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Learning and Memory
Learning and Memory
Learning
relatively permanent change in behavior
as a function of training, practice or
experience
excludes behavioral changes resulting
from sensory adaptation or fatigue
Learning and Memory
memory is implicit in the definition of
learning
“relatively permanent” implies a lasting
alteration of nervous system structure and/or
function
Two aspects of learning
1. acquisition of a response in presence of a
stimulus
2. suppression of responses in its absence
Two fundamental questions
1. What are the neural mechanisms of
learning? (cellular level)
2. What is learned? (where in the brain;
organizational level)
To answer the first question,
simplified systems have been
studied.
• Simple forms of learning
• Simple systems
e.g. invertebrates, simplified mammalian
preparations
Types of Learning
• Non-associative
– Habituation, sensitization
– perceptual in nature
– recognition of objects and situations
• Associative
– Classical conditioning
• S-S learning
– Operant Conditioning
• R-S learning
Associative learning may
involve
• acquisition of new motor behaviors
• acquisition of associations between new
stimuli and existing behaviors
• acquisition of new associations between
familiar stimuli and existing responses.
Non-associative learning
habituation
•
•
repeated exposure to a stimulus results in
reduced responding to that stimulus
generally neutral, non-noxious stimuli
sensitization
• repeated exposure to a stimulus results in
increased responding to that stimulus
• generally biologically relevant, strong hedonic
valence (+ or -)
• also refers to augmentation of responding
following exposure to a second stimulus
Associative learning
classical conditioning; Pavlovian
conditioning; respondent conditioning
•A neutral stimulus is paired with a stimulus that
reliably elicits a response. Conditioning is indicated
when the previously neutral stimulus elicits a
response.
CSUSUR
CR
US
UR
CR
CS
Associative learning
operant conditioning; instrumental
learning
•repeated presentation of a stimulus after emission of
a designated response increases (reinforces) or
decreases (punishes) the likelihood of that response
CRUSUR
Modeling, latent learning
Usually defined as learning in the
absence of a reinforcer (US or
conditioned reinforcer such as money)
Problem: One can always posit a “hidden reinforcer,”
a negative hypothesis that cannot be disproven. The
burden of proof therefore falls on those to make
hidden reinforcers evident.
The existence of mirror neurons seems to provide a
neurobiological basis for imitative learning.
Research on neural mechanisms has focused on
non-associative learning and classical conditioning.
Eric Kandel and his
collaborators used Aplysia
to unravel synaptic
mechanisms for short- and
long-term habituation,
short- and long-term
sensitization, and classical
conditioning.
Aplysia
Eric Kandel won the 2000 Nobel Prize for Physiology and
Medicine for this work.
Habituation in Aplysia
Sensitization in Aplysia
Kandel termed the
neural mechanisms
that underlie behavioral
sensitization,
“heterosynaptic
facilitation,” which is
produced by a second
(usually strong)
stimulus
heterosynaptic
facilitation
5-HT released onto
presynaptic terminal
(metabotropic synapse)
c-AMP mediated closure
of K+ channels
Prolongation of
depolarization at the terminal
greater Ca2+ influx
more neurotransmitter released onto target neuron
Memory in Aplysia also involves structural
changes in the presynaptic terminal
group
active zones active zone
area
vesicles/
active zone
vesicles/
neuron
naïve
100%
.16 um
13
2250
habituated
28
.11
4
500
sensitized
183
.28
20
6300
LTP in the hippocampus:
A mammalian model for learning
Hippocampus most studied because of it's organization-• complete circuitry represented in thin slices (100-400um thick)
• can be placed in a dish for in vitro electrophysiological experiments
• also thought to be important for memory consolidation in vivo
20
Copyright © 2004 Allyn and Bacon
LTP in the hippocampus:
A mammalian model for learning
typical LTP experiment
1. stimulate neuron A, record PSP from neuron B
2. stimulate neuron A tetanically (e.g. burst of stimuli @ 100 Hz)
3. record PSP from B w/test pulses at varying intervals
4. PSP augmented for several days or even up to months
5. this augmentation is what is called LTP
LTP in the hippocampus:
A mammalian model for learning
CaMKII: Calcium/calmodulin
dependent kinase II
PKA, PKC: Protein kinase A, C
CREB: cAMP-responsive
element-binding protein
Low-frequency stimulation
results in small increases in
[Ca2+] in the postsynaptic cell,
which in turn results in fewer
AMPA channels opening in
response to glutamate. This is
called low-frequency
depression and is a
mechanism for weakening
synaptic strength.
