2 Neuroscience2.7 Neural circuits
2.7.1.7 Learning and computational models
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Written by Resident Doctor Committee · Published
Hebbian plasticity
Hebbian plasticity, proposed by Hebb in 1949, means that when a presynaptic neurone repeatedly takes part in firing a connected postsynaptic neurone, the synapse between them is strengthened.
The slogan is "neurones that fire together wire together", but the rule needs the presynaptic cell to help cause the firing, not merely to fire at the same time.
NMDA receptors detect this coincidence: they open only when glutamate binds and depolarisation removes the magnesium block, and calcium then enters.
The size and timing of the calcium rise decide the outcome.
Calcium rise | Cascade | Result |
|---|---|---|
Large and rapid | CaMKII, insertion of AMPA receptors, spine enlargement | Long-term potentiation |
Smaller and prolonged | Phosphatases such as calcineurin, AMPA receptors internalised | Long-term depression |
Inhibition, synaptic scaling and heterosynaptic depression stop runaway strengthening.
Non-associative and associative learning
Form | What happens |
|---|---|
Habituation | A response declines to a repeated harmless stimulus; it is stimulus-specific, unlike fatigue |
Sensitisation | A response increases after strong stimulation and can generalise |
Classical conditioning | The animal learns which stimulus predicts which outcome |
Operant conditioning | The animal learns which actions lead to which consequences |
Reward prediction error
A reward prediction error is the gap between an outcome and what was expected, and many midbrain dopamine neurones signal it.
Unexpected reward increases their firing, predictive cues come to carry the burst, and an omitted expected reward produces a pause.