Spatial Vision

Question 1

What is visual system’s main task?

Answer 1

To extract information from the pattern of light delivered to the eye that is used to understand and interact with the environment.

Question 2

What is a ‘pixel by pixel’ measurement of luminance inefficient?

Answer 2

We would need finer resolution, more pixels and a much larger optic never to get a good representation.

Question 3

When might only coarse spatial information be necessary?

Answer 3

To get the ‘gist’ of a scene.

Question 4

Give the name for patches of sinusoids.

Answer 4

Luminance gratings.

Question 5

Give the equation associated with sinusoids.

Answer 5

Luminance = sin(x).

Question 6

Give the most effective formula for frequency.

Answer 6

Frequency = cycles/degree (cpd).

Question 7

Give the formula for amplitude.

Answer 7

Amplitude = max - min.

Question 8

Give the formula for contrast.

Answer 8

Contrast = max - min/max + min.

Question 9

Which sinusoid characteristic always has a value between 0 and 1?

Answer 9

Contrast.

Question 10

What are the 4 sinusoid characteristics?

Answer 10

Spatial frequency, amplitude (contrast), orientation and phase.

Question 11

Coarse and fine scales are processed by (the same/different) populations of neurons.

Answer 11

Different.

Question 12

Give 2 reasons sinusoids are used to describe and study the visual system?

Answer 12

Images are decomposed into patterns of sinusoids, allowing it to be described using a mathematical ‘language’ and measuring their responses allow prediction of other responses.

Question 13

Give the formula for contrast sensitivity.

Answer 13

Sensitivity = 1/contrast threshold.

Question 14

Why is the c.s.f. an inverted U shape?

Answer 14

Processing of high spatial frequencies is limited by the eye’s optics, as very high spatial frequencies are beyond the resolution of the eye.

Question 15

Give 2 advantages of acuity tests.

Answer 15

Quick to administer and most deficits are optical.

Question 16

Give 4 advantages of CSF tests.

Answer 16

Predict object visibility in any scene, some diseases show deficits in specific CSFs, measures acuity, and allows us to predict how the brain responds to different spatial frequencies.

Question 17

How do retinal ganglion cells respond when light is shone?

Answer 17

Their response can increase or decrease, depending on the specific location the light shines on.

Question 18

Why is a weak retinal ganglion cell response evoked by a uniform field of light?

Answer 18

The excitatory and inhibitory regions affect a cell approximately equally.

Question 19

How are edges detected by retinal ganglion cells?

Answer 19

Changes in luminance create a strong response, and contrast between borders is represented.

Question 20

What do ‘simple cells’ in V1 respond maximally to?

Answer 20

High contrast border of bars of the correct orientation, in specific locations.

Question 21

Give a 3rd reason sinusoids are used to describe and study the visual system, accounting for receptive fields.

Answer 21

Neurons with different sized receptive fields process the scene at different scales, so computation is akin to sinusoidal decomposition.

Question 22

What does the low spatial frequency limit of receptive fields inform us about?

Answer 22

The largest receptive fields.

Question 23

Spatial frequency channels are sensitive to a (narrow/wide) range of spatial frequencies.

Answer 23

Narrow.

Question 24

Why does a ‘notch’ in c.s.f. show after prolonged exposure to one stimuli?

Answer 24

Only one channel has adapted to it, due to independence of channels.

Question 25

How do we represent the whole range of spatial frequencies?

Answer 25

Ratios of activity are compared and if one changes, the visual system measures this as a change in spatial frequency of the stimulus.

Question 26

Why after adaptation do we perceive two things that are the same to be different?

Answer 26

Because the ratio of activity has changed and so it is interpreted as a different stimulus.