Role of long-term synaptic modification in short-term memory

One way that some types of short-term or working memory may be implemented in the brain is by using autoassociation networks that recirculate information to maintain the firing of a subset of neurons in what is termed an attractor state. We describe how long-term synaptic modification is necessary to set up the appropriate stable attractors, each one of which corresponds to 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 which 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 prediction that if this type of implementation is used for working memory, then long-term synaptic modification may be necessary only during an acquisition phase of a task, and once the task has been acquired, the performance of the working memory task should be unimpaired if no further synaptic modification 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 the hypothesis that blocking synaptic modification in the hippocampus impairs the acquisition, but not the later performance, of hippocampal-dependent working memory tasks.