1.7 Learning and Memory I

procedural memory

A type of long-term memory of how to perform different actions and skills. Essentially, it is the memory of how to do certain things.

do rats have episodic memory

Experiment 1: Rats remember where their preferred food was cached • What - pretzel vs. cheese • Where - which arm of the maze • Rats will store food in the arms of the maze to come back to • At test they choose the arms where they stored cheese first Experiment 6: Rats do not remember when food was cached • What - pretzel vs. cheese • Where - which arm of the maze • When - 2 delays (1h and 25h) • At 25h delay the cheese is degraded using quinine • If rats can remember what, where and when they should search the cheese arms at the 1 h delay and the pretzel arms at the 25h delay • Rats preferred the cheese arms in both conditions

for long lasting memory we need long lasting LTP

For this we need: • Short term LTP (insertion of AMPA receptors into postsynaptic membrane) • A chemical tag at the synapse to attract proteins • Proteins to stabilise the changes induced in short term LTP

Case of H.M. (Henry Molaison)

H.M. had brain surgery in 1953 when he was 27 yrs. old. The surgery involved removal of part of the brain known as the hippocampus to alleviate the severe symptoms of epilepsy. Although the surgery controlled the epileptic seizures H.M. suffered serious and debilitating memory impairment as a side effect. His short-term memory was normal but he was completely unable to transfer any new information into his long-term memory. He showed almost no knowledge of current affairs because he forgot any news item as soon as he had read about it; he knew nothing of recent family events including moving house and the death of his father. Despite being able to remember people he had known long ago he was never able to store information about new people he encountered and they remained forever complete strangers to him. In many respects H.M. seemed cognitively 'normal' as he was able to learn and remember perceptual and motor skills although he needed reminding of what he was able to do. This case and others illustrate the highly selective nature of the problems of anterograde amnesia following brain damage. There is no general deterioration of memory function but specific deficits in which some abilities such as learning new information are severely impaired whilst others, including language and memory span are quite normal

Hebb's Law

Neurons that fire together Wire together, their synaptic connections become more effective

short vs long term memory

short term memory - Short term memory also called working memory is the gateway for information to enter long term memory long term memory - Declarative memory and procedural memory are the two types of long-term memory. - Procedural memory consists of how to do things. - Declarative memory consists of facts, general knowledge, and personal experiences. The two subtypes of declarative memory are episodic memory and semantic memory

declarative vs non declarative memory

• Declarative memory: memory that can be consciously recalled (facts, events) - semantic memory: memory for facts - episodic memory: memory of events • Non-declarative memory: automatic learning not under conscious control (riding a bike)

do animals have episodic memory?

• Definition of episodic memory: - "receives and stores information about temporally dated episodes or events and the temporal-spatial relations among these events" (Tulving, 1972) • What happened, where it happened and when it happened • Autonoetic consciousness added as part of the definition later on • Can we test self-awareness in animals? • Can we test the contents of episodic memory for what, where and when? Clayton & Dickinson (1998) Nature, 395, 272-274 • Birds prefer wax worms • If birds remember what, where and when they will retrieve the wax worms when they were cached recently but not when they were cached a long time ago • The birds choose the worms when they are fresh but not when they have learnt that they will be bad • They can remember what food they stored, where they stored it and when that was

episodic memory

• Episodic memory is the first symptom of mild cognitive impairment (MCI) which often precedes Alzheimer's disease • Episodic memory deficits have also recently been described in Huntington's disease, following chemotherapy and after ECT treatment for depression To understand the mechanisms underlying these disorders we must understand the neural circuitry that supports episodic memory • Hippocampus involved - Studies of HM suggest that episodic memory is dependent on the hippocampus - Episodic memory is impaired in patients with hippocampus damage

priming

• Improvement in identifying or processing a stimulus as a result of its having been observed previously • Perceptual priming - Words presented quickly and subjects asked to identify them (e.g. fragrance, telephone etc.) - Some time later subjects are presented with word stems (e.g. "fra_____") and they are asked to complete them - Priming is the increased probability of choosing previously presented words (even if they were presented too quick to consciously process)

summary

• In humans the hippocampus is critical for episodic memory • Episodic like memory can be tested in both rats and birds • In rats the hippocampus is critical for episodic memory Cellular mechanisms for memory consolidation - synaptic plasticity • Learning must involve changes in synaptic function • For long term memory these changes must be long lasting • Long term potentiation (LTP) • Stimulate inputs to the hippocampus (perforant path) • Record signal (Excitatory Post-Synaptic Potential -EPSP) from groups of neurons in hippocampus

Chemistry of short term LTP

• LTP is dependent on NMDA receptors which allow Ca2+ ions into the cell once activated • Ca2+ ions set in motion a cascade of reactions that result in more AMPA receptors being inserted into the postsynaptic membrane • More AMPA receptors result in a stronger response to stimulation from the presynaptic cell • Synapse strengthened for a few hours- short term LTP • Long lasting LTP (over a few hours) requires proteins to stabilise the short term changes already produced • These are captured by tags that are set at the relevant synapse

non-associative learning

• Learning that does not involve the association of two stimuli to illicit a behavioural response • Habituation - decrease in level of response through repeated exposure to a stimulus • Sensitisation - increase in level of response through repeated exposure to a stimulus

Clssical Conditioning

• Previously neutral stimulus (conditioned stimulus - CS) is paired with a positive (or negative) stimulus (unconditioned stimulus -US) that produces a response (unconditioned response UR) • In subsequent trials the CS can illicit an conditioned response which is very similar to the UR in the absence of the US

rats and episodic memory in context

• Rats (like the rest of us) like new things - they explore novel objects • Memory for object (what) • They also like familiar objects in new places • Memory for object in place (what and where) • They also like familiar objects in new contexts / environments • Memory for object in context (what and which) • They also like familiar objects in new places within familiar contexts • Memory for what, where and which (episodic like memory) Rats with hippocampus lesions cannot remember the combination of what, where and which - deficit in episodic-like memory