learning and memory lecture.ppt

Biology of Learning and
Memory

Learning, Memory, Amnesia, and Brain
Functioning
• An early influential idea was that physical
changes occur when something new is
learned or a memory is formed.
• Explanations was that connections grew
between areas of the brain.
– Led to the search for localized
representations of memory

Functioning
• Ivan Pavlov pioneered classical conditioning
in which pairing of two stimuli changes the
response to one of them.
– A conditioned stimulus (CS) is paired with
an unconditioned stimulus (UCS) which
automatically results in an unconditioned
response (UCR).
• After several pairings, response can be
elicited by the CS without the UCS,
– New response is now called a conditioned
response (CR).

Functioning
• In operant conditioning, responses are
followed by reinforcement or punishment that
either strengthen or weaken the behavior.
– Reinforcers are events that increase the
probability that the response will occur
again.
– Punishment are events that decrease the
probability that the response will occur
again.

Functioning
• Pavlov believed that conditioning
strengthened connections between the CS
center and UCS center in the brain.
• Karl Lashley set out to prove this by
searching for such engrams, or physical
representations of what had been learned.
– Believed that a knife cut should abolish the
newly learned response.

Functioning
• Lashley’s studies attempted to see if
disrupting certain connections between
cortical brain areas would disrupt abilities to
learn associations.
• Found that learning and memory did not
depend entirely on connections across the
cortex
• Also found that learning did not depend on a
single area of the cortex.

Functioning
• Lashley proposed two key principles about
the nervous system:
• Equipotentiality – all parts of the cortex
contribute equally to complex functioning
behaviors (e.g. learning)
• Mass action – the cortex works as a
whole, not as solitary isolated units.

Functioning
• Richard F. Thompson et. al. suggested that
the engram for classical conditioning is
located in the cerebellum, not the cortex.
• During conditioning, changes occur in the
lateral interpositus nucleus (LIP) of the
cerebellum
– Responses increase as learning proceeds
– necessary for learning and retention
• However, a change in a brain area does not
necessarily mean that learning took place in
that area.

Functioning
• Suppression of activity in the LIP led to a
condition in which the subject displayed no
previous learning.
• As suppression wore off, the animal began to
learn at the same speed as animals that had
no previous training.
• But suppression of the red nucleus also led to
a similar condition.
• Later assumed that the learning did occur in
the LIP, as it was the last structure that
needed to be awake for learning to occur.

Functioning
• Psychologist differentiate between learning
and memory.
• Hebb (1949) differentiated between two types
of memory:
• Short-term memory – memory of events
that have just occurred.
• Long-term memory – memory of events
from times further back.

Functioning
• Differences between STM and LTM
– Short-term memory has a limited capacity;
long-term memory does not.
– Short-term memory fades quickly without
rehearsal; long-term memories persist.
– Memories from long-term memory can be
stimulated with a cue/ hint; retrieval of
memories lost from STM do not benefit
from the presence of a cue.

Functioning
• Researchers propose all information enters
STM where the brain consolidates it into LTM.
• Later research has weakened the distinction
between STM and LTM
• Working Memory
– Proposed by Baddeley & Hitch as an
alternative to short-term memory.
– Emphasis on temporary storage of
information to actively attend to it and work
on it for a period of time.

Functioning
• Common test of working memory is the
delayed response task.
– Requires responding to something you
heard or saw a short while ago.
• Research points to the prefrontal cortex for
the storage of this information
• Brain may use elevated levels of calcium to
potentiate later responses

Functioning
• Older people often have impairments in
working memory.
• Changes in the prefrontal cortex assumed to
be the cause.
• Declining activity of the prefrontal cortex in
the elderly is associated with decreasing
memory.
• Increased activity is indicative of
compensation for other regions in the brain.

Functioning
• Amnesia is the loss of memory.
• Studies on amnesia help to clarify the
distinctions between and among different
kinds of memories and their mechanisms.
• Different areas of the hippocampus are active
during memory formation and retrieval.
– Damage results in amnesia.

Functioning
• H.M. is a famous case study in psychology
who had his hippocampus removed to
prevent epileptic seizures.
• Afterwards H.M. had great difficulty forming
new long-term memories.
• STM or working memory remained intact.
• Suggested that the hippocampus is vital for
the formation of new long-term memories.

