Perception 2 Flashcards
What is spatial frequency?
Number of pairs (1dark+1white bar) within a given distance on retina
How does the size (or visual angle) of the retinal image cast by an object depends on the distance of that object from the eye?
As the distance between eye and object = decreases
=> the objects image subtends a greater visual angle
A grating of high spatial frequency has
Many cycles within each 1 degree of visual angle and NARROW bars
A grating of low spatial frequency
has few cycles within each 1 degree of visual angle-> contains WIDE bars
Spatial frequency is defined based on visual angle, does that change?
So the spatial frequency changes with viewing distance
As the distance decreases, what happens?
Distance decreases - each bar=larger image -> the gratins spatial frequency decreases as distance decreases
A pair of dark and bright bars
A cycle
The size (width) of a cycle(1D+1W pair) ?
A period
Degrees
Spatial frequency formula
1/period cpd (how many cycles are there in 1 degree)
How to calculate spatial frequency of a grating???
Count how many pairs= e.g 2.5
Over what distance? E.g 0.5deg -you actually need how many pairs in 1 degree, e.g 2.5x2=5 pairs or cycles or period in 1 degree which is SPATIAL FREQ
Now if you do 1/period e.g 1/5= 0.2 deg this is actually spatial period
If you know Snellen how to do SFreq?
S.Freq= 600/ S.Denominator
Opposite is (S.D = 600/ S.Freq )
Other way: for 20/40
GET MAR : 40/20 = 2arc min
Multiply by 2: 2arc minx2= 4arc min
Convert to degree: 4/60= 0.06
S.Freq is 1/spatial period: 1/0.06
Convert from S.Freq to Snellen acuity
S.D= 600/ S.Freq
Or
MAR is half the spatial period : X/2
Convert to arc min by X x 60
20/ (20x MAR) = answer
Does Blue degrade letter acuity or grating acuity more?
Letter acuity
20/20 0.25
20/30 0.50
20/40 0.75
What is contour interaction?
Acuity impaired by presence of nearby contours
What is crowding or separation difficulty?
Acuity impaired in some people like amblyopes by nearby letters on chart
Bailey Lovie chart controls contour interaction?
Yes. Has proportional spacing between adjacent letters and lines.
So less contour interactions among letters
What is the Bailey- Lovie logMAR chart ?
5 letters per line
British letters
Each line has different size in constant factors like 1.26x so 0.1log steps
Line spacing, letter spacing is proportional to letter size
Advantages of logMAR scale
Error is constant for all acuities and testing conditions
Equal changes in acuity
LogMAR charts scale with viewing distance
Contrast and acuity
Acuity improves gradually with contrast
Small changes in chart contrast don’t affect clinical acuity measurements
Luminance and acuity
Acuity improves with both scotopic and photopic target luminance
In clinic it’s like LogVA 0.5 and Log Retinal Illuminance 3-5 and 20/63??
Retinal eccentricity and acuity
Photopic acuity best at fovea and declines with increased retinal eccentricity.
When expressed in min arc = change with eccentricity is linear. So acuity is getting worse in periphery.
Also changes with eccentricity depending at different rates for different kinds of acuity tasks. Not uniform.
At Mesopic luminances, acuity is …… across a range of retinal eccentricities
Approximately constant
Scotopic acuity is best outside of fovea, but …… eccentrically than the region of highest rod density
LESS
It’s kinda expected to be higher at 20 degree eccentricity cause rods but it’s actually not the peak height, instead it’s at 5!!! Cause scotopic acuity is limited by neural convergence (increases with eccentricity) NOT by rod photoreceptors spacing like it’s the case with CONES.
Photopic letter acuity is limited by ……
The cone photoreceptor spacing.
Whereas scotopic acuity is limited by neural convergence (increases with eccentricity) not by rod photoreceptor spacing
Does visual acuity change with image motion from normal fixational eye movements
Doesn’t hurt but doesn’t help.
VA is neither aided or degraded by the image motion from normal eye fixational eye movements
Does visual acuity change with abnormal eye movements or when head motion is not compensated for?
Yeah. Acuity is degraded systematically by image velocity greater than a few deg/s.
Does acuity improve with target duration?
Yeah. Primarily because of temporal summation, acuity improves with target duration to a few hundred msec.
So acuity improves with longer duration
