as the visual icons originally studied by Sperