Learning and the Brain
What happens in the brain
when we learn?
Learning and the Brain: Review
1.
Learning is often synonymous with memory in cognitive neuroscience and, in
biological terms, change (plasticity) in brain structure/connectivity/activity can
be called (in biological terms) learning - including decreases!
2.
All animals learn and useful learning can be achieved by quite small networks
when they can change the efficiency of their connections appropriately – e.g.
“Hebbian” synapses – respond to simultaneous pre/post excitation
3.
Biological connections between neurons are called synapses - chemical
synapses of particular interest in neurocognition.
4.
Early and late processes of long-term potentiation (LTP) provide a biological
basis for the Hebbian Synapse and, therefore, learning Working memory may be
due to persistent neuronal firing
5.
Memory (and working memory) are distributed in the brain
6.
Evolutionary psychology = another perspective on categorizing different types of
learning
7.
Neuroplasticity: When we learn, it is not just connectivity (and function) that
changes in the brain, but structure can change also.
Pavlov’s dogs – classical conditioning
of reflexive behaviors …….
"Pavlov's Dog". Nobelprize.org.
Nobel Media AB 2014. Web. 6 Oct 2014.
<http://www.nobelprize.org/educational/medicine/pavlov/index.html>
Demonstration
Animals can
often appear
to make
“reasonable”
decisions.
Some of these
may be based
on classical
conditioning….
© Nicolas P. Rougier / Wikimedia
Commons / CC-BY-SA 3.0
The case of “Clever Hans” the counting horse
By Karl Krall (Karl Krall,
Denkende Tiere, Leipzig 1912,
Tafel 2) [Public domain], via
Wikimedia Commons
But how about non-reflexive responses that require
anticipation of behavioural consequence?
Operant Conditioning = Instrumental conditioning
- response to reward/punishment - anticipating
behavioural consequence
- requires more of a cognitive construction
Also: How do rats know how to take the second best route
when the best is closed off?
How would neurons have to behave to produce such
behaviour i.e. make educated guesses in new situations from
a “cognitive map”?
Networks of neurons – with the “right”
connections – can produce complex
behaviour …………
•Neurons generate & mediate electrical signals in a
complex and interconnected manner.
Image © Paul Howard-Jones 2014
Artificial parallel networks have human-like learning curves
and can even make good guesses in novel situations. But
how do networks learn those “right” connections?
By en:User:Cburnett [GFDL
(http://www.gnu.org/copyleft/fdl.html) or CC-BYSA-3.0 (http://creativecommons.org/licenses/bysa/3.0/)], via Wikimedia Commons
The Synapse
-a neural connection
Neurotransmitter
Synaptic vessicle
By Synapse_Illustration2_tweaked.svg:
Nrets derivative work: Looie496
(Synapse_Illustration2_tweaked.svg) [CCBY-SA-3.0
(http://creativecommons.org/licenses/bysa/3.0/) or GFDL
(http://www.gnu.org/copyleft/fdl.html)],
via Wikimedia Commons
Neurotransmitter
transporter
Voltage gated
Ca++ channel
receptor
Presynaptic
terminal
Synaptic
cleft
Dendrite
1. Action potential reaches the synapse.
2. Depolarizes synapse membrane -> channels open and calcium ions flow through.
3. Activation of calcium-sensitive proteins attached to vesicles (containing neurotransmitter)
4. Proteins change shape, vesicles fuse with presynaptic membrane and open
5. Some neurotransmitter escapes, some binds to receptor molecules in postsynaptic membrane.
6. Receptor molecule activates postsynaptic cell in some way (e.g. exciting, inhibiting)
7. Neurotransmitter molecules loosen and drift off
8. Neurotransmitter either reabsorbed in presynaptic cell or broken down.
For the “right” connections:
“neurons that fire together wire together” (anon.)
Donald Hebb (1949):
* When an action potential in the presynaptic neuron reaches
the synapse, it causes release of neurotransmitter
* This influences the postsynaptic neuron – and that may fire
also
* If the postsynaptic neuron was firing when the presynaptic
neuron was releasing neurotransmitter, then the synaptic
efficiency would be increased.
