Resolution and acuity Flashcards
define spatial acuity
the smallest spatial detail that can be detected, resolved or identified
what is spatial acuity different from and how
different from temporal acuity which changes over time
how significant is the measure of spatial acuity
one of the most common and important of psychophysical measures of the visual system
list and describe the 4 types of spatial acuity
- detection acuity: diameter of the smallest spot or width of the smallest line that can just be seen
- hyperacuity: smallest spatial offset or difference in location that can be resolved. aka, localisation acuity and vernier acuity
- resolution acuity: smallest angular separation of two points or lines that can be discriminated as two
- identification/letter acuity: identify letters or other optotypes by resolving their details
requires form vision and acuity
define detection acuity
angular size of the smallest spot or width of line that can just be detected
what type of psychophysical procedure is detection acuity carried out as
a 2AFC, is it there or isn’t it? test
what is detection acuity dependent on for small objects
detection is dependent on illuminance of the retinal image and not the size
i.e. how bright the dot is on the retinal image compared to the background, if it is not bright enough then it won’t reach threshold to allow us to see
what do you need a certain amount of, for detection acuity
a certain amount of contrast
what is detection acuity as way of measuring
a way of measuring weber’s contrast
what is the equation of weber’s contrast (for detection acuity)
difference in luminance/luminance background
what is retinal image size limited by for small objects
diffraction and aberration, so can fall below this limit however small the object, as it is limited by pupil size
how can you calculate retinal image size
using nodal points, since the angle in = the angle out in the human eye
N’F’ = 16.67mm
if you convert the angle subtended by something as far away as a star to radians, which = 1.57x10-8, the size of this on the retina = 2.6x10-4 microns, this is 10x smaller than a cone diameter, so is a very small retinal image size
what can an image size never be below due to diffraction and what does this indicate
image size can never be below 2.72 microns
so to see something small, it depends on how much light it produces and how much light that ends up in this diffracted image compared to the background and whether it reaches enough threshold for us to see
what is increment threshold ΔL
the ability to detect if one stimulus intensity differs from another
with which optometric test carried out in practise, is the principle of increment threshold used ΔL
when measuring visual fields, which shows that weber’s law depends on stimulus size, duration, wavelength and retinal location
describe 3 different types of targets that can be used to measure increment threshold ΔL, i.e. to see if one differs from the other
- annulus = rings
- bipart = fields split L and R
- two separate targets
what does deVries - rose law show about webers fraction ΔL/L against background luminance (brightness)
the performance is constant over a range of background luminances and then it starts to fall down to the end and this is where the constant = weber’s law ΔL/L
weber’s law says here under optimum conditions ~2% is right for the ratio ΔL/L and we will be able to detect the difference at moderate photopic levels, so weber’s contrast is more relevant to targets against a larger background
at what value does the weber-fechner fraction ΔL/L reach a minimum value at photopic levels and what is this known as
minimum value of 2% at photopic levels
this constant is known as weber’s law
what is the smallest size spot we are able to detect
0.25-0.33’ arc
what is the smallest size line we are able to detect
0.0083-0.017’ arc
why do we do better at detecting lines that we do with spots as shown by detection acuity results
because of the increased/better spatial contrast, there is summation along the length of a line.
