One way that some types of short-term or working memory may be implemented
in the brain is by using autoassociation networks that recirculate informat
ion to maintain the firing of a subset of neurons in what is termed an attr
actor state. We describe how long-term synaptic modification is necessary t
o set up the appropriate stable attractors, each one of which corresponds t
o a memory of a particular item. Once the synapses have been modified, any
of the short-term memory states may be triggered by an appropriate input wh
ich starts the neurons firing in one of the attractors, and then the firing
is maintained in that attractor by the already modified synapses, with no
further synaptic modification necessary. This analysis leads to the predict
ion that if this type of implementation is used for working memory, then lo
ng-term synaptic modification may be necessary only during an acquisition p
hase of a task, and once the task has been acquired, the performance of the
working memory task should be unimpaired if no further synaptic modificati
on is allowed. We show that a considerable body of research findings on the
effects of agents that block synaptic modification on working memory tasks
can be understood in this way. Many of the findings are consistent with th
e hypothesis that blocking synaptic modification in the hippocampus impairs
the acquisition, but not the later performance, of hippocampal-dependent w
orking memory tasks. Hippocampus 2001;11:240-250. (C) 2001 Wiley-Liss, Inc.