It’s Professor Dave, let’s learn about learning.
In our study of biology, we began to learn about simple living organisms and how they
function.
In anatomy and physiology we extended this to examine ourselves, the human species, and
we saw precisely how complex the human body is.
But the most remarkable thing about us is our minds.
We don’t just run and jump and climb.
We think.
We learn.
We communicate, and we remember.
We have experiences that literally change the structure and functionality of the brain.
This is called learning, and the storage of these changes is called memory.
How exactly does this work?
Now that we have a basic understanding of neuroplasticity, we are ready to find out.
There are many different kinds of memories.
Of course we can remember things from the past.
The names and faces of people we know, or places we have been to.
We can remember the melody of a song.
Many of these are long-term memories, but we can also produce short-term memories, like
briefly memorizing sequences of numbers.
We can remember how to ride a bike even if we haven’t ridden one for many years.
This is called procedural memory.
Though these are different ways of describing memory, they all refer to specific neuronal
connections that are produced and reinforced.
We have gained an understanding of the discrepancies between these types by studying amnesia patients.
Someone with amnesia has significant loss of memory, often due to brain damage or even
surgical removal of parts of the brain due to extreme epilepsy or other medical issues.
First, we understand that the medial temporal lobes are crucial for memory, specifically
the hippocampus, which is sort of like the brain’s search engine.
Second, we understand that there is a difference between explicit memories, which are conscious
long-term memories, and implicit memories, which correlate more with learned motor skills
that are acquired through practice.
When an amnesia patient experiences severe impairment in one form of memory and not the
other, this demonstrates that there is a strong distinction between the two, and explains
how that patient could develop procedural memory for a particular skill while having
no recollection of practicing it.
We also know that there is a process by which short-term memories are translated into long-term
memories, which we call memory consolidation.
Let’s examine explicit memories more closely.
These are the ones that include remembering what kind of cake you had at your ninth birthday party.
These come in two varieties, semantic memories, and episodic memories.
Semantic memories involve facts and information.
We can think of them as being associated with singular bits of data.
Episodic memories involve events.
These are bit more complex and may combine different sensory information.
The distinction is exhibited by patients with brain damage that can remember facts like
their birthday, home town, names of schools they attended, people they know, and so forth,
while they are unable to remember almost a single personal event or experience.
We should also note the difference between anterograde amnesia, which is an inability
to create new memories after the onset of the amnesia while old memories remain intact,
and retrograde amnesia, which is a loss of memories acquired prior to the onset of amnesia.
There are many other types of amnesia apart from the medial temporal lobe amnesia we’ve
been describing, and we will mention more of them in the pathology course.
For now let’s get back to understanding memory.
Given that forms of amnesia can affect short-term memory while leaving long-term memories intact,
this supports the idea of memory consolidation, whereby memories are transferred from the
short term to the long term.
This means that the two methods of storage are different.
Short-term memory involves neural activity reverberating in closed circuits.
Over time, structural changes occur with the synapses involved that allow for long-term storage.
Experiments with electroconvulsive shock are working to understand precisely how long this
time frame is, by disrupting this closed-circuit reverberation in a controlled manner, but
evidence suggests that the activation of a memory solidifies the circuit and links it
to other memories, which increases resistance to disruption of the circuit by some kind
of trauma.
Some studies suggest that new memories are temporarily stored in the hippocampus until
they can be transferred to a more stable cortical storage system, but our understanding of the
role the hippocampus plays in memory is still evolving.
Many tests have been done with both rats and monkeys whereby specific lesions to the brain
are administered and performance in tests involving object recognition or maze navigation
are used to refine our understanding.
One thing to consider is the distinction between place cells, or neurons that respond only
when a subject is in specific locations, and concept cells, which are neurons that respond
to ideas or concepts rather than specific details of things.
This is best understood when considering the evolution of the hippocampus, which initially
aided in spatial navigation, or learning assocations between different spatial locations, but it
soon also evolved the ability to learn associations between more abstract entities, like learning
that a particular sound is associated with a particular animal.
In addition, our efforts to understand precisely where memories are stored have turned up several
conclusions.
First, it seems that memories are storied in a diffuse manner, meaning they are not
relegated to an individual area of the brain, and second that memories become more resistant
to disruption over time.
But the main areas of memory storage include the hippocampus and medial temporal cortex
that we have mentioned, the mediodorsal nucleus, basal forebrain, inferotemporal cortex, amygdala,
prefrontal cortex, cerebellum, and striatum.
The function of each region is slightly different.
The inferotemporal cortex seems to be associated with the storage of visual memories.
The amygdala is associated with the emotions tied to memories.
The cerebellum is associated with the learning of motor skills, and the striatum is associated
with relationships between stimulus and response, or habit formation.
So what are the mechanisms by which learning takes place?
To answer this we must look more closely at the synapse.
Essentially, synaptic transmission between two neurons can be strengthened over time
through a process called long-term potentiation.
This requires synaptic plasticity, the idea that the strength of a synapse can increase
or decrease over time.
The increase in strength of specific synapses through repeated signal transmission is correlated
with learning and memory in an intimate way, in that the induction, maintenance, and expression
of long-term potentiation are what is fundamentally happening when we learn, memorize, and recall
different things.
We can say that what fires together, wires together, which means that when we form certain
associative pathways repeatedly, these become myelinated, and thus more efficiently connected,
so any kind of neuroplasticity associated with the act of learning to execute certain
thought patterns or motion patterns, like playing a musical instrument, has a lot to
do with pruning and myelinization in order to facilitate the required patterns of transmission.
We should also take a moment to distinguish between classical and operant conditioning,
which are two contrasting behavior modification techniques that both result in learning.
With classical conditioning, a stimulus is used to produce an involuntary response by
association.
This would be like the classic example of Pavlov’s dogs, which were taught to associate
the ringing of a bell with mealtime, and thus eventually began to salivate simply upon hearing
the sound of the bell.
With operant conditioning, it is a voluntary behavior that is associated with a specific
consequence, either through reinforcement or punishment.
This would be like training a dog to perform a trick or follow a command, by offering a
treat when the dog succeeds, and withholding the treat when the dog fails.
The dog will associate the desired behavior with the treat, which leads to learned behavior.
Research is still ongoing in this area, so we will have to return to it a bit later.
For now, let’s move on to other topics in biopsychology.