Functioning
• H.M. showed massive anterograde
amnesia after the surgery.
• Two major types of amnesia include:
• Anterograde amnesia – the loss of the
ability to form new memory after the
brain damage
• Retrograde amnesia – the loss of
memory events prior to the occurrence
of the brain damage.

Functioning
• Patient HM also displayed greater “implicit”
than “explicit” memory.
• Explicit memory – deliberate recall of
information that one recognizes as a
memory.
• Implicit memory – the influence of recent
experience on behavior without realizing
one is using memory.

Functioning
• H.M. had difficulty with episodic memory and
declarative memory.
– Episodic memory: ability to recall single
events.
– Declarative memory: ability to state a
memory into words.
• H.M.’s procedural memory remained intact.
– Procedural memory: ability to develop
motor skills (remembering or learning how
to do things).

Functioning
• Research of the function of the
hippocampus suggests the following:
1. The hippocampus is critical for
declarative memory functioning
(especially episodic).
2. The hippocampus is especially important
for spatial memory.
3. The hippocampus is especially important
for configural learning and binding.

Functioning
• Research in the role of the hippocampus in
episodic memory shows damage impairs
abilities on two types of tasks:
• Delayed matching-to-sample tasks – a
subject sees an object and must later
choose the object that matches.
• Delayed non-matching-to-sample tasks–
subject sees an object and must later
choose the object that is different than
the sample.

Functioning
• Damage to the hippocampus also impairs
abilities on spatial tasks such as:
• Radial mazes – a subject must navigate
a maze that has eight or more arms with
a reinforcer at the end.
• Morris water maze task – a rat must
swim through murky water to find a rest
platform just underneath the surface.

Functioning
• Hippocampus may also be important for
contextual learning
• Remembering the detail and context of an
event
• suggests that the hippocampus is important
in the process of “consolidation”.
• Damage to the hippocampus impairs recent
learning more than older learning.
– The more consolidated a memory
becomes, the less it depends on the
hippocampus.

Functioning
• Reverberating circuits of neuronal activity
were thought to be the mechanisms of
consolidation.
• Consolidation is also influenced by the
passage of time and emotions.
– Small to moderate amounts of cortisol
activate the amygdala and hippocampus
where they enhance storage and
consolidation of recent experiences.
– Prolonged stress impairs memory.

Functioning
• Different kinds of brain damage result in
different types of amnesia.
• Two common types of brain damage
include:
1. Korsakoff’s syndrome
2. Alzheimer’s disease

Functioning
• Korsakoff’s syndrome – brain damage
caused by prolonged thiamine (vitamin B1)
deficiency
• impedes the ability of the brain to
metabolize glucose.
• Leads to a loss of or shrinkage of neurons
in the brain.
• Often due to chronic alcoholism.
• Symptoms include apathy, confusion, and
forgetting and confabulation (taking
guesses to fill in gaps in memory).

Functioning
• Alzheimer’s disease is associated with a
gradually progressive loss of memory often
occurring in old age.
• Affects 50% of people over 85.
• Early onset seems to be influenced by
genes, but 99% of cases are late onset.
• About half of all patients with late onset
have no known relative with the disease.

Functioning
• Alzheimer’s disease is associated with an
accumulation and clumping of the following
brain proteins:
• Amyloid beta protein 42 which produces
widespread atrophy of the cerebral
cortex, hippocampus and other areas.
• An abnormal form of the tau protein, part
of the intracellular support system of
neurons.

Functioning
• Accumulation of the tau protein results in:
– Plaques – structures formed from
degenerating neurons.
– Tangles – structures formed from
degenerating structures within a neuronal
body.

Functioning
• A major area of damage is the basal forebrain
and treatment includes enhancing
acetylcholine activity.
• One experimental treatment includes the
stimulation of cannabinoid receptors that
limits overstimulation by glutamate.
• Research with mice suggests the possibility
of immunizing against Alzheimer’s by
stimulating the production of antibodies
against amyloid beta protein.

Functioning
• Lessons from studying amnesiac patients
include:
– There can be deficiencies of very different
aspects of memory.
– There are independent kinds of memory.
– Various kinds of memory depend on
different brain areas.

