lecture 8- synaptic plasticity in simple systems

Question 1

what was plato’s beliefs on learning

Answer 1

nativist= all knowledge is innate and not acquired.

Question 2

what was aristotles beliefs on learning?

Answer 2

empiricist= mind is a blank slate, where experience is drawn on it.

Question 3

what are the two main physical basis of memory according to Rose (1995)?

Answer 3

1. plasticity- physical change in the brain leading to memory formation. HOW does it work?
2. Engram- the physical manifestation of memory in the brain. WHERE are they?

Question 4

what is connectionism?

Answer 4

memory is distributed in networks of neurons, memory is the strength of connections between neurons.

Question 5

what can memory be encoded by?

Answer 5

experience-dependent changes in the ability of one neuron to excite its neighbours.
a simple network can be trained to change its responses.

Question 6

what is Long Term Potentiation (LTP)?

Answer 6

the long lasting enhancement in efficacy of the synapse between two neurons.

Question 7

what is Long Term Depression (LTD)?

Answer 7

alternative form of plasticity in which there is a decrease in the efficacy between two neurons.
(better way to code memory).
LTD is the exact mirror of LTP.

Question 8

what do LTP and LTD form?

Answer 8

the physiological basis of memory in the brain.

Question 9

what is the LTP theory? as in how does it work?

Answer 9

1. application of a tetanus (high frequency burst of APs) induces repeated pre and post synaptic neurons.
2. results in an increased efficacy between pre and post synaptic neurons.

Question 10

what is associative LTP?

Answer 10

tetanus will not elicit LTP when applied to a weak input alone.
when a tetanus stimulus is applied to both inputs simultaneously it gives rise to LTP in both.
supports the view that LTP represents a neural mechanism underlying learning.

Question 11

what do synapses in LTP have?

Answer 11

1. Rapidity- LTP induced v fast
2. Cooperativity - LTP can be induced either by strong tetanic stimulation or cooperatively via weaker stimulation- many weaker inputs generate more APs.
3. Associativity - strong stimulation of another pathway will induce LTP when weaker stimulation is insufficient.
4. Input Specificity - specific to the place LTP is applied, specific to those synapses.

Question 12

explain connectionism…

Answer 12

memory distributed in networks of neurons - experience is encoded in the strength of the connections between them.

Question 13

what is Parallel Distributed Processing (PDP)?

McLelland et al, 1986

Answer 13

massively parallel networks- acquired knowledge is distributed across the network rather than at any single point.
every input unit is connected to every hidden unit (recipients) which is connected to every output unit.
learns statistical regularities between inputs and outputs.

Question 14

how do connectionist networks PDPs learn?

Answer 14

presented with training sets containing inputs and outputs. the difference between the actual and desired outputs= the degree of the error.
connections have values, the error signal is propagated backwards - until the errors are minimal.

Question 15

what are the advantages of connectionism? give 4.

Answer 15

1. it gives biological realism.
2. networks learn through experience
3. If lesions occur memory can still be useful - graceful degradation.
4. analytic solutions are not required.

Question 16

what are the disadvantages of connectionism? give 4.

Answer 16

1. can learn anything given enough training- not how real brains work.
2. info is sub-symbolic - no symbol manipulation, just APs.
3. training the network to a competent level is v time consuming.
4. networks forget learned material fast when trained on other data sets.