Plasticity

Question 1

What is brain plasticity

Answer 1

A feature of the brain whereby its structure and function change all the time, through perception, task performance and after damage.

Question 2

What are some consequences of plasticity

Answer 2

Cortical reorganisation - a piece of cortex that loses its function then takes up a new task
Cortical expansion - the size of the represented body part in the cortex depends on the amount of use of that part.
Memory - synapses can be strengthened or weakened over various time scales.

Question 3

What is the central governing rule of plasticity

Answer 3

‘Hebbian learning’ - postulated by Donald Hebb
‘Neurons wire together if they fire together’
Synapses weaken if the activity of the neurons are not correlated.

Question 4

What is plasticity like in infants

Answer 4

It is non-selective, all stimuli induce plastic changes = critical period of development.

Question 5

What is plasticity like in adults

Answer 5

It is selective, controlled by task demands and context.

Question 6

How do salient stimuli induce plasticity

Answer 6

1. Stimuli in the focus of attention (i.e. in working memory)
2. Surprising stimuli
3. Stimuli which are associated with a reward.

Question 7

What is the role of neuromodulators in plasticity

Answer 7

Plasticity is enhanced by neuromodulators (a messenger released from a neuron to affect another group of neurons that have an appropriate receptors) released from the limbic system (a collection of mostly subcortical areas regulating emotion and memory).

Question 8

How are areas of the limbic system activated

Answer 8

Areas of the limbic system are activated in context and task-dependent ways
E.g. the nucleus basalis releases acetylcholine which selectively amplifies input in the focus of selective attention, and weakens others.
Connections supporting the representation of attended stimulus become stronger.
Nuclei in the limbic system project to all parts of the cortex.

Question 9

Explain cortical reorganisation in the motor cortex (examples of plasticity)

Answer 9

• Motor neurons in a rat that control the whiskers are cut.
• Motor cortex reorganisation occurred within a few weeks - the other brain areas took over the area that controlled whiskers, so that area gained a different new function since the function to control whiskers was no longer needed.

Question 10

Explain cortical reorganisation in the visual cortex (examples of plasticity)

Answer 10

• Echolocation recruits in the visual cortex.
• Two PPs; echolocation expert and control PP
• Listened to background sounds; clicks and echoes vs just clicks
• Echolocation expert: increases in brain activity in occipital lobe and other areas.
• No contrasts in control subject.

Question 11

Explain cortical expansion in motor cortex (examples of plasticity)

Answer 11

• String players vs control
• Somatosensory stimulation of thumb (D1) and little finger (D5) of left and right hand.
• Dv was the distance between areas activated by D1 and D5
• String players has a larger representation

Question 12

Describe how plastic changes have been found to be reversible

Answer 12

• Tonotopic mapping; different tone frequencies are mapped to different parts of the brain area in an orderly way
• In young rats, tonotopic mapping showed that neurons are narrowly tuned to tone frequency
• In aged rats, it showed that many neurons are broadly tuned - this fits with the symptom of ageing; lower freq discrimination ability - decline in the discrimination between stimuli.
  After conducting simple discrimination training on the aged rats for 1 hour a day for a month, the auditory cortex in old rats was restored to a ‘youthful’ state.

Question 13

What are the main approaches for treating neurological impairment and psychiatric illness

Answer 13

1. Pharmalogical
2. Targeting behaviour
   There are also promising areas including
   1 . recover from injury or stroke
3. hemispatial neglect
4. schizophrenia
5. ageing
   Brain plasticity offers a new approach: fixing the underlying faulty brain mappings through specific training.

Question 14

What are some varieties of synaptic plasticity

Answer 14

• Long-term potentiation (LTP)
• Long-term depression (LTD)
These two are hours to months
• Short-term synaptic depression (STSD)
• Short-term synaptic facilitation (STSF)
These two are milliseconds to seconds.

Question 15

Describe LTP and LTD in action

Answer 15

LTP is Hebbian learning in action: discovered by Bliss & Lomo (1973) in a rat hippocampus.
They stimulated an electric potential to a granule cell and then measured the response across the synapse to measure the effects of repetitive stimulation.
The baseline assessment was just a single pulse.
The condition was multiple pulses at either 100 Hz or 5 Hz.
When there was fast stimulation it led to long-term depression which reduced the post-synaptic potential.
Essentially, LTP strengthens synapse connections while LTD weakens them.

Question 16

Describe short-term synaptic plasticity

Answer 16

STP (i.e. dynamical synapses): synaptic efficacy changes over time in a way that reflects the previous use of the synapse, i.e. the history of presynaptic activity.
Its duration is tens of milliseconds to several seconds.
It is in contrast to Hebbian learning (LTP and LTD)

Question 17

What is the synaptic plasticity and memory (SPM) hypothesis

Answer 17

Activity-dependent synaptic plasticity is induced at synapses
during memory formation (learning).
Memory representations are
distributed across synapses and encoded as synaptic strengths.

Question 18

How does memory reside in the synapses

Answer 18

Memory representations are encoded and distributed over multiple synapses.
Memory is an emergent property

Question 19

How does memory representation take insight from computational modelling

Answer 19

1. Hebbian learning in an artificial neural network.

2. STP in a model of auditory cortex.

Question 20

Explain the neural network representing visual images

Answer 20

At each time point, each neuron has a certain activity level in the 0-1 range.
The state of the network corresponds to a unique image.
As the neurons interact with each other, the sate of the network changes.
The light intensity of each pixel represents the activation value of the neuron.

Question 21

What is the Hopfield network

Answer 21

Hopfield (1982, 1984) showed how point attractors can be ‘manufactured’ in neural networks through plasticity.
Modifying synpatic strengths according to Hebbian learning and the current state of the system turns this state into an attractor.
These attractors function as associative memory representations.
Partial or nosy input leads to original representation.
We have content-addressable associative memory.

Question 22

How does the attractor equate to memory representation.

Answer 22

The original image is encoded as a system attractor.
The system “recognizes” a corrupted stimulus.
It takes the stimulus and associates it with a memory representation by reconstructing the state it was in when it underwent learning.

Question 23

How is memory distributed

Answer 23

It is distributed across all the synaptic connections.
Synaptic weights determine where the attractor is.
If you change any one of the weight values, the attractor changes location and the memory representation is changed.
The same network can have many attractors.
The same set of connections can encode multiple memory representations.
The attractor is an emergent property of the system which emerges when the units interact with each other.

Question 24

How can the auditory cortex be translated into computational models

Answer 24

Computational unit: cortical column

Connections with short-term synaptic depression.

Question 25

Describe the replication of MMN in simulations

Answer 25

A complex 5-tone sequence; deviant 3rd and 4th tone flipped.
Deviant produces a clear MMN sequence.
Removing synaptic plasticity removes adaptation and the MMN.
So temporal binding as reflected in MMn needs STP.

Question 26

Describe the temporal binding responses

Answer 26

Columns respond to speech stimuli as temporal entities.
1. strong response to complete word
2. weak or no responses to parts presented in isolation.
Once again, temporal binding requires short-term synaptic depression.

Question 27

Describe synaptic depression as memory.

Answer 27

Each stimulus changes the cortical network uniquely and thus leaves a memory trace.
Ergo: each stimulus is also met by a system which has been changed by the previous stimulation.
Thus responses become dependent on historical context.