Brain and memory

RM107 at PHX.CAM.AC.UK RM107 at PHX.CAM.AC.UK
Sat Mar 21 06:53:38 EST 1992


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)





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