Nature of Plasticity in Eyeblink Conditioning

Question 1

Now that we have established the sites of plasticity, what is the next step?

Answer 1

Find out how the plasticity works

Question 2

What did Jirenhead et al. (2007) find regarding the firing of Purkinje cells?

Answer 2

PC firing reduced in learning

Reduction in firing of Purkinje cells in HVI eyeblink area after learning (first trial compared to last trial) shown by electrophysiological recording

Question 3

Explain the structure of the cerebellum before conditioning

Answer 3

CS tone received by granule cells
Granule cells connected to Purkinje cell dendrites via parallel fibres (parallel fibre - purkinje cell synapse)
Purkinje cell dendrites –> Purkinje cell inhibits –> cerebellar nuclei
Climbing fibre also connects to Purkinje cell dendrites

Question 4

What effect does the CS tone have on PC firing rate and CN firing rate in the cerebellum before conditioning?

Answer 4

CS tone produces no change in PC firing rate
CS tone produces no change in CN firing rate i.e., no conditioned response

Question 5

What can be implied from the fact that a CS tone produces no consistent change in PC simple spikes?

Answer 5

Implies that the direct excitatory effect via the synapses between parallel fibres and Purkinje cells balances the inhibitory effect via inhibitory interneurons (stellate and basket cells)

Question 6

Explain the cerebellum during conditioning

Answer 6

1. During conditioning, CS is paired with US - so firing in parallel fibres is paired with firing in climbing fibre
2. Theory predicts that the PF-PC synapses become less effective by a process termed long term depression (LTD)
3. It is assumed that the inhibitory pathway is unchanged
4. Therefore, PC receives a net INHIBITORY input when CS comes on
5. PC pauses, thereby releasing firing in the anterior interpositus nuclei

Question 7

How does synaptic plasticity produce classical conditioning?

Answer 7

Models predict that LTD of synapses between parallel fibres and Purkinje cells produces conditioned responses

Question 8

Who formally modelled the role of LTD in classical conditioning?

Answer 8

Michael Mauk’s group

Question 9

Is there evidence for LTD?

Answer 9

YES - work done by Ito (1984, 2002) in cerebellar slices
Stimulated parallel fibres and climbing fibres together
Record Purkinje cell response to parallel fibre stimulation
Findings = as time from pairing of climbing fibre and parallel fibre stimulation increases, Purkinje cell response decreases

Question 10

What are the disadvantages of slice work?

Answer 10

1. Slices are ‘unnatural’ - no afferents, no blood supply - cannot assume that processes observed in slices are necessarily present in the intact brain
2. Even if the LTD observed in slices were present in the intact brain, it might not necessarily be there for learning - it might have a protective function

Question 11

Can we make conclusions from slice work?

Answer 11

Results from slices lead to hypotheses about functions in the whole animal
Need experiments that link biochemistry and behaviour to test those hypotheses

Question 12

What is one way of testing these hypotheses from slice work?

Answer 12

Knock out mice - breed mutant mice that specifically lack the necessary cellular machinery

Question 13

What has been found about LTD in knock out mice?

Answer 13

Knockout mice lack LTD in slices but show normal eyeblink conditioning

Question 14

What conclusion can be made regarding LTD in classical eyeblink conditioning?

Answer 14

LTD is not involved in classical eyeblink conditioning

Question 15

Mice lacking ____?____ do not show classical conditioning

Answer 15

Long term potentiation

Question 16

What is long term potentiation in terms of cerebellar learning?

Answer 16

When pairing a stimulus with a climbing fibre discharge, it results in the firing of parallel fibre synapses, increasing in strength

Question 17

What is the covariance learning rule?

Answer 17

Covariance learning rule for supervised learning

If input signal is positively correlated with error, decrease its influence

If negatively correlated, increase its influence

Correlation treated as cause

Question 18

How does the covariance learning rule relate to the cerebellum?

Answer 18

Increasing the influence of signals positively associated with error appears to be accomplished by increasing the influence of the inhibitory interneurons - stellate and basket cells

Individual synapses are either excitatory or inhibitory - hence need for parallel pathways with excitatory and inhibitory outputs

Question 19

How is the covariance learning rule related to motor control in general?

Answer 19

Know when an error has occurred e.g., fallen over
–> Change control signal in appropriate direction

Question 20

What device is used in control?

Answer 20

Adaptive filter - most models of cerebellar function are now some form of adaptive filter

Takes relevant information about the motor command to be carried out and the context in which it is to be carried

If the result is an error (movement is inaccurate) this is signalled and changes made to the filters connectivity

Question 21

What is an adaptive filter?

Answer 21

A signal processing device that seeks to decorrelate its main inputs from a teaching or error signal

Question 22

Why is the cerebellum considered an operating system?

Answer 22

Point and click - carries out high-level instructions without you being aware of what is going on

These instructions originate elsewhere e.g., cerebral cortex

Frees up system so that you can e.g., walk out of the room whilst thinking of what you are going to do or say next