

Answer 16

. resolution relies entirely on high spatial frequency signals ( perceiving detail )

. vernier acuity is all about localization , for which even low spatial frequencies are sufficient

Question 17

what does loss of high spatial frequency cause?

Answer 17

. hard to resolve fine detail

Question 18

explain the argument against the pooling of information?

Answer 18

. vernier acuity works with dots

. even a microdot will activate multiple photoreceptors due to nystagmus

Question 19

explain the argument for the pooling of information?

Answer 19

. you get better at vernier acuity with practice

Question 20

what family is vernier acuity part ?

Answer 20

. vernier acuity is part of a family of localisation acuities known as hyperacuities

Question 21

what is hyperacuity resistant to ?

Answer 21

. hyperacuity is resistant ( though not immune ) to optical degradation e.g. blur

Question 22

how to demonstrate that hyperacuity is resistant to blur?

Answer 22

. by measuring wavefront aberration using aberrometry

. this shows that people with more aberration get worse at landolt acuity test

Question 23

what vernier acuity used to measure?

Answer 23

. vernier acuity is an attractive way of measuring retinal function in the presence of ocular media anomalies ( corneal opacity , cataract, vitreous opacity)

Question 24

why is hyperacuity ideal screening tool for AMD?

Answer 24

. hyperacuity is quite stable with respect to changes in contrast and luminance. Plus uses a small, localised stimulus that needs central vision

Question 25

what is stereoacuity?

Answer 25

. another member of the hyperacuity family
. the smallest detectable depth difference that can be seen in binocular vision
. important for judging object distance
. important clinically for detecting e.g. amblyopia

Question 26

what is recognition acuity?

Answer 26

. the smallest sized letter the observer can correctly identify
. non-verbal/non-literate flavours also available

Question 27

what is the outcome measure of recognition acuity?

Answer 27

. the logarithm of the minimum angle of resolution

Question 28

what is the relationship of recognition ( optotype ) acuity, and resolution
( grating acuity )

Answer 28

. when snellen E is placed horizontal to a vertical grating

. the spacing of letter corresponds to black and white strips in the grating

Question 29

does it make sense to map between logMAR and cycles per degree?

Answer 29

. the recognition ( optotype ) acuity and resolution
( grating ) acuity are not correlated perfectly
. they can differ by up to 5 lines

Question 30

what is the difference between resolution acuity and recognition acuity?

Answer 30

. unlike resolution acuity, recognition acuity involves low spatial frequencies ( the overall form/shape of the letter)

Question 31

how do we know that a letter chart doesn’t measure resolution acuity?

Answer 31

. if a letter chart was measuring reolution acuity, then blurring it ( i.e. removing all fine spatial detail ) would make lower rows completely illegible

Question 32

how to make recognition acuity and resolution acuity similar?

Answer 32

. you can make the two similar by filtering out low spatial frequencies ( highpass acuity )

Question 33

why is recognition acuity preferred?

Answer 33

1. letters are preferred as you are less likely to guess the correct answer by chance
2. real world relevance: less likely to guess the correct answer by chance
3. ease of printing
4. convention/precedent
5. normative data

Question 34

what are disadvantages of letter charts?

Answer 34

. 95% test re-test repeatability is approximately +/- 4 to 8 letters
. this means 95% of the time repeat measurement of VA will fall within +/- 1 line
. some letters are easier to recognise than others due to their shape and contour
. letter scoring improves repeatability over line scoring

Question 35

what is distance acuity?

Answer 35

. VA testing performed at optical infinity (6m)

Question 36

when is distance acuity a problem?

Answer 36

. this is insensitive to hyperopia ( 3% of children aged 5-15 years) , which if untreated can lead to headaches, binocular dysfunction , deficits in self-esteem , reduced educational outcomes

Question 37

what is near acuity test ?

Answer 37

. same principle as distance acuity but handheld chart performed at 40cm

Question 38

what is temporal acuity?

Answer 38

. acuity can also measure the finest temporal detail by flickering a stimulus at different temporal frequencies

Question 39

what is critical flicker fusion?

Answer 39

. the fastest detectable flicker frequency at maximum contrast is temporal acuity