knowledge that result from recent experience, even if the recent experience is not recalled. For example, a gunner who saw a picture of a particular weapon in a magazine the previous evening would be more likely to notice that same weapon under battlefield conditions in the field the following day, even if the magazine event were totally forgotten and unavailable.

Although the existence of implicit memory effects is well established, the importance of including them in military models and simulations is not yet clear. Therefore, the focus here is on episodic and generic storage and retrieval, both of which are essential components of any simulation of human behavior. It is important to note that retrieval in all episodic tasks relies on a combination of generic and episodic access. For example, an externally provided retrieval cue, such as a word, will first be used to access generic, lexical, and semantic knowledge, which will join the surface features in a probe of episodic traces. A soldier prompted by a loud sound to recall the position of a recently seen tank, for example, will automatically be prompted to access long-term memory for associations with such a sound, perhaps recovering knowledge of the sorts of weapon systems that produce such a sound. The issue of whether both types of retrieval occur in episodic retrieval is largely independent of whether both must be included in a simulation; that answer depends on the particular application.

Short-term memory is the term used most often when one is interested more in the retention properties of the short-term system than in the control operations involved. Conversely, working memory is the term used most often when one is concerned with the operations carried out in active memory. These are not universal usages, however, and there is no hard and fast line between the two terms. The retention properties of the short-term systems are still an active area of study today, and there is no shortage of mathematical and simulation models of the retention of item and order information (e.g., Schweikert and Boruff, 1986; Cowan, 1993). Models of working memory that address the control of cognition are still in their infancy, and until very recently dealt only with certain specific control processes, such as rehearsal, coding, and memory search. What seems to be clear is that a successful model of short-term memory (and of memory in general) must deal with both the retention and control properties of the system. It seems clear that a model useful for military simulations must incorporate both aspects. Recent models of working memory are increasingly modular, with greater limitations of capacity within rather than between functional modules. Examples are the Soar architecture (e.g., Laird et al., 1987), the adaptive control of thought (ACT)-R model (Anderson, 1993), and the recent executive-process interactive control (EPIC) model of Meyer and Kieras (1997a); these are discussed in detail in Chapter 3.

Having pointed out the need for such a model, we must also note that a