What is memory and where is it done?
Reading the correspondence with Tom about memory I was
initially inclined to let things pass. But as someone who
has worked on human disorders of memory and is
(reasonably) up to date with debate in that area I feel I
ought to just offer a few comments. My apologies if this
eats up bandwidth, but I hope it is interesting and
moderately illuminating Comments and criticisms
welcome.
1. Short term versus long term memory. For people
working with human disorders short term memory has a
very specific meaning -- it is the type of short duration
memory that is used in retaining veridical or verbatim
information. So if I introduce you to a series of people
around the table or if I dictate a telephone number to you
then you will almost certainly be using short term retention
to hold that information for long enough to work on it and
perhaps convert it into a more durable form (e.g. in writing
or in your head). This type of memory can be selectively
disrupted in brain damage either because the patient cant
establish a strong enough or large enough set of
representations -- or because strategic control and
modulation of information (e.g. holding operations) have
been disrupted. In the case of verbal STM the storage type
of disorder is associated with parieto-temporal damage to
the left hemisphere -- at least in the case of spoken word
lists. The attentional and modulatory types of disorder
appear to implicate the frontal lobes. Remarkably, STM is
not the gateway to all long term memory representations.
You can have a very reduced short term span and yet have
normal ability to learn new information and maintain an
ongoing autobiographical record.
2. Hippocampal and other amnesias Amnesic patients fail
to lay down new episodic memories -- in the severe case
they will fail to recognize the examiner following an
interruption of a couple of minutes in the course of a
session of several hours duration. Bilateral damage to the
hippocampus (from stroke, alzheimer-type degeneration,
tumor etc.) is one of the major causes of the amnesic
syndrome -- the other main cause is damage to thalamic
regions and the mammillary bodies. The thalamic-
mammillary body type of amnesia also arises in people who
have had stroke etc. but it has been most widely studied in a
syndrome in which the patient has sustained damage as a
consequence of alcohol abuse combined with a diet low in
thiamin (AlcoholicKorsakoff syndrome).
In functional terms, amneisa appears to be caused by
disruption of a closely integrated circuit that links those
anterior regions of the brain required for attentional and
problem solving operations with posterior (parieto-
temporal-occipital) areas that hold stored vocabularies or
encyclopedias of knowledge. This circuit is also closely
allied to the emotional and motivational zones of the brain
(and indeed, animal studies initially suggested that the
whole circuit - Papez circuit- was an emotional
powerhouse). We do not really know as yet whether there
are different types of amnesic disorder associated with
damage to different components of this circuit. Certainly
any difference between patients is not obvious or easy to
ascertain.
People did initially think that damage to this circuit just
blocked entry of information into storage but this view has
been refined. Amnesic patients can learn some types of
information - provided it can be tested by techniques that
do not require explicit access to a prior event. They can
acquire new skills, they show perceptual learning,
conditioning and a variety of priming (prior exposure
facilitation) effects. The whole question of spared learning
in amnesia has become a research field in itself. People are
still discussing whether this type of spared knowledge is the
product of the way memory is tested or the way memory is
represented and stored. Not easy questions to disentangle!
Amnesic patients also characteristically have problems in
recalling some information that was stored before their
injury. It is another matter of current debate whether
impairment in laying down new memories is separable
from problems in recalling old information. There is also
much discussion about the types of old memory that are
spared: Are these memories simply old and tough or are
they different in kind? Patients with organic (as contrasted
with psychogenic) amnesia typically do not forget their
names or occupations, they also remember whether they are
parents or not and whether they are married.
3. Other types of memory? There are many skills and
abilities that depend on some form of long-term stored
knowledge and these types of stored knowledge can be
disrupted in highly specific ways by brain damage.
Memory is both distributed (in many places) and local
(specific places seem to carry out particular types of
computation/operation). In the case of the human brain the
evidence for local specialization is very powerful indeed
coming both from the study of neurological patients and
from investigations of the regional activation of the brain in
normal volunteer subjects. Within these regions of local
specialisation it seems plausible to postulate some form of
distributed processing and there is a lot of very exciting
work being carried out to try and model this level of
representation.
In addition to local specialisation, we do tend to think of
sets of representations as being integrated within larger
systems. So in the case of reading, areas that process the
visual forms of words are separable from those that are
implicated in the assignment of sound to print; these are
also separable from the areas required for the assignment of
meaning to print. These are all memory dependent
operations, and there is no question of the brains being
hardwired for such a recently developed skill. Normal
reading makes use of different processing elements as
constituents of a larger reading system. The precise
configuration of that system may vary in different ways
depending on the task at hand. Incidentally, the reading
system tseems to fractionate along even more specific lines
than I have let on here -- but that is a whole other area for
debate.
Roz McCarthy
RM107 at UK.AC.CAM.PHX (within U.K.)
RM107 at PHX.CAM.AC.UK (from the world)
