Perception 2

Question 1

What is spatial frequency?

Answer 1

Number of pairs (1dark+1white bar) within a given distance on retina

Question 2

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?

Answer 2

As the distance between eye and object = decreases
=> the objects image subtends a greater visual angle

Question 3

A grating of high spatial frequency has

Answer 3

Many cycles within each 1 degree of visual angle and NARROW bars

Question 4

A grating of low spatial frequency

Answer 4

has few cycles within each 1 degree of visual angle-> contains WIDE bars

Question 5

Spatial frequency is defined based on visual angle, does that change?

Answer 5

So the spatial frequency changes with viewing distance

Question 6

As the distance decreases, what happens?

Answer 6

Distance decreases - each bar=larger image -> the gratins spatial frequency decreases as distance decreases

Question 7

A pair of dark and bright bars

Answer 7

A cycle

Question 8

The size (width) of a cycle(1D+1W pair) ?

Answer 8

A period
Degrees

Question 9

Spatial frequency formula

Answer 9

1/period cpd (how many cycles are there in 1 degree)

Question 10

How to calculate spatial frequency of a grating???

Answer 10

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

Question 11

If you know Snellen how to do SFreq?

Answer 11

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

Question 12

Convert from S.Freq to Snellen acuity

Answer 12

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

Question 13

Does Blue degrade letter acuity or grating acuity more?

Answer 13

Letter acuity
20/20 0.25
20/30 0.50
20/40 0.75

Question 14

What is contour interaction?

Answer 14

Acuity impaired by presence of nearby contours

Question 15

What is crowding or separation difficulty?

Answer 15

Acuity impaired in some people like amblyopes by nearby letters on chart

Question 16

Bailey Lovie chart controls contour interaction?

Answer 16

Yes. Has proportional spacing between adjacent letters and lines.

So less contour interactions among letters

Question 17

What is the Bailey- Lovie logMAR chart ?

Answer 17

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

Question 18

Advantages of logMAR scale

Answer 18

Error is constant for all acuities and testing conditions
Equal changes in acuity
LogMAR charts scale with viewing distance

Question 19

Contrast and acuity

Answer 19

Acuity improves gradually with contrast

Small changes in chart contrast don’t affect clinical acuity measurements

Question 20

Luminance and acuity

Answer 20

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??

Question 21

Retinal eccentricity and acuity

Answer 21

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.

Question 22

At Mesopic luminances, acuity is …… across a range of retinal eccentricities

Answer 22

Approximately constant

Question 23

Scotopic acuity is best outside of fovea, but …… eccentrically than the region of highest rod density

Answer 23

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.

Question 24

Photopic letter acuity is limited by ……

Answer 24

The cone photoreceptor spacing.

Whereas scotopic acuity is limited by neural convergence (increases with eccentricity) not by rod photoreceptor spacing

Question 25

Does visual acuity change with image motion from normal fixational eye movements

Answer 25

Doesn’t hurt but doesn’t help.

VA is neither aided or degraded by the image motion from normal eye fixational eye movements

Question 26

Does visual acuity change with abnormal eye movements or when head motion is not compensated for?

Answer 26

Yeah. Acuity is degraded systematically by image velocity greater than a few deg/s.

Question 27

Does acuity improve with target duration?

Answer 27

Yeah. Primarily because of temporal summation, acuity improves with target duration to a few hundred msec.

So acuity improves with longer duration