Memory is a persistent change to large areas of the cortex
- Memory involves most of the cortex, linked by associative systems
- Most memories are spread across the cortex (not localised). Every time you remember something it reinforces that memory
- Also associating digits with objects/colours etc allows you to recall them more easily
- Destroying parts of cortex typically causes gradual degradation of memory rather than abrupt loss of unitary memories.
- Memory persists through disruptive changes in consciousness such as sleep, coma, seizure or surgical anaesthetic.
- This indicates that memory must involve permanent structural change in the nervous system rather than just ongoing activity patterns.
- NB; amnesia is quite rare
- there are permanent changes to the neurons to make memories
The physical substrate of memory is structural change in the nervous system
- Memory is a fundamental part of nervous system
- experience modifies the functional structure of all nervous systems
- the act of remembering uses the same parts of cortex as when the memory was formed
- the mechanism by which patterns of neural activity produce permanent or temporary changes in brain structure is plasticity
- plasticity works by modifying the synapse
- waking up quiet synapses
- silencing active synapses
- forming new synapses or destroying old
- Learning occurs when two connected neurons are active simultaneously in a way that strengthens the synaptic connection
- cells that fire together, wire together (get connected together if they're made to fire together; also your brain prunes back connections that aren't being used)
- neurons that are out of synch, lose their link
Long term potentiation (LTP)
- A form of synaptic plasticity
- Discovered: in 1973 in rabbit hippocampus
- Connections in the rabbit hippocampus could be strengthened by high frequency electrical pulses
- Defined: a stable, long‐lasting increase in magnitude of response of hippocampal neurons after bursts of high‐frequency electrical stimulation of their afferents
- HFS forms a memory trace in the hippocampus
Hippocampal slice preparation to test synaptic plasticity
- Stimulate Schaffer collaterals (fibres converging on the one neuron), and then measure their target neuron
- If you stimulate at 1 Hz, you get the same size fast EPSP, but if you stimulate it faster, you get a higher amplitude EPSP (therefore that neuron is more likely to fire in future and contribute to the overall electrical activity)
What is LTP
- A phenomenon of the nervous system that may allow for complex behaviours such as memory and task learning
- Formed from a combination of mechanisms that allow synapses to increase synaptic efficacy that vary in time course
- Very hard to determine from one neuron how the whole brain is functioning
- Increases synaptic efficacy (fast EPSP that is larger and more likely to form an AP)
- This long term change in neuronal activity occurs in all brain structures
- Amygdala - fear learning is strong and easy to invoke.
Does LTP = memory?
- No, but it is part of the initial formation of memories
- Lasts a long time
- Labile consolidation - when you first try to invoke it, it's not stable yet (can disrupt it with a drug)