http://www.sumanasinc.com/webcontent/anisamples/neurobiology/receptors.html
Organizational Aspects of
Learning and Memory
Three gross stages of learning and memory
Acquisition
Storage
Retrieval
Organizational Aspects of
Learning and Memory
There are also qualitatively different kinds
of learning
Declarative
episodic
semantic
Procedural or non-declarative
skill learning
priming
conditioning
Organizational Aspects of
Learning and Memory
Memory can be categorized according to
its duration or persistence
Sensory traces
e.g. iconic (visual) and echoic (auditory)
at most a few seconds in duration
Organizational Aspects of
Learning and Memory
Memory can be categorized according to
its duration or persistence
Sensory traces
Short-term memory (STM) or working
memory
from LTM
information held in “consciousness”
phonological loop (sub-vocal rehearsal)
visual imagery
episodic buffer
can be initiated by current event or by recall
controlled by “central executive”
Organizational Aspects of
Learning and Memory
Memory can be categorized according to
its duration or persistence
Sensory traces
Short-term memory (STM) or working
memory
Intermediate-term memory
distinguishable from STM and LTM?
up to a few days in duration
Organizational Aspects of
Learning and Memory
Memory can be categorized according to
its duration or persistence
Sensory traces
Short-term memory (STM) or working
memory
Intermediate term memory
Long-term memory
indefinite duration, up to days
Organizational Aspects of
Learning and Memory
Memory can be characterized by its
underlying physical bases
Sensory traces
persistent activity in sensory pathways
Organizational Aspects of
Learning and Memory
Memory can be characterized by its
underlying physical bases
Sensory traces
Short-term memory (STM) or working
memory
reverberating circuits
electrical basis
limited capacity
can be disrupted by intrusion of other activity
displays both primacy and recency effects
Organizational Aspects of
Learning and Memory
Memory can be characterized by its
underlying physical bases
Sensory traces
Short-term memory (STM) or working
memory
Intermediate term memory
biochemical basis?
accessible by working memory
eventually irretrievable (storage or retrieval
failure?)
Organizational Aspects of
Learning and Memory
Memory can be characterized by its
underlying physical bases
Sensory traces
Short-term memory (STM) or working
memory
Intermediate term memory
Long-term memory
structural or anatomical basis
protein synthesis essential
unlimited(?) capacity
Organizational Aspects of
Learning and Memory
Memory can be characterized by its
underlying physical bases
Sensory traces
Short-term memory (STM) or working
memory
Intermediate term memory
Long-term memory
The case of Henry Molaison
(H.M.)
H.M. suffered from epilepsy, with epileptic foci in
both temporal lobes. It did not respond to drugs
available at the time and was life-threatening.
Therefore, it was decided to remove the tissue in
which the epileptiform activity originated.
H.M.’s surgery removed the amygdala, the
hippocampus, and some of temporal lobe cortex.
Patient H.M.
EC: Entorhinal cortex
H: Hippocampus
Cer: Cerebellum
PH: Parahippocampal
cortex
Following surgery, H.M.’s memory of events prior
to the surgery was intact (no retrograde amnesia).
But, he did display anterograde amnesia, the
inability to form declarative memories after the
surgery.
H.M.’s memory deficit was confined to verbal
tasks, with motor learning (non-declarative
memory) unaffected.
However, H.M. DID display anterograde amnesia
for spatial tasks, e.g. navigating new
environments
There Are Several Kinds of
Memory and Learning
Studies in animals and other humans with brain
damage showed that the H.M.s deficits were unlikely
due solely to hippocampal damage. Other temporal
lobe structures and the thalamus form a larger
functional circuit necessary for consolidation of
declarative memories.
Damage to other areas can also cause memory
loss.
Patient N.A. has amnesia due to accidental
damage to the dorsomedial thalamus, which
is part of the same memory circuit affected
by H.M.’s surgery.
Like Henry Molaison, he has short-term
memory but cannot form declarative longterm memories.
Patient N.A.
damage of the dorsomedial thalamus
damage to floor of third ventricle, mammillary bodies
Korsakoff’s Syndrome
Korsakoff’s syndrome is a memory deficiency caused
by lack of thiamine (vitamin B6)–often seen in
chronic alcoholism.
Brain damage occurs in mammillary bodies and
basal frontal lobes, which produces anterograde
amnesia.
Patients often confabulate–fill in a gap in memory
with a false memory that they fully believe to be true.
Organizational Aspects of
Learning and Memory
Three gross stages of learning and memory
Acquisition
Storage
Retrieval
STM is susceptible to disruption prior to
consolidation
• electroconvulsive shock (or ECT in clinical settings)
can disrupt long-term memory by pulling those memories
back into working memory at the time of ECT, which prevents re-consolidation
• head trauma generally results in retrograde amnesia, whose magnitude is
inversely related to the age of the memory, by disturbing ongoing electrical activity
• traumatic events may also induce amnesia similar to head trauma or ECS
e.g. placing rats in ice water immediately after training trial results in forgetting,
or amnesia
• may be a model for the loss of memory following psychological trauma (e.g. rape)
in humans
LTM can be prevented by interfering with
protein synthesis
Protein synthesis inhibitors block LTP and formation of memories in behavioral tasks.
The role of protein synthesis in response to environmental stimulation is very evident
in the phenomenon of environmental enrichment
“Enrichment” studies are really studies of
varying degrees of impoverishment. Rats
living outdoors in natural envronmental
enclosures exceed “enriched” rats on all
measures