NB synaptic efficiency = likelihood of being stimulated into
further firing = strength of connection
Artificial neural networks with this
mechanism build impressive
learning networks:
•human-like learning curves
•human-like errors
•pattern recognition (inc.
language)
•best guesses and cognitive maps
•graceful degradation
Parallel distributed learning (PDP)
•etc
By en:User:Cburnett [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0
(http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
(Artificial) Neural Networks
But we still need a biological mechanism by which neurons
change their synaptic efficiency when adjacent neurons
increase their activity - i.e. we need a “Hebbian” synapse.
Long-Term Potentiation (LTP)
– an enduring increase in the size of the post-synaptic
potential.
Bliss and Lomo (1973) in Hippocampus (visuospatial
memory) of rabbits – but now known to be common
throughout the brain.
Lomo (2003): “As for me, it was certainly not prior interest in
possible memory mechanisms that led me to a discovery that was in
some ways accidental and in other ways the result of an intuition
that has often, I feel, brought me to look for or see phenomena that
turn out to be new and interesting.”
EPSP (Excitatory PostPynaptic Potential measured at REC) is low until electrical
stimulation (tetanus at STIM).
Green square shows EPSP immediately high (PTP – post-tetanic potentiation).
The blue/grey squares show elevated EPSP measurements 3 to 60 minutes later (LTP
or long-term potentiation).
By Synaptidude at English Wikipedia [GFDL (www.gnu.org/copyleft/fdl.html) or CC-BY-SA3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
LTP type is classified
according to pre- and
postsynaptic activity
required:
Hebbian LTP requires
simultaneous pre- and
postsynaptic depolarization (“fire
together, wire together”)
Non-Hebbian LTP does not
require such simultaneous
depolarization.
LTP depends on a postsynaptic glutamate receptor: NMDA
Normally: EPSP (excitatory postsynaptic potential) mediated by AMPA receptors
(another glutamate receptor). NMDA receptors blocked by magnesium (Mg) ions.
But: when sufficient incoming glutamate generates enough EPSP, it depolarises the
postsynaptic membrane and allows Mg++ ions to be repelled from NMDA receptors
-> Na+ and Ca++ ions let through.
Coincidence detector (Pre + post synaptic activity -> EPSP up)
Happens if: * post-synaptic activity for depolarised membrane (gate open)
* AND neurotransmitter at synaptic cleft (something to go thro’ gate)
But….early LTP only lasts a few hrs/wks ….
Late LTP:
In the late phase of LTP, calcium enters the cell
and triggers Calcium modulated protein
(calmodulin) -> chain of events-> the nucleus of
the cell -> processes leading to protein synthesis
and structural changes, i.e., the formation of new
synapses and receptors
Long Term Potentiation = Learning?
* We know STM -> LTM transition needs protein synthesis to
maintain synaptic enhancement. Suppression of protein
synthesis using drugs suppresses learning.
* mice treated with NMDA receptor blockers are unable to
learn their way around a water maze.
* Using rats conditioned to freeze to a neutral sound, synaptic
potentials from the amygdala became enhanced only while
the learned response continued, and this enhancement
mediated by glutamate receptors & dependent upon NMDA
receptors.
demo
Other learning mechanisms...
•Also LTD (Depression) – e.g. reduction in firing rates (e.g.
in the perirhinal cortex - involved in face recognition) with
habituation.
•New neurons can be born – at least in the hippocampus –
we don’t know yet how neurogenesis relates to learning
So where is memory in the brain?
Psychologist attempted simple memory dichotomies: declarative/procedural,
explicit/implicit , memory/habit, semantic/episodic.
Biologically, declarative/non-declarative seem useful:
MEMORY
Declarative
Facts
Events
Non-declarative
Procedural Priming and
(skills and perceptual
learning
habits)
Simple
classical
conditioning
Emotional
responses
Medial Temporal Lobe Striatum
Neocortex
Amygdala
Nonassociative
learning
Skelatal
responses
Cerebellum
Reflex
pathways
We walked
across the
Clifton
Suspension
Bridge!
Location:
parietal
Working
memory
buffer
Shape:
Inferior
temporal
Colour:
Temporal
occipital
However we categorise types of memory (e.g. semantic, conceptual,
procedural, episodic, visual, spatial) a simple stimulus (e.g. a word) can cue
all/many of these at once.
