Sensory systems and sensory input

Question 1

What 4 things does a visual neuron encode?

Answer 1

1) Contrast
2) Colour
3) Frequency
4) Orientation

Question 2

What is contrast?

Answer 2

An increase or decrease in light intensity from a certain level

Question 3

What is visual system interested in, rather that light intensity?

Answer 3

CHANGES in light intensity (contrast) in time/space

Question 4

What frequency does the visual system encode?

Answer 4

Spatial/temporal frequency:

• How quickly neighbouring pixels change
• How quickly the intensity changes in time

Question 5

Why is orientation important in an environment?

Answer 5

It encodes the EDGES of an object

Question 6

What information does the visual system use from the environment in order to arrange itself?

Answer 6

• Contrasts are not random
• Colours are not random
• Orientations are not random
• Neighbouring pixels are similar in brightness and colour

Question 7

Why is colour not random in the environment?

Answer 7

• Blue dominates the UPPER part of the visual field (sky)

- Green dominates the LOWER part of the visual field (grass)

Question 8

Why do neurons adjust their sensitivities to contrasts?

Answer 8

To be able to process very bright and very dim pixels at the same time

Question 9

What 2 orientations dominate the visual scene?

Why is this the case?

Answer 9

Vertical and horizontal

Due to gravity - stable orientations are either stood up (vertical) or lay down (horizontal)

Question 10

Describe the distribution of pixel brightness in the environment?

Answer 10

Very wide

Question 11

Describe the distributions of the key properties of the natural scenes

Answer 11

They are NOT random

They have certain statistics

Question 12

How do the retinal neurons respond to the environment? (graphically, where X axis is property of the environment and Y axis is response amplitude of the neuron)

What does this show

Answer 12

Sigmoidal response:

• Most information is encoded in the middle (where small changes in the property of the environenment has large changes in the response amplitude)
• Curve saturated - very large changes in property has very small changes in response amplitude

Question 13

What problem does the broad distribution of a property cause?

Answer 13

Problem for the encoding of the sensory neuron:

• The very edges of the distribution (very high contrast, very low contrast), which are more rare are still VERY important
• Most edges have very high contrast - visual system is tuned to process edges
• However, these lie of the flattened portion of the tuning curve (large changes in property, small/no changes in intensity)

Question 14

How are the problems of the distribution/tuning curve matching overcome?

Answer 14

1) Tuning

2) Adaptation

Question 15

Describe the tuning strategy

Answer 15

Different neurons tuned to different parts of the distribution:
- Some neurons - high sensitivity, respond to low stimulus intensity

• Others - intermediate
• Others - Less sensitive, respond to high contrast/intensity

Question 16

Describe the adaptation strategy

Answer 16

Neurons have plastic curves, that can become more/less sensitive

Question 17

How does the retina use the information from the environment (colour is not random)?

Answer 17

Green cones - located at the TOP of the eye

Blue cones - located at the BOTTOM of the eye

Question 18

Where the ganglion cells that respond to approaching objects (increasing in size) present?

Why are they needed?

Answer 18

In the bottom of the retina

Sky is static - need to know if anything changes (eg. approaching bird)

Birds usually in the sky

Question 19

Does the retina process negative or positive contrasts better?

Why?

Answer 19

Negative better

There are more negative contrasts in the environment

Question 20

How does the retina better process negative contrasts?

Answer 20

More OFF cells than ON cells

Better to calculate decreases in light intensity

Question 21

What calculation is used to calculate the amount of information that can be transmitted by a neuron?

Answer 21

C = B log2 (1 + S/N)

Question 22

What is S/N?

What happens when this is increased?

Answer 22

Signal to noise ratio

When increase - more info can be transmitted

Question 23

What limits the amount of information that can be transmitted by a neuron?

How is this calculated?

Answer 23

N - noise

Noise = individual response - signal

Question 24

What is the main source of noise in a neuron?

Answer 24

• Spontaneous opening of channels (Na, K)
• Affects spike rate
• Spontaneous opening of Ca2+ channels that is NOT triggered by the release of membrane potential

Question 25

What happens to the amount of information processed per spike when the spike rate of a neuron increases?

Answer 25

Increase spike rate - smaller amount of information that can be processed per spike

Question 26

Why is it beneficial to keep the spike rate of a neuron low?

Answer 26

So that more information can be processed per spike

Each individual spike costs the neuron lots of ATP molecules

Question 27

How is information in a neuron carried?

Answer 27

In ‘bits’

Question 28

What happens to the cost per bit if more information is transmitted by a neuron?

Why is this a problem?

Answer 28

More cost per bit

Small number of spikes per second

Question 29

What did Gollisch state?

Answer 29

Information is encoded by LATENCY of spikes NOT the frequency