SD206 Tutorial 6 - Revision notes on learning & memory (Books 1, 2 & 4)

Amnesia

Forms of learning (Book 1)

Forms of memory (Bk 2, Ch 11)

Biological Basis of Memory (Book 4, Ch 5)

STM normally 'decays' within a few seconds (§5.3.1)

Studies of passive avoidance learning in chicks (§5.3.2) indicate that consolidation of STM into a more permanent form (resistant to electroconvulsive shock) occurs in about 10 mins. In rats, consolidation occurs in around 15 mins. (This of course assumes that the ECT did not disrupt memory recall, rather than the memory storage)

Conversion of STM into LTM is believed to involve changes in the brain (LTM can last for years). These are likely to involve changes in the synapses between neurons.
 

But what is memory?

Hebb proposed that increased activity in a neural circuit leads to increased 'efficiency' of the circuit.
This 'strengthening' of synapses could provide the neural basis of memory.

TWO CELLULAR MECHANISMS are hypothesized for associative changes in synaptic strength during learning. The pre-post coincidence mechanism proposed by Donald O. Hebb in 1949, posits that coincident activity in the presynaptic and postsynaptic neurons is critical for strengthening the connections between them The pre-modulatory coincidence mechanism proposed in 1963, based on studies in Aplysia, holds that the connection can be strengthened without activity of the postsynaptic cell when a third neuron, the modulatorv neuron, is active at the same time as the presynaptic neuron. Stripes denote neurons in which coincident activity must occur to produce the associative change.
 

Structural changes in memory (§5.5)
Evidence for specific synaptic changes in memory is provided by studies of chicks after passive avoidance learning (PAV). Following PAV, changes can be seen in specific brain areas, the intermediate hyperstriatum ventrale (IMHV) and lobus parolfactorius (LPO).
The changes included increased numbers of synapses, increased density (numbers of) dendritic spines, increased length of the synaptic apposition zone (the region of synaptic contact) and increased numbers of synaptic vesicles. These changes are specific to the PAV learning. and not due to some other, unrelated events.
 

Biochemical approaches to memory (§5.6)
The structural changes seen after learning must also have a biochemical basis. Studies may be:
correlative: after training, associated biochemical changes are sought.
interventive: attempts are made to block memory formation by specific drugs. The commonest approach is to block synthesis of the proteins involved in the altered synaptic structure. Inhibitors of protein synthesis, e.g. puromycin or cycloheximide, have been shown to disrupt memory formation.

Protein synthesis requires the activation (expression) of certain genes (§5.8). The particular genes involved here are immediate early genes. Immediate early genes are activated in chicks within 30 mins of training on a passive avoidance task.
 
 

Animal models and techniques for studying learning (§5.7)
A number of specific learning processes in particular animals have been studied:-

a. Habituation in Aplysia. Specific changes are evident in the synapses involved in habituation of the gill withdrawal reflex.

b. Mutualistic insects, which lack the genes ( and biochemical processes) necessary for learning.

c. Long Term Potentiation (LTP) is a phenomenon observed in the hippocampus. Single inputs produce a response in hippocampal neurons. If a burst of high frequency inputs is given, the response to a single input is increased (potentiated). This increase lasts for hours, and sometimes even longer. This is long term potentiation.
LTP involves specific changes in hippocampal neuronal synapses.
The hippocampus is involved in specific memories, such as spatial learning (Book 1.8). One way to demonstrate spatial learning in rats, is using the Morris tank. Here, rats quickly learn to find a platform submerged in a tank of murky water.
A drug AP5, prevents the occurrence of LTP. This drug blocks a particular receptor for the transmitter glutamate - the NMDA receptor.
AP5 impairs the ability of rats to find the submerged platform in the Morris tank. This impairment is dose-related to the amount of AP5 given. If LTP is studied in rats that were given AP5 for the Morris tank experiment, the extent of LTP produced is affected by the amount of AP5 given. The higher the dose of AP5, the less the amount of LTP produced. Thus, AP5 interferes with spatial learning and also with the induction of LTP. The interpretation is that the same mechanism is involved in spatial learning and LTP
There is now some understanding of the mechanisms involved in LTP, and these seem to be very similar to the mode of action suggested by Hebb.

d. Passive avoidance learning. Increased neural activity requires energy in the form of glucose. Glucose uptake in the brain can be studied using 2-deoxyglucose (a modified form of glucose that cells can't use, so it accumulates in cells trying to use glucose). If the 2-DG is radioactively labelled. its presence in cells can be demonstrated by autoradiography. During passive avoidance learning in chicks, there is an increase in glucose uptake in particular parts of the brain: the IMHV and LPO (see above). This is consistent with the structural studies mentioned above. It appears that the IMHV is involved in acquiring and storing the memory of passive avoidance learning, whilst the LPO is involved in storing the memory formed in IMHV.
 

Location of memory (§5.10)

Studies on chicks reveal the role of IMHV and LPO in passive avoidance memory.

Where are specific memories stored?

Wilder Penfield:     localised memories  (evidence from focal stimulation studies)

Karl Lashley:         diffuse memory location (maze running & cortical ablation studies)

Leading to the notion that memories could be 'stored' in several parallel circuits, that can be activated in different ways, and so there is an element of redundancy ('belt and braces').
 

These are not necessarily contradictory: each was studying different things in different ways.

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