“A memory” is distributed across many different brain regions – some more
associated with particular aspects of a memory than others
Working Memory ...
Persisting neuronal activity of a prefrontal cortical neuron:
Visual Fixation
cue
point
Correct Responses
cue
delay period
response
Visual Fixation
cue
point
Incorrect Responses
cue
delay period
response
Working memory also distributed,
but particularly associated with
dorsolateral prefrontal cortex
(DLPFC)
By Natalie M. Zahr, Ph.D., and Edith V. Sullivan, Ph.D.
[Public domain], via Wikimedia Commons
Evolutionary & Species perspectives
Our learning mechanisms have evolved – this provides
insights at a functional (backwards) design level – if we
think of evolution as a “designer”:
Issue
(prehistoric
environment)
Specification
(what’s needed
to survive)
Design (how
the mind &
brain work
Product
(mind &
brain)
Understanding the prehistoric issues and specification
helps us understand the design that underlies our
mind/brain?
Or…..was there a specification that would have justified the
design we think underlies our mind/brain product and how
we think it is designed?
- If not, we probably misunderstand the design/product!
Learning: What/why was the
reason (or “design specification”)?
Conditioned/instrumental/“slow” learning:
avoids spurious connections – associations of more
frequently linked phenomena are more useful.
Although……
Garcia effect: - an exceptional type of learning
mechanism (or – preparedness for learning) to avoid
poisons
By Evan-Amos (Own work) [Public
domain], via Wikimedia Commons
Phobias: Innate species-specific knowledge to avoid
potentially dangerous animals. (but mostly redundant
in UK) – more preparedness for knowledge.
http://www.flickr.com/photos/8373783@N07
/ Snakecollector [CC-BY-2.0
(http://creativecommons.org/licenses/by/2.0)
], via Wikimedia Commons
Neural basis of Garcia, and species-specific phobias are unknown
Brain is ready for significant (even
structural) change in response to
changing environment
Taxi drivers have larger posterior hippocampi
Maguire et al. (1997)
By ed g2s • talk (Own work) [GFDL
(http://www.gnu.org/copyleft/fdl.html),
CC-BY-SA-3.0
(http://creativecommons.org/licenses/bysa/3.0/) or CC-BY-SA-2.5-2.0-1.0
(http://creativecommons.org/licenses/bysa/2.5-2.0-1.0)], via Wikimedia Commons
But such plasticity is not species-specific
Avian food storers such as the scrub jay lose their
expanded hippocampi when they are denied the
freedom to store and retrieve food.
By Aphelocoma_californica_in_Seattle.jpg: Minette
Layne derivative work: Samsara
(Aphelocoma_californica_in_Seattle.jpg) [CC-BY-SA2.0 (http://creativecommons.org/licenses/bysa/2.0)], via Wikimedia Commons
Structural change over only 3 months
Draganski et al. (2004)
1
By Backlit (Own work) [CC-BY-SA-3.0
(http://creativecommons.org/licenses
/by-sa/3.0)], via Wikimedia Commons
2
3
1:before training
2:After 3 m practise
3: 3 m since practised
Images © Bodran Draganski 2012
Learning and the Brain: Review
1.
Learning is often synonymous with memory in cognitive neuroscience and, in
biological terms, change (plasticity) in brain structure/connectivity/activity can
be called (in biological terms) learning - including decreases!
2.
All animals learn and useful learning can be achieved by quite small networks
when they can change the efficiency of their connections appropriately – e.g.
“Hebbian” synapses – respond to simultaneous pre/post excitation
3.
Biological connections between neurons are called synapses - chemical
synapses of particular interest in neurocognition.
4.
Early and late processes of long-term potentiation (LTP) provide a biological
basis for the Hebbian Synapse and, therefore, learning Working memory may be
due to persistent neuronal firing
5.
Memory (and working memory) are distributed in the brain
6.
Evolutionary psychology = another perspective on categorizing different types of
learning
7.
Neuroplasticity: When we learn, it is not just connectivity (and function) that
changes in the brain, but structure can change also.