PSY290H5 Lecture Notes - Lecture 8: Visual Cortex, Depth Perception, Long-Term Memory
PSY290 Lecture 8
Part II: Learning at the Neuronal Level
How does learning take place as a neuronal representation?
Next class: how information is stored
Looking at learning, we realize that there are over 150 definitions of learning
oConditions necessary for learning
oHow learning is captured, represented, and declared
Learning as a new neuronal representation that can then change behaviour over time
If we believe that the brain and the neuron is the fundamental unit of behaviour we should see
learning as a neuronal activity
Neurons store and retrieve information
What is learning?
Learning as a new neuronal representation that can then change behaviour over time
What place in the brain do you think most captures where learning happens?
Top Hat #1: What is the sign in the brain that you would look for as a change in response to learning?
What is learning?
Francis Gal: the size of the skull reflects activation in a certain area of the brain
oIf we have areas in great expertise, we do see some changes in representation in their brain
oViolin players have string representation of their fingers from the motor cortex
In the beginning, when the neuron hypothesis started, people thought that if the neuron is the
fundamental unit of the brain, we should be able to see change in the neuronal connection as result
of this
Donald Hebb in the 1930s: he wrote down in a paper and said that if two cells are active at the
same time and in the same space then the connection between them should change. In learning,
they should strengthen and in forgetting, they should weaken in that connective strength. That has
been changed colloquially to “cells that fire together wire together” and “cells that won’t don’t”.
Donald Hebb’s result: we should be able to see changes in strength of the connection between
two neurons
Top Hat #2: if we want to measure neuronal changes, what part of a neuronal respond would we
measure?
What do we expect might reflect a change in EPSP or IPSP? What is indicative of change?
oAction Potentials
Rate (frequency) of firing
Rate of action potential doesn’t change as much as we think
Not very indicative of change in neural responded
Action potential is NOT a reliable way to detect neuronal changes as a
response to learning
Speed of firing
oGraded Potentials
Increased number of graded potentials
Bigger graded potentials
If there is enough graded potentials, it’ll more likely reach threshold and trigger an
action potential
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MORE and BIGGER graded potentials is what is indicative of change in neuronal
change
Question: what is the neuronal change that is indicative of neuronal activity?
MORE and BIGGER graded potentials
When Donald Hebb said fire together wire together, he referred to an axon making a connection
with a dendrite and causing a depolarization
What Hebb said is that if the blue cell triggered a graded potential in the white neuron, over time,
we might be able to see change in the quantity and the magnitude of EPSP
oIf that happens frequently enough and repeatedly, what we tend to see if that given enough
stimulations, we will observe a change in magnitude of EPSP (it will grow in strength)
o“wiring together” --> changes in magnitude of graded potentials (EPSP)
if they do not fire together, we expect weakening
oif the stimulation never produces EPSP of any substantial magnitude, over time, the
magnitude of EPSP should actually get smaller if there is no realisable depolarization
happening
Hebb’s model sets the notion of how changes in a neuron should reflect learning and forgetting
oBoth exist in equal contributions
What happened is that Donald Hebb did not have the tools to validate ideas
10 years later: physiologist was really interested in studying this at the neuronal level and get
presented to someone studying squids with huge neurons seen with the naked eye. They ended up
seeing if they could stimulate an neuron or an axon and if they could then record the
magnitude of depolarization form the dendrites or the cell body
oone of the things they saw was that they could replicate instances of learning and
forgetting
oif we look at the magnitude of the depolarization in the initial test, we record information,
we see before repeated stimulation that we get a depolarization of a certain magnitude and
after you give a number of stimulations, we know the strength of that connection actually
grows and if you don’t reliably stimulate one neuron and the other the strength gradually
decreases
Summary: in an experiment, Huchkn and Husly stimulated a neuron at the
presynaptic cell and recorded the magnitude of depolarization occurring at the
post-synaptic cell and kept recording changes in the EPSP when numerous
stimulations are given at an instance. They found that after repeated stimulations,
the strength of the connection increases and this is noted by the increased number
and magnitude of EPSPs
oHuchkn and Husly in 1940s ^
oHow we could represent learning at the neuronal level: change in magnitude of EPSP
after continuous stimulation
oQuestion: does this really represent learning?
Learning Recap: we have established that learning takes place on a neuronal level in the brain. How is
this done? This is done according to the Hebbian synapses where neuronal connections strengthen upon
learning and this is evident in increased NUMBER and MAGNITUDE of EPSPs (excitatory graded
potentials) of the connection between 2 neurons after repeated stimulation.
Can we actually see this during the process of an animal learning something an behaviourally
adapting to a situation? --> the answer took another 30 years
1930: Donald Hebb
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1940: Huckn
1960s: Eric Candell
oI will try to put together an experiment that links behaviour with changes in neurons
oWorked with vertebrates
oThe aplysia could be up to 30 cm in size and weigh 10 pounds
oThe aplysia has the characteristics when we study the basic properties of neurons
First: Behaviourally, sea slugs are capable of demonstrating learning (one of the
important features)
We know that they are capable of demonstrating basic forms of learning
(nonassociative – Pavlov’s work)
Basic reflexive responses that can change over time in 2 ways
oIncrease in magnitude
oDecrease in magnitude
Example: eye blink reflex where a puff of air is shot in the eye the natural
reaction is close the eye
Example: you’re being poked and at first, you touch your hand but with
time, you just start to ignore it
Example: allergic reaction or a rash: you become more sensitive to
something over time
Second: the neurons of an aplysia are absolutely massive and visible to a human
eye
The overall width is over a 1 mm in diameter
Third: The gill-siphon withdrawal response (GSWR)
Siphon: Delicate structure that senses out things on the bottom of the sea
and determines whether or not its food
Gill: muscle that surrounds siphon to protect it
This reflex is a protective response reflex
The longer it remains retracted, the more it tells us that it has learned
An aplysia that has been taught, stays retracted for a longer time
This aplysia is being stimulated with an electrical shock and that shock is
painful and if something is painful, the animal would sensitize to it.
They are measuring the length of time that the siphon stays retracted
Initial touch: 11 seconds
Post-stimulation: 45 seconds
With sensitization, we are growing the size of response
Behaviourally, this is a change with experience
Fourth : an aplysia is really simple
They only have 20,000 neurons in their nervous system
There are only 2 neurons that separate this gill-siphon withdrawal reflex
oSensory neuron on siphon
oMotor neuron on gill that causes retraction
Scientists wanted to see if they can induce sensitization and habituation and
note a neuronal difference
oInduce forgetting: they touched the aplysia with a typical pencil
Behaviourally, we have an initial response to a touch (20 to
30 seconds)
After the course of days, the reaction time started to decrease
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Looking at learning, we realize that there are over 150 definitions of learning: conditions necessary for learning, how learning is captured, represented, and declared. Learning as a new neuronal representation that can then change behaviour over time. If we believe that the brain and the neuron is the fundamental unit of behaviour we should see learning as a neuronal activity. In the beginning, when the neuron hypothesis started, people thought that if the neuron is the fundamental unit of the brain, we should be able to see change in the neuronal connection as result of this. Donald hebb in the 1930s: he wrote down in a paper and said that if two cells are active at the same time and in the same space then the connection between them should change. In learning, they should strengthen and in forgetting, they should weaken in that connective strength.
