What is Working Memory?
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different process from long term memory. For example, the famous
neurological patient, H.M. who has not formed an explicit long term
memory since the day of an operation to remove his hippocampi in 1954,
has intact working memory. If you or I were to meet him, we could
interact with him and sustain a normal conversation about Eisenhower
or that new gadget television until a door slammed or something
distracted his attention; at that point, we would have to begin again.
Long term memory of the sort that H.M. is missing is operative over
long periods such as hours, days, or even a lifetime. It has been
clinically and experimentally well studied, and has been shown to
involve brain regions such as the hippocampus. It is thought to be
mediated by changes in cell functioning, such as long term
potentiation (LTP).
In sharp contrast, working memory seems to be something profoundly
different. Scientists, particularly psychologists and cognitive
scientists, have long been curious about working memory because of its
involvement in all cognitive processes. Early psychological work in
the 1950's and 1960's led to the hypothesis of 'short term memory'; a
process of limited capacity and only operative over a few seconds. The
concept of 'working memory' is an extension of this idea, with the
added idea that short term memory is woven together with higher
cognitive processes, such as learning, reasoning, and comprehension.
Unlike long term memory, which has a large clinical body of research,
working memory has only recently become the focus of intense clinical
study. It is often assayed in intelligence or cognitive examinations
using span tests, in which patients are asked to repeat a set of
digits in reverse order (if I read "8-9-3-2-1-9", you would say
"9-1-2-3-9-8") or alphabetize a group of words that had been read
aloud. Studies of patients with various frontal lobe lesions do not
show a systematic deficit in storage. These studies indicate that
working memory is not one process; rather, it is made up of several
separable processes.
A Psychological Perspective
Alan Baddeley, in his landmark book Working Memory, captures three
decades of psychological work on working memory systems. Many working
memory experiments simply consist of stimuli that are to be remembers
for a few seconds. A typical task might ask you to remember a few
letters, numbers, or features of an object. Typically, there is a
brief delay, after which the subject is 'probed', or asked what he or
she remembers. From extensive studies like these, Baddeley proposed a
model of working memory that involved three distinct subsystems. The
best described is the 'phonological loop', a system that draws upon
speech resources. For example, if I wanted to remember a set of
numbers, I might catch myself whispering to myself -- it turns out
that speech systems are an integral part of working memory. The second
component is the visuospatial sketchpad, a parallel system akin to an
artist's sketchbook for stimuli that cannot be verbalized, such as
spatial information. The third main unit is the central executive, a
system responsible for supervisory attentional control and cognitive
processing. This last system, though poorly defined, is most alluring
because it represents the very stuff of thought.