Re: types of longterm memory

Anders Sandberg (
Thu, 14 Aug 1997 15:11:09 +0200 (MET DST)

On Wed, 13 Aug 1997, Darren Reynolds wrote:

> Could you post a brief explanation and / or a link for those who weren't at
> the conference? (And for those who made inadequate notes! :-| )

My talk will soon appear in the proceedings and on the web, but here
is an extract. Feel free to comment on it:

The Structure of Memory

Memory is not an unitary concept; it contains several different
subsystems that have different functions, properties and neural

The most well-known division is between Short-Term Memory (STM) and
Long Term Memory (LTM); information enters STM where it is stored for
a comparatively short period of time, and then is either forgotten or
moved into LTM, a process called memory consolidation.

Short consolidation Long
Term ------------------> Term
Memory Memory

Short term memory acts as our working memory, our current context. It
has a fairly small capacity, the famous "7 plus or minus 2" chunks of
information (Miller 1956), and if we are distracted we easily forget
what we were doing.

The border between STM and LTM is a bit unclear, while some memories
(like a name at a cocktail party) can be forgotten in seconds, others
remain for minutes, hours or even days before being forgotten. The
importance and emotional content of a memory will determine how well
it will be consolidated; shocking, unusual or personal memories will
be stored quite clearly, while the tedious details of daily life just
fade away. Studies in amnesia patients whose memory consolidation
process has been damaged hint that it may take days to weeks until a
memory is permanently stored.

The Medial Temporal Lobe (MTL) memory system has been a favorite
research subject ever since it was discovered that it is deeply
involved in memory consolidation (Squire & Zola-Morgan 1991). The MTL
consists of several brain structures, most importantly the
hippocampus, a tubular fold of cortex on the inner surface of the
temporal lobe. People with damage to the hippocampus have normal
short term memory, intact memories of their lives before the accident
or illness, but lack the ability to learn new episodic or semantic


Exactly how the MTL works is as present controversial, but one theory
which seems to be gaining support is the complementary encoding
theory of McClelland, McNaughton and O'Reilly (1995).

The basic idea is this: in order to learn, the brain needs to be
plastic, but if it is too plastic new memories will also overwrite
old memories and we will forget what we have learned; this is the
stability-plasticity dilemma which crops up in a lot of machine
learning. But we can remember single experiences which have not been
repeated, and at the same time retain memories for decades. This
could be explained if there was an intermediary memory system with
high plasticity (fast learning/forgetting) which stored recent
experiences, and then played them back to the less plastic cortex
several times (possibly during sleep or other mental activity),
gradually storing them in permanent memory. This could be the
function of the MTL, which we know is more plastic than the cortex,
and it would explain why damage to the MTL prevents patients from
learning new information. The MTL acts as the sketchbook of the

Long term memory is permanent; while STM seems to rely on neural
activity and chemical changes at the synapses, LTM appears to be
based on actual changes in cell morphology in the cortex. This means
that it is not easily wiped, and since it is distributed even fairly
large lesions will not remove specific memories.

LTM can be divided into several subsystems. Three of the most
well-defined and useful systems are episodic memory, semantic memory
and procedural memory.

/ | \
/ | \
/ | \
/ | \
/ | \
Episodic | Semantic
Memory | Memory

* Episodic memory is our memories of our lives, what we have done and
experienced. It is interesting since we usually learn it in a single
trial: we can experience something once, and then remember it forever.

An important and at present poorly understood subsystem of episodic
memory is prospective memory: the ability to remember things in the
future, like what to do tomorrow or what people will come and see you
on Tuesday (Sellen et al 1996). While it appears that we usually forget
more about the past than the future, forgetting to do things is usually
a bigger problem (Eldrige et al 1992).

* Semantic memory is our memory for facts and general knowledge, such as
Paris being the capital of France and that glutamate is a
neurotransmittor. This is what is traditionally learned in school, and
the area where study techniques or memory devices work best.

* Procedural memory is our memory of skills: how to do things
practically, how to swim or dance. Unlike episodic memory it usually
requires a lot of training, and it is not consciously declarative like
the previous two memory systems; this may be due to the fact that it is
strongly tied to the basal ganglia and the cerebellum, while semantic
memory and episodic memory (declarative memory) both appear tied to the
temporal lobe.


Eldridge, Margery, Sellen, Abigail, Bekerian, Debra, (1992) Memory Problems
at Work: Their Range, Frequency and Severity Rank Xerox Research Centre
Technical Report EPC-1992-129,

McClelland, J.L, McNaughton, B.L., O'Reilly, R. C. (1995) Why there are
complementary learning systems in the hippocampus and neocortex: Insights
from the success and failures of connectionist models of learning and
memory, Psychological Review, 102, 419-457.

Miller, G. A. (1956) The magic number seven, plus or minus two. Some limits
on our capacity of processing information. Psychological Review, 63, 81-97

Sellen, A.J., Louie, G., Harris, J.E., Wilkins, A.J. (1996), What Brings
Intentions to Mind? An In Situ Study of Prospective Memory, Rank Xerox
Research Centre Technical Report EPC-1996-104,

Squire, Larry R., Zola-Morgan, Stuart (1991) The medial temporal lobe memory
system, Science, 253, 1380-1386.

Anders Sandberg Towards Ascension!
GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y