Measuring visual acuity

Question 1

Explain visual angle

Answer 1

alpha: visual angle subtended at the eye by height of image, h, viewed from distance, d

Question 2

Explain why, using in vision research, is it a more useful measure of image size than linear units such as cm?

Answer 2

-height of letters on bottom row of Smellen chart, subtend visual angle of 1/12 of a degree (5 arc min).
-Each horizontal element of the E would subtend 1 arc min
-the height, h’ of retinal image formed from letter of height, h subtending visual angle of a h’ = 2d’ tan(a/2) where d’ is diameter of eyeball

Question 3

what is 1 degree?

Answer 3

60 min of arc

Question 4

visual angle formula

Answer 4

h’ = 2d’ tan(a/2)

Question 5

what is the peak cone density in retina?

Answer 5

160k per mm^2

Question 6

what is the cone spacing in central fovea?

Answer 6

1-4um

Question 7

what is the size of retina? how thick?

Answer 7

50x50mm
0.4mm thick

Question 8

what is diameter of central fovea?

Answer 8

(foveola, rod-free, capillary free)
0.3mm

Question 9

1 degree visual angle =

Answer 9

0.3mm on retina

Question 10

what is spatial frequency

Answer 10

number of cycles per degree
1cm st 57cm is 1 degree

Question 11

what is contrast?

Answer 11

difference between min and max luminance

Question 12

what is 1 arc min?

Answer 12

30 cycles/degree

Question 13

what is the role of receptor field ganglion cells?

Answer 13

-send us around organisation and are set up to detect changes

Question 14

how does contrast sensitivity vary with spatial frequency?

Answer 14

-contrast decreases logarithmically along y-axis
-spatial freq increases log along x-axis

Question 15

what is contrast threshold?

Answer 15

minimum contrast required to detect target

Question 16

what is contrast sensitivity

Answer 16

reciprocal of contrast threshold

Question 17

contrast equation

Answer 17

Contrast = Im - In / Im + In
-max and min intensities

Question 18

what is the contrast-sensitivity function

Answer 18

-all visible contrasts and freq are inside the curve
-sensitivity is reasonably constant for intermediate freq
-highest spatial freq require high contrast

Question 19

explain the tilt-after effect

Answer 19

1. system does not need an orientation sensitive cell for every possible orientation
2. orientation could be estimated by averaging cells with similar preferences
3. comparison of signals across cells with different tuning
4. adaptation of cells occurs with prolonged stimulation
5. if cell is adapted to one orientation then response will be decreased to same or similar orientation immediately afterwards
6. will introduce error in estimation of orientation
7. this predicts that the effect will only be elicited by adapting to an orientation close to the test orientation so that neural tuning curves overlap

Question 20

describe population coding of orientation

Answer 20

-adapt to grating then measure threshold contrast required to detect vertical test grating.
-plot as function of adapting grating orientation
-effect declines and disappears as difference in adapting orientation increases
-implies population coding of orientation

Question 21

Psychological evidence for orientation columns in humans

Answer 21

• relies on adaptation
• contrast sensitivity decreases adaptation
• adapts to orientation

Question 22

ORIENTATION TUNING

Answer 22

• orientation-sensitive neurons do not respond to just a single orientation (to range)

Question 23

tuning curve:

Answer 23

describes the range and selectivity with which a neutron responds to orientations

Question 24

idealised orientation tuning curve

Answer 24

-what does a firing rate of 10Hz mean?
-high contrast 25 deg away from preferred orientation?
-or low contrast at preferred orientation?
-faint line increases contrast line away from vertical or preferred orientation