Science

Learning and Memory All What You have to know

Memories are central to our individuality. What each of us
remember is different from what others remember, even of
situations we have been in together. Yet, in our distinct
ways, all of us remember events, facts, emotional feelings
and skills – some for a short time, others for a lifetime.
The brain has multiple memory systems with different
characteristics and mediated by different neuronal
networks. The formation of new memories is now widely
thought to depend on synaptic plasticity, as described in
the last chapter, but we are still uncertain about the
neural mechanisms of information retrieval. While we all
complain about our memories, they are in the most part
pretty good, only starting to fail in old age or certain
neurological diseases. It might be good to try to improve
our memory, but doing so could be at the cost of
remembering many things that it is as well to forget

The organisation of memory


There is no single brain area to which all the information we
ever learn is shuttled for storage. Working-memory holds
information in your mind for a short time in an active
conscious state. The much larger, more passive storehouse
of information is called long-term memory.

Working Memory

Like a pad on a desk for jotting down names or telephone
numbers that we need to remember only briefly, the brain has
a system for holding on to and working with small amounts of information very accurately. We use it to remember
speech for long enough to interpret the flow of conversation,
for doing mental arithmetic, and for remembering where and
when we put our keys down a moment ago. Fidelity is central
to the system – a feature that comes at the cost of limited
capacity and persistence. It is often said that you can
remember 7 ± 2 items in working memory; this is why so
many telephone numbers are no longer than 7 or 8 digits.
But remembering these accurately is essential. You can
demonstrate the capacity and limited persistence of
working memory in a simple experiment you can do with
your friends.

An Experiment on Short-Term Memory
A simple test of short-term or working memory is
called “letter-span”. You need a minimum of 2
people, although it works better with the whole
class. Privately, one of you writes down a series of
letters beginning with as few as 2, taking care they
do not spell out a word (e.g. XT). This person then
produces further letter strings, one letter longer
at a time (e.g. a 5-letter string such as QVHKZ and
a 10-letter string such as DWCUKQBPSZ).
The experiment begins after these are prepared.
The other person (or class) listens to each letter
string in turn and, after about 5 seconds, tries to
write down the letters in the correct order from
memory. Starting with the easy 2-letter string,
the memory test moves on to longer ones. Most
people can do it perfectly up to about 7 or 8 letters
– and then errors creep in. Very few can do 10 let￾ters correctly. The capacity of short-term memory
has been described as “the magical number 7 plus or
minus 2”.

A central executive system controls the flow of information,
supported by two additional memory stores. There is a
phonological store alongside a silent rehearsal loop – the bit
of your brain that you use to say things to yourself. Even if
you read words or numbers visually, the information will be
transcribed into a phonological code and stored for a short
while in this two-part system. There is also a visual
sketchpad that can hold on to images of objects for long
enough for you to manipulate them in your mind’s eye.
Working memory is largely located in the frontal and parietal
lobes. Brain imaging studies (see p. 41) using PET and fMRI
brain imaging indicates that the auditory parts of working￾L memory are generally lateralized to the left frontal and parietal lobes where they interact with neuronal networks involved
in speech, planning, and decision-making. These are activities
for which a good working-memory is essential. The visual
sketchpad is in the right hemisphere
How did working-memory evolve? Animals, even most mammals,
probably do not have quite the same sort of
a short-term memory system as we have, and it clearly didn’t
evolve to help early hominids remember telephone numbers!
Studies with young children point to a critical role for working￾memory in learning language, suggesting that this memory system may have co-evolved with speech. The precision
required for keeping track of words and their order in a
the sentence is critical for accurately working out the correct
meaning.

Long-term memory

Long-term memory is also sub-divided into different
systems located in widely dispersed networks of the brain.
The different networks do very different jobs. Broadly
speaking, information enters sensory systems and then
passes down pathways that provide increasingly specialized
processing. For example, information entering the visual
the system passes down a so-called ventral pathway from the
striate cortex to the medial temporal lobe through a
cascade of networks that work out the shape, color, object
identity, whether the object is familiar or not, until finally,
some kind of memory is formed of this particular object and
when and where it has been seen.

The cascade of brain areas through which visual information is first processed perceptually and then for the purpose of memory.

There are several ways of thinking about this cascade of
analysis. First, there are areas in the cortex that extract a
perceptual representation of what we are looking at.
This is used to store and later recognise things around us.
Our ability to identify familiar people in newspaper cartoons,
such as politicians, reflects this system. Very closely
related is a system called semantic memory – the vast
storehouse of factual knowledge that we have all accumulated
about the world. We know that Paris is the capital of France,that DNA encodes genetic information as a sequence of base
pairs, and so on. The critical property is that facts are
organised into categories. This is vital for memory retrieval as
the search process can then shuttle through tree diagrams in
this storehouse to find things efficiently. If semantic memory
were organised in the way that many people organise things in
the attic of their houses – pretty randomly – we would have
terrible trouble remembering anything. Fortunately, the brain
sorts the information that we encode into categories, though
it helps to have a skilled teacher for the complex things we
learn at school. Indeed, gifted teachers build these
structures in their pupils effortlessly.

The facts we know about animals are organized in a tree structure. We do not yet know how the networks of the brain do this.

