lecture 3- spacial frequency

Question 1

state- the greater number of waves, the greater the number of stripes (higher spatial frequency ie they get narrower)

Question 2

state- the greater the amplitude (depth) of the wave, the greater the contrast (blackness to whiteness) of the stripes

Question 3

contrast C=

Answer 3

(Lmax - Lmin)/(Lmax + Lmin)

Question 4

what can gratings also vary in

Answer 4

orientation

Question 5

what are the three characteristics of gratings?

Answer 5

-spatial
-contrast
-orientation

Question 6

what can these grating characteristics be manipulated to form?

Answer 6

visual pattern

Question 7

why sinusoidal gratings?

Answer 7

it can be shown mathematically (fourier analysis) that any wave pattern can be broken down into a series of sine waves with different frequencies and amplitudes

Question 8

how can a lens be evaluated?

Answer 8

a lens can be evaluated by measuring the luminance intensity of the transmitted stripes in the image in comparison with the original intensities of the stripes in the object

Question 9

why are the measurements done?

Answer 9

the measurements are done for a series of different spatial frequencies

Question 10

what is the relationship of the ratios of image to object intensities against spatial frequency called?

Answer 10

modulation transfer function (MTF) of the lens

Question 11

what is the perceptual transfer function also called?

Answer 11

contrast sensitivity function

Question 12

what does the perceptual transfer function depend on?

Answer 12

optical transfer function and the neural transfer function

Question 13

age and the CSF

Answer 13

• infants have a different window of visibility than adults
• the CSF of the infant gradually improves during the first year of life
• the CSF changes again later in life

Question 14

CSFs of other species

Answer 14

• are human CSFs of non-human primates similar to the CSF of humans?
• do birds of prey have better acuity than most animals?

Question 15

visibility of gratings of different frequencies

Answer 15

• blakemore and campbell (1969) adapted observers for several minuets to a high contrast gratings of 7 cycles/degrees
• then observer viewed a low contrast test grating and adjusted its contrast till it could just be seen
  -the experiment was repeated (adaptation followed by test grating) with several different frequencies of test grating
  -the experiment was also repeated with different adaptation grating frequencies

Question 16

orientation of gratings

Answer 16

a classic experiment was conducted by campbell and kulikowski (1966) using 10 cycles per degree horizontal masking grating (always present) and variously orientated test gratings flashed on and off for 1 second periods
- the observer adjusted the contrast of the test grating so that it can be seen overlapping the masking grating
- the masking grating was also changed in contrast between experiments

Question 17

orientation conclusions

Answer 17

• very little masking of the test grating when it is 90 degree to the masking grating
• as the angle between the test and masking gratings decreases, the effect of masking increases (ie increased contrast of test grating is needed for it to be seen
• most interference occurs when gratings are parallel

Question 17

the structural basis of the CFS

Answer 17

• selective adaptation effect suggest that different neural channels are used to detect different spatial frequencies
• detection of any spatial target depends on responses in sets of neurons tuned to a certain spatial frequency and orientation