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Lecture 2

Long Term Memory: Lecture 2 Summary - The Search for the engram

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Lecture 2: The Search for the Engram Donald Hebb  Radical behaviourism = No thoughts, ideas or emotions. o Stimulus -> Conditioned response o No thought whatsoever associated with the brain.  Hebb rejected radical behaviourism. His hypothesis was a PLAUSIBLE NEURAL MECHANISM that makes ideas and memories possible. o Multiple presynaptic neurons must fire to reach the threshold to cause postsynaptic neuron to fire o Correlated signal – Some presynaptic neurons are almost always associated with the activation of the post synaptic neuron. So for eg. For a set of 3 presynaptic neurons: Each time the postsynaptic neuron fires, the middle presynaptic neuron always fires as well, so the firing of the middle presynaptic neuron is associated with the postsynaptic neuron.  Hebb’s learning Rule: If a presynaptic neuron repeatedly participates in causing a postsynaptic neuron to fire, some growth or process occurs to make it easier for the presynaptic neuron to cause the postsynaptic neuron to fire in the future. “NEURONS THAT FIRE TOGETHER, WIRE TOGETHER.”  Hebb’s original idea: o Cell assembly = Set of neurons that have learned to fire as a unit because of Hebb’s learning rule. This assembly can recreate the firing pattern caused by a stimulus even when the stimulus is not present. Neural basis for ideas and memories.  Long-term potentiation - 1973 o The more a neuron is caused to fire, the more the signal will be amplified over time (the connections gets stronger) eg. More receptors embedded on postsynaptic membrane to allow more signals/NT to be received. o If cells repeatedly fire together, connection is made stronger. Reconciling Lashley vs. Hebb’s ideas:  Cell assembles may be very large and redundant  So the memory production occurs all over the brain, just like Lashley proposed, but are still localised within the cell assemblies.  The cell assemblies may be largely spread out and complex, redundant  Localised yet distributed at the same time  At the cortical level, the memories are stored over large regions distributed all over the brain, while at the cellular level, specific localised molecular changes occur. Patient H.M. (Henry Molaison)  Proved that damage to specific areas DID in fact result in memory loss  Suffered epilepsy, couldn’t work, had several seizures per day – drugs didn’t help  Underwent bilateral (BOTH SIDES) removal of medial temporal lobe in 1953.  Severity of seizures reduced, totally alert with normal intelligence and emotion.  RESULTED in profound amnesia  Can’t find his way home, remember significant details eg. Death of loved ones Two Types of Amnesia  Caused by a discrete event  Anterograde amnesia  can’t form any new memories since the amnesia causing event  Retrograde amnesia  inability to remember what happened in the past before the amnesia causing event. o Very dense immediately after the event, but the older the memories were, the more he could remember them. o Retrograde amnesia is graded Older memories are more resistant to this amnesia. Oldest memories are less susceptible to amnesia.  H.M suffered from severe anterograde amnesia and moderate retrograde amnesia (to a lesser extent). The Medial Temporal Lobes  Hippocampus
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