We also learn skills and acquire emotional feelings about
things. Knowing that a piano is a piano is one thing: being able
to play it is another. Knowing how to ride a bicycle is useful,
but being aware that certain situations on the road can be
dangerous is no less important. Skills are learned through
deliberate and extensive practice, whereas emotional learning
tends to be much more rapid. Often it has to be fast,
particularly for the things we learn to be afraid of. Both are
types of learning called conditioning. Specialised brain areas
are involved – the basal ganglia and cerebellum being very
important for skill learning, and the amygdala for emotional
learning. Many animals learn skills – it is very important for
their survival.

Chimpanzees have learned the skill of fishing for termites using a stick. Young chimpanzees learn this by watching their parents.

Memory failure and the localization of
episodic memory in the brain

The last type of memory system in the brain is called episodic
memory. It is what you use to keep track of personal experience. Remembering events is different from learning
facts in one very important respect – events happen only
once. If you forget what you ate at breakfast today (unlike￾ly), or what happened last Christmas (possibly), or all the
things that happened on your very first day at school
(probably), you cannot re-run any of these events like an
extra lesson in class. This system learns quickly because it
has to.
We have learned a lot about what episodic memory is by
studying neurological patients who, following a stroke, brain
tumors or viral infections such as herpes encephalitis,
sometimes have very specific deficits in this type of memory.
Studying such patients carefully has been the major way to
work out the anatomical organization of this and other
memory systems.
Amazingly, amnesic patients can learn some things that they
cannot consciously remember! They can be taught motor
skills or to read backwards very quickly.
Training to read backward quickly takes a while
This is true for amnesics no less than for us, but whereas
we would remember being taught to do this, they do not.
This is a fascinating dissociation in their conscious
awareness. Amnesics are certainly conscious when they
learn, but are later unaware of having learned. They cannot
recover conscious awareness from the past.
The damage that causes this distressing condition can occur
in a number of brain circuits. Areas of the midbrain called
mamillary bodies and the thalamus seem to be critical for
normal memory, as is a structure in the medial temporal lobe
called the hippocampus. Damage in these regions seems
particular to affect the formation of episodic and
semantic memories.

“It is not so much the injury that captures our
attention as how, through injury or disease, normal
function is laid bare.”
(Sir Henry Head – 20th C Neurologist).

People affected by a condition known as amnesia cannot
remember meeting other people only half an hour earlier.
They cannot remember whether they have recently eaten a
meal or ought to have one, and even such simple necessities
of life as where things have recently been put down around
the house. Shown a complex drawing – such as the one in the
inset – they can copy it accurately but they cannot draw it
as well as most of us could do from memory as little as 30
minutes later. Often, they cannot remember things that
happened before they became ill. This is called retrograde
amnesia.
Such a life lacks all structure in time and place and has been
described by one extensively studied amnesic patient as like
continually “waking from a dream”.

Amnesics (A) can see just fine and copy complex drawings like this one quite accurately, but they cannot remember them for very long compared to normal control subjects (NC).

Yet this same person retains his command of language and the meaning of words, and enough working-memory to carry on a sensible conversation. It is not until one has exactly the same conversation with him a few minutes later that the devastating isolation of his existence is revealed.

Two structures are very important for episodic memory – the perirhinal cortex (PRH) which mediates the sense of familiarity about the past and the hippocampus (HIPPO) which encodes events and places.

 

Other memory systems

Damage elsewhere in the brain affects other memory
systems. Degenerative conditions, such as certain types of
semantic dementia (a type of Alzheimer’s Disease), can
cause fascinating patterns of the breakdown of semantic
memory. Early on, patients will be quite capable of telling you
that the pictures they are being shown in an experiment are
of a cat, or a dog, or of a car, or a train. Later on in the disease
, they may hesitate to call a picture of a mouse a
mouse, saying instead that it is a dog. What this confirms is
that factual information is organised categorically, with
animate information stored together in one place well away
from inanimate information.
The neurobiology of memory
Studying neurological patients carefully helps us to discover
where memory functions are in the brain, but finding out how
They work in terms of neurons and chemical transmitters
involves carefully conducted research using laboratory
animals.
Neuroscientists now believe that many aspects of the
fine-tuning of neural connections in the developing brain are
also used during early learning. The attachment that
develops between an infant and its mother has been studied
in young chicks in a process called imprinting. We now know where this learning process takes place in the young chick’s
brain and the chemical transmitters that are released to act
on receptors involved in storing some kind of an ‘image’ of the
mother. This image is quite precise, such that the young
chick will follow its mother but not another. Young animals
also need to know what foods are safe to eat by tasting
small amounts of food at a time, and learning those that
taste bad. This cannot be left to genetic predispositions
alone – developmentally tuned learning mechanisms are at
work. Downstream of the receptors activated during
imprinting or the tasting of food, a cascade of second￾messenger chemicals transmit signals to the nucleus of
brain cells where genes are activated to make special
proteins that can literally fix the memory.
Place cells are another important discovery. These are
neurons in the hippocampus that fire action-potentials only
when an animal explores a familiar place. Different cells code
for different parts of the environment such that a
population of cells is involved in mapping a whole area. Other
cells in a nearby brain area code for the direction the animal
is moving in. The two areas working together – the map of
space and the sense of direction – help the animal learn to
find its way around the world. This is clearly very important
for animals, because finding food and water and then their
way back to the burrow, nest, or other home is vital for their
survival. This navigational learning system relates to both
semantic and episodic memory. Animals form a stable
representation of where things are in their territory – just
like the factual knowledge we acquire about our world. And
this map of space provides a memory framework in which to
remember events – such as where a predator was last seen.
Place cells may code more than just place – they may help
animals to remember where events have happened.

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