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 limy 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

What is the formula to work out contrast?

Answer 9

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

Question 10

What is contrast threshold?

Answer 10

The minimum contrast required to detect a target

Question 11

What is contrast sensitivity?

Answer 11

The reciprocal of contrast threshold

Question 12

What is the contrast sensitivity function?

Answer 12

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

Question 13

What is the evidence for orientation columns in humans?

Answer 13

• Reliant on adaptaiton
• Contrast sensitivity decreases following adaptation

Question 14

What do orientation sensitive neurons sensitive to?

Answer 14

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

Question 15

What is population coding?

Answer 15

Idea that from a single neuron, information can be ambiguous

Question 16

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

Answer 16

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

Question 17

Explain the tilt after effect

Answer 17

• 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