complete model of short-term memory (in this general sense, which includes all the control processes used to store, retrieve, and decide) is tantamount to a complete model of cognition. The field is a long way from having such a model. Thus for the foreseeable future, short-term memory simulations will need to be tailored carefully to the requirements and specifications of the particular task being modeled.

Long-Term Memory and Retrieval

For most modelers, long-term memory denotes a relatively permanent repository for information. Thus models of long-term memory consist of representation assumptions (e.g., separate storage of exemplars, distributed representations, networks of connected symbolic nodes) and retrieval assumptions. The latter assumptions are actually models of a certain set of control and attention operations in short-term memory, but it is traditional to treat them separately from the other operations of short-term memory.

For episodic storage and retrieval, the best current models assume separate storage of events (e.g., the search of associative memory [SAM] model of Raaijmakers and Shiffrin, 1981; the ACT-R model of Anderson, 1993; the retrieving effectively from memory [REM] model of Shiffrin and Steyvers, 1997; Altmann and John, forthcoming); however, there may be equivalent ways to represent such models within sparsely populated distributed neural nets. Retrieval of all types is universally acknowledged to be driven by similarities between probe cues and memory traces, with contextual cues playing an especially significant role in episodic storage and retrieval.

It seems likely at present that different modes of retrieval are used for different tasks. For example, free recall (recall of as many items as possible from some loosely defined set) is always modeled as a memory search, with sequential sampling from memory (e.g., Raaijmakers and Shiffrin, 1981). Recognition (identification of a test item as having occurred in a specific recent context) is almost always modeled as a process of comparing the text item in parallel with all traces in memory (e.g., Gillund and Shiffrin, 1984). Cued recall (recall of one of a pair of studied items when presented with the other) is still a matter of debate, though response time data favor a sequential search model for episodic tasks (e.g., Nobel, 1996).

Most current models, and those with an elaborated structure, have been developed to predict data from explicit memory tests. Models for retrieval from generic memory are still in their infancy. One problem hindering progress in this arena is the likelihood that a model of retrieval from generic memory may require, in principle, an adequate representation of the structure of human knowledge. Except for some applications in very limited domains, the field is quite far from having such representations, and this state of affairs will probably continue for some years to come.



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