our neural systems tends to integrate the information along the length of a line i.e. the contrast difference, and so we do better with lines as looks at performance along a whole line than just a spot
since the size of the retinal image doesn’t decrease in detection acuity, what does decrease as the object gets smaller
retinal illuminance decreases as the object gets smaller, so extremely small spots/lines can be detected if they are sufficiently intense i.e. difference in contrast has to meet the threshold
list three downsides in implementing detection acuity
- it is clinically impractical because of very small sizes (black on white)
- it is clinically impractical to achieve sufficient intensity of the target (white on black)
- 2AFC tests may not be best since it takes time to build up the psychometric function (frequency of seeing) curve and determine the threshold
what is hyperacuity
the smallest spatial offset (that we can detect) or difference in location that can be resolved
list the 4 hyperacuity tasks where performance is better than predicted from static identification visual acuity
- displacement
- vernier
- tilt
- stereo acuity
= tasks we’re good at doing
what does displacement acuity measure
relative location or displacement, usually of dots or lines
how is line displacement displayed when measuring someones displacement acuity
a central line is closer to one side than it is to the other, or it can be in the middle
how is a 3 dot displacement displayed when measuring someones displacement acuity
compare two flanking dots with a central dot
how is a 2 dot displacement displayed when measuring someone displacement acuity
notice if one dot is out of alignment to the other
what does vernier acuity measure
the smallest spatial offset or difference in location, that can be resolved
what is a vernier alignment task and what can it be done by doing
alignment of two lines to each other. it is a practical task for alignment, done by looking on a main scale and a line above it e.g. by using vernier callipers have a main scale and another that runs along it which lets you work out the sub mm divisions so you can measure the nearest 0.1mm rather than 1mm by looking at the alignment on the main scale = using the vernier acuity to make a judgement
what does tilt measure
the smallest spatial difference in location that can be resolved
e.g. one grating is slightly tilted with respect to the other = observe if line is tilted compared to flanking lines
what type of task is detecting the tilt of a line or grating
a hyper acuity task = do better hyper acuity tasks
what acuity can you get down to under optimal conditions with a hyper acuity task, and how does this compare to resolution acuity
under optimal conditions i.e. with high contrast, can get down to 2-6” arc which is 10-30x better than resolution acuity
how does a hyperacuity task detect where a line is from a foveal cone mosaic and a blurred line very accurately
- a photoreceptor under this line will have an idea of where the line is in terms of left to right position
- but they may be at slightly different positions of the first photoreceptor
- so when you average out the two signals, you get a better and more accurate indication of where it is
- if you average more indications of where it is from photoreceptors further up the line, then you get a better indication of where the line is
- but these hyperacuity tasks work with dots as well as lines and we can’t integrate along a dot, so only get a good idea for lines, so there aren’t any definitive answers
what does resolution acuity detect
the smallest operation of two points or lines that can be discriminated
how high can resolution acuity be under optimal conditions
0.5’ arc ~6/3
what does resolution acuity vary with
as the cone mosaic varies, as we don’t expect to get the same performance every time
for which 3 reasons is measuring resolution acuity difficult to clinically apply
it is difficult to do because:
- it requires lines of different thicknesses or spots of different sizes which are not known to the subject - otherwise they can predict the results without actually resolving the gap
- high luminance levels causes smaller pupil sizes and more diffraction which reduces the resolution
- astigmatism means that the test with lines would need repeating at different orientations to make a valid test
what can be used to solve the problem of measuring resolution acuity
use resolution acuity grating
which type of gratings is best to use when measuring resolution acuity and why
sine wave gratings, because they have a single spatial frequency, so are more precise - as the bars get closer together, the spatial frequency is increasing
when is the sine wave grating for measuring resolution acuity usually tested and why
it is usually tested in the CSF, especially towards the bottom right end of the CSF where you have the high contrast and high spatial frequency information, which is the closest thing to letter acuity
why can square wave gratings also be used to measure resolution acuity, and state a disadvantage of using them
square wave gratings can be used because they care closer to letters
but they contain a range of different spatial frequencies and so is not a pure test
why are using letters much better than using gratings when testing resolution acuity
with gratings: as the blur is increasing in dioptres, the grating drops but doesn’t drop so quickly, which shows its relatively insensitive to blur
you also get aliasing (error) when using gratings to test resolution or you get spurious (false) resolution. due to spurious resolution, you can detect gratings in eccentric rising sometimes, and other times you can’t
with letters: you can test you are getting the right end point as letters are more sensitive to blur
when will you use gratings to measure resolution acuity
- preferential looking in young children
- test for illiterates
- test for retinal acuity (NCT) - without using computer based systems
list four advantages to using identification/letter acuity
- highly familiar an easily identified targets
- quick and reliable
- sensitive to ametropia
- less affected by guessing as you have to name a letter ‘identification’ and not just say if its there or not (like with gratings = 2AFC/4AFC)
what do most low contrast charts tend to be available in and why
25%, 19%, 15%, 11% and 9% because it wants to test different points and allows for variation in the stimulus or lighting without affecting the results significantly
between which % of contrast does your LogMar score not fall that much
between 20-100% contrast
from which contrast does your LogMar score start to fall
from below 25%
what confidence interval does a test re-test repeatability performance of letter charts have
95% confidence interval of ± 4-6 letters.
this means 95% of the time your VA will fall within ± 1 line
which type of scoring is better than line scoring and where is this used
letter scoring as it increases repeatability over line scoring
used in LogMar charts
how many different types of acuity are there
4
which types of acuity are regularly test in clinic
stereo acuity and identification