Functioning
• Other subcortical brain areas and the cortex
important in learning:
• Amygdala associated with fear learning
• Parietal lobe associated with piecing
information together
• anterior and inferior region of the temporal
lobe and semantic memory
– semantic dementia (loss of semantic
memory)

Functioning
• Other areas of the cortex important in
learning (con’t):
• Prefrontal cortex and learning about rewards
and punishments
– Basal ganglia, anterior cingulate cortex
also involved

Storing Information in the Nervous
System
• Activity in the brain results in physical
changes.
• Patterns of activity leave a path of physical
changes.
• Not every change is a specific memory as
was once originally believed.
• Many ideas originally believed to be true have
been refined.

System
• A Hebbian synapse occurs when the
successful stimulation of a cell by an axon
leads to the enhanced ability to stimulate that
cell in the future.
– Increases in effectiveness occur because
of simultaneous activity in the presynaptic
and postsynaptic neurons.
– Such synapses may be critical for many
kinds of associative learning.

Hebb. Learn.
Hebbian Learning
•Hebbian Learning is a learning rule which is the
oldest and most famous of all learning rules. It was
postulated by Donald Hebb (1949) in his book (The
Organisation of Behaviour):
“ When an axon of cell A is near enough to excite a cell B
and repeatedly or persistently takes part in firing it, some
growth process or metabolic changes take place in one or
both cells such as A’s efficiency as one of the cells firing
B, is increased”
•Hebb proposed this change as a basis of
associative learning. We may expand this as a two-
part rule:

Hebb. Learn.
Hebbian Learning-1
•If two neurons on either side of a synapse
(connection) are activated simultaneously then the
strength of that synapse is selectively increased.
•If two neurons on either side of a synapse are
activated asynchronously, then that synapse is
selectively weakened or eliminated.
•Such a synapse is called a Hebbian synapse.
More precisely, we define a Hebbian synapse as a
synapse that uses a time-dependent, highly local,
and strongly interactive mechanism to increase
synaptic efficiency as a function of the correlation
between the pre-synaptic and post-synaptic
activities.

Hebb. Learn.
Hebbian Learning-2
• We analyse the four key mechanisms mentioned
above:
1. Time dependent mechanism: This mechanism
refers to the fact that the modifications in the
synapse depend on the exact time of occurrence
of the pre-synaptic and post-synaptic signals;
2. Local mechanism: By its very nature, a synapse is
the transmission site where information-bearing
signals are in spationtemporal contiguity. This
locally available information is used by the
synapse to produce a local modification that is
input specific;
3. Interactive Mechanism: The occurrence of a
change in a synapse depends on signals on both

Hebb. Learn.
Hebbian Learning-3
sides of the synapse. That is, a Hebbian form of
learning depends on a “true interaction” between
the pre- and post-synaptic signals in the sense
that we cannot make any prediction from either
one of these two activities by itself. The
interaction may be deterministic of stochastic;
4. Correlational mechanism: The condition for a
change in synaptic efficiency is the co-occurrence
of pre- and post-synaptic signals. The correlation
over time between the two signals is responsible
for the synaptic change.

Hebb. Learn.
Hebbian Learning-4
• We may classify synaptic modifications of a
synapse as:
• Hebbian: which is a synapse that increases its
strength when positively correlated pre- and post-
synaptic signals are present and decreases its
strength when these signals are either
uncorrelated or negatively correlated;
• Anti-Hebbian: Such a synapse weakens positively
correlated pre- and post-synaptic signals and
strengthens negatively correlated signals;
• Non-Hebbian: It does not involve, in the
modification of a synapse, any mechanism that is
time dependent, highly local and strongly
interactive in nature (as in the previous cases).

Hebb (1949)
“When an axon of cell A is near enough to
excite a cell B and repeatedly or persistently
takes part in firing it, some growth process or
metabolic change takes place in one or both
cells such that A’s efficiency, as one of the
cells firing B, is increased”
From: The organization of behavior.

Hebb’s rule
• Each time that a particular synaptic
connection is active, see if the receiving cell
also becomes active. If so, the connection
contributed to the success (firing) of the
receiving cell and should be strengthened. If
the receiving cell was not active in this time
period, our synapse did not contribute to the
success the trend and should be weakened.

