Lecture 2: The Search for the Engram
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
The Medial Temporal Lobes