Measuring Visual Acuity

Question 1

What is the visual angle?

Answer 1

• The angle that an image subtends at the eye (α)
• The image size depends on the viewing distance so if you move closer the visual anfle is larger

Question 2

What is 20:20 vision?

Answer 2

When α is 5 mins of arc at a viewing distance of 20 ft

Question 3

What would limit the resolving power of the eye?

Answer 3

The spacing of the photoreceptors

Question 4

What is spatial frequency?

Answer 4

The number of cycles per degree

• More cycles per degree means higher spatial frequency and so the detail is finer

Question 5

What is the relationship between high spatial frequency and resolving power?

Answer 5

Higher the spatial frequency, the higher the resolving power

Question 6

What do we use for high resolution vision?

Answer 6

Combination of red and green cones in fovea

Question 7

What is contrast?

Answer 7

The difference between the maximum luminance and the minimum luminance

Question 8

What is luminance?

Answer 8

The amount of light coming off of an image and this is low from black lines and high from white lines

Question 9

the luminance graph

Answer 9

A cycle= going from black to white or visa versa
The more cycles per degree, the higher the spatial frequency, i.e. the finer the detail, higher the
resolving power

Question 10

what is contrast

Answer 10

Contrast: difference between min and max luminance

Question 11

what is The ultimate upper limit of the resolving power of the eye and spatial frequency,

Answer 11

assuming the optics
are perfect (the eye is correctly focusing the image as well as it can), is the spacing of the
photoreceptors

Question 12

what is the cone spacing in the central fovea

Answer 12

• Cone spacing in central fovea: 160k per mm2

Question 13

What is the formula to work out contrast?

Answer 13

lm-ln / lm+ln, where lm is the maximum intensity and ln is the minimum intensity

Question 14

What is contrast threshold?

Answer 14

The minimum contrast required to detect a target

Question 15

What is contrast sensitivity?

Answer 15

The reciprocal of contrast threshold

Question 16

What is the contrast sensitivity function?

Answer 16

• All visible contrasts and frequencies are inside the curve
• Sensitivity is reasonably constant for intermediate frequencies
• Highest spatial frequencies require high contrast

Question 17

what are orientation columns

Answer 17

• It’s the main property of neurones in the primary visual cortex
• They’re tuned to particular orientations
• are excited by visual line stimuli of varying angles
• span multiple cortical layers

Question 18

What do orientation sensitive neurons sensitive to?

Answer 18

• Orientation but not just a single orientation, can respond to a range

Question 19

What is population coding?

Answer 19

Idea that from a single neuron, information can be ambiguous

Question 20

What can a reduction in the signal from a neuron be due to?

Answer 20

• Bars orientation stayed the same but changed in contrast
• Bars tilted but contrast stayed the same

Question 21

what is the tilt-after effect

Answer 21

Stare at bar for long enough, then look at bars that are vertically orientated, there will be an illusion that
the bars are orientated in the opposite direction
* You adapt to that orientation but not to others so that when you’re presented with a new orientation,
the decoding mechanism is biased because of the previous adaptation

Question 22

Explain the tilt after effect

Answer 22

• System does not need an orientation-sensitive cell for every possible orientation
• Orientation can be estimated by averaging cells w similar preferences
• Adaptation of cells occurs w prolonged stimulation
• If a cell is adapted to one orientation, then the response will be decreased to the same or a similar orientation immediately afterwards
• It will introduce error in the estimation of orientation
• This predicts that the effect will only be elicited by adapting to an orientation close to the test orientation so the neural tuning curves overlap

Question 23

what is orientation tuning

Answer 23

Orientation-sensitive neurons do not respond to just a single orientation, they respond to a rangethey’re broadly tuned
* E.g. have a max response a 45 degrees but still respond at 35 degrees

Question 24

what is the tuning curve

Answer 24

• Tuning curve: describes the range and selectivity with which a neuron responds to orientations