• Hebb’s Rule:
neurons that fire together wire together
• Long Term Potentiation (LTP) is the biological
basis of Hebb’s Rule
• Calcium channels are the key mechanism
LTP and Hebb’s Rule
strengthen
weaken

Chemical realization of Hebb’s
rule
• It turns out that there are elegant chemical processes
that realize Hebbian learning at two distinct time scales
– Early Long Term Potentiation (LTP)
– Late LTP
• These provide the temporal and structural bridge from
short term electrical activity, through intermediate
memory, to long term structural changes.

Calcium Channels Facilitate
Learning
• In addition to the synaptic channels
responsible for neural signaling, there are
also Calcium-based channels that facilitate
learning.
– As Hebb suggested, when a receiving neuron
fires, chemical changes take place at each
synapse that was active shortly before the event.

Long Term Potentiation (LTP)
• These changes make each of the winning synapses
more potent for an intermediate period, lasting from
hours to days (LTP).
• In addition, repetition of a pattern of successful firing
triggers additional chemical changes that lead, in
time, to an increase in the number of receptor
channels associated with successful synapses - the
requisite structural change for long term memory.
– There are also related processes for weakening synapses
and also for strengthening pairs of synapses that are active
at about the same time.

The Hebb rule is found with long
term potentiation (LTP) in the
hippocampus
1 sec. stimuli
At 100 hz
Schafer collateral pathway
Pyramidal cells

System
• Studies of how physiology relates to learning
often focus on invertebrates and try to
generalize to vertebrates.
• The aplysia is a slug-like invertebrate that is
often studied due to its large neurons.
• This allows researchers to study basic
processes such as:
– Habituation
– Sensitization

System
• Habituation is a decrease in response to a
stimulus that is presented repeatedly and
accompanied by no change in other stimuli.
– Depends upon a change in the synapse
between the sensory neurons and the
motor neurons.
– Sensory neurons fail to excite motor
neurons as they did previously.

System
• Sensitization is an increase in response to a
mild stimulus as a result to previous exposure
to more intense stimuli.
• Changes at identified synapses include:
– Serotonin released from a facilitating
neuron blocks potassium channels in the
presynaptic neuron.
– Prolonged release of transmitter from that
neuron results in prolonged sensitization.

System
• Long-term Potentiation (LTP) occurs when
one or more axons bombard a dendrite with
stimulation.
– Leaves the synapse “potentiated” for a
period of time and the neuron is more
responsive

System
• Properties of LTP that suggest it as a cellular
basis of learning and memory include:
– Specificity
– Cooperativity
– Associativity

System
– Specificity – only synapses onto a cell that
have been highly active become
strengthened.
– Cooperativity – simultaneous stimulation
by two or more axons produces LTP much
more strongly than does repeated
stimulation by a single axon.
– Associativity – pairing a weak input with a
strong input enhances later responses to a
weak input.

System
• Long-term depression (LTD) is a prolonged
decrease in response at a synapse that
occurs when axons have been active at a low
frequency.
– The opposite of LTP

System
• Biochemical mechanisms of LTP are known
to depend on changes at glutamate and
GABA primarily in the postsynaptic neuron
• This occurs at several types of receptor sites
including the ionotropic receptors:
–AMPA receptors
–NMDA receptors

System
• LTP in hippocampal neurons occurs as
follows:
– Repeated glutamate excitation of AMPA
receptors depolarizes the membrane.
– The depolarization removes magnesium
ions that had been blocking NMDA
receptors.
– Glutamate is then able to excite the NMDA
receptors, opening a channel for calcium
ions to enter the neuron.

System
– Entry of calcium through the NMDA
channel triggers further changes.
– Activation of a protein that sets in motion a
series of events occurs.
– More AMPA receptors are built and
dendritic branching is increased.
• These changes increase the later
responsiveness of the dendrite to incoming
glutamate.

System
• Changes in presynaptic neuron can also
cause LTP.
• Extensive stimulation of a postsynaptic cell
causes the release of a retrograde transmitter
that travels back to the presynaptic cell to
cause the following changes:
– Decrease in action potential threshold
– Increase neurotransmitter release of
– Expansion of the axons.
– Transmitter release from additional sites.

System
• LTP reflects increased activity by the
presynaptic neuron and increased
responsiveness by the postsynaptic neuron
• Understanding the mechanisms of changes
that enhance or impair LTP may lead to drugs
that improve or block memory.
– Increasing production of hormones
increased by LTP
– Ginkgo biloba
– Etc.

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