Limits on Vision: Spatial Resolution and Acuity

Question 1

What is resolution acuity and what three stimuli may it be measured using?

Answer 1

Resolution acuity is the smallest angular size at which observers can discriminate the separation between critical elements of a stimulus pattern. It can be measured using stimuli that exist as dots, gratings or checkerboards.

Question 2

How may dot stimuli be used to test for resolution acuity?

Answer 2

In terms of dots, resolution acuity would be measured as: - The smallest separation of two points or lines that can be discriminated

Question 3

How may gratings be used as a stimulus to test for resolution acuity?

Answer 3

In terms of gratings, resolution acuity would be: -The highest spatial frequency gratings (thinnest grating lines) that the observer can see (e.g., correctly identify the orientation of).

Question 4

How may checkerboards be used as a stimulus to test for resolution acuity?

Answer 4

In terms of checkerboards, resolution acuity-The highest spatial frequency checkerboard (smallest squares) that the observer can see.

Question 5

Can we identify resolution acuity from the contrast sensitivity function?

Answer 5

Yes! Look at the outermost point which shows you how many cycles per degree a px can observe at high contrast- this is your resolution acuity - a.k.a the smallest distance at which two points can be distinguished.

Question 6

What is the difference between resolution acuity and detection acuity?

Answer 6

Resolution acuity: the highest spatial frequency for which spatial patterns are perceived veridically- i.e. that you can tell they are vertical. (This is essentially your ability to resolve things)

Detection acuity: the highest spatial frequency for which luminance gratings can be discriminated from a uniform field. (This is your ability to detect something being there even if you can’t resolve).

Question 7

On what pxs are acuity cards used and why?

Answer 7

They are used on babies/children/people who can’t talk in order to identify their detction/resolution acuity.

Question 8

In what cases is the resolution acuity different to the detection acuity and why?

(Surely if we can detect the presence of a grating we can resolve it to- i.e. tell what direction the lines are going in?!?!?!?)

Answer 8

In peripheral vision resolution acuity and detection acuity are two different things.

In the periphery if neural sampling is insufficent (i.e. that the photoreceptor mosaic pattern is too spread out (not close enough)), the presence of gratings may be detecting (i.e. detection resolution) but the brain cannot correctly identify the orientation of the lines (i.e. resolution acuity).

Question 9

In what circumstance is resolution acuity equal to detection acuity?

Answer 9

In central vision - when the limiting factor is NOT neural sampling but optics!

In this case photoreceptors are close enough to also resolve the orientation to the same extent as they can detect the presence of the grating in the first place.

Question 10

What is aliasing?

Answer 10

When detection acuity is greater than resolution acuity and as a result the px can detect gratings but not resolve them and so falsely identifies the orientation of lines. This process is called aliasing.

Question 11

What is Vernier acuity?

Answer 11

The ability to discern a “disalignment” among two objects.

(I.e. to literally tell if something is to the e.g. right or left, of something else).

Humans have a crazily high vernier acuity!

Question 12

Why is our vernier acuity so much higher than our resolution acuity?

Answer 12

Long story short - you don’t need fine spatial detail to detect localisation - low spatial detail is enough.

“Vernier acuity and recognition/resolution acuity are fundamentally different tasks (despite sharing the name “acuity”)

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 13

True or False- Vernier acuity is part of a family of hyperacuities.

Answer 13

True - all the different types of hyperacuities with their stimuli can be seen in the slide.

Question 14

True or False- Hyperacuity is resistant(though not immune) to optical degradation

Answer 14

True - have a look at the picture despite blur we can see the relative side of the top line in relation to the bottom line. I.e no matter how blurred the lines are we can always tell the top line is to the right of the bottom line.

Question 15

Why is there a lot of interest in using hyperacuity to screen for retinal function?

Answer 15

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

Therefore, it is an extremely attractive means of measuring retinal function in the presence of ocular media anomalies (corneal opacity, cataract, vitreous opacity, etc.)

Plus…

Hyperacuity is also quite stable with respect to changes in contrast and luminance.

It uses a small, localised stimulus that needs central vision (so if seen fovea must be intact) thus could be the

Ideal screening tool for AMD or DMO.

Question 16

What is stereoacuity?

Answer 16

The smallest detectable depth difference that can be seen in binocular vision

Question 17

True or False- Stereoacuity belongs to the hyperacuity family

Answer 17

True

Question 18

What is recognition acuity?

Answer 18

The smallest sized letter (technically: smallest sized letter features) the observer can correctly identify.

Question 19

Although interchangeably used - why may a px’s recognition acuity be greater than a px’s resolution acuity?

Answer 19

Essentially we don’t need absolute fine spatial resolution to identify letters (or at least all aspects of letters) - thus you can blur a letter and even at low spatial frequency still work out it was the letter ‘H’.

However with a grating as you blur it becomes distinctly harder to identify whether it is a grating pattern or a uniform field (i.e. one colour). Resolution with gratings relies on high spatial frequency.

Question 20

If you wanted to convert your recognition acuity test into a resoltuion acuity test, how would you do that?

Answer 20

You put a ‘Highpass’ filter on the letter chart. A highpass filter removes low spatial frequency.

This would give SIMILAR results to a resoltuion acuity test i.e. the grating test.

Question 21

If standard recognition acuity gives different answers to resolution acuity, why is recognition acuity preferred?

Answer 21

Practical reasons (“10AFC”)

less likely to guess the correct answer by chance as there is a 1 in ten chance of choosing the right letter whereas with direction of gratings there is a 1 in 4 (i.e. up,down,left or right)

Real world relevance – px quality of life would be affected more by not being able to read than detecting gratings

Various other reasons

Ease of printing (gratings are very difficult to print because of edge artefacts)

Letts are Convention/precedent

We have lots of values for Normative data regarding letters

Question 22

What are some disadvantages of using recognition acuity to measure vision?

Answer 22

95% test re-test repeatability is approximately ± 4 to 8 letters

This means 95% of the time repeat measurement of VA will fall within ±1line

Some letters easier to recognise than others

Letter scoring improves repeatability over line scoring

Question 23

At what distance is near acuity measured at?

Answer 23

40 cm

Question 24

What is temporal acuity?

Answer 24

The fastest detectable flicker frequency at maximum contrast is temporal acuity (also called critical flicker fusion).

AKA the fastest flickering stimuli at high contrast that you can detect.

Question 25

What is near acuity useful for screening?

Answer 25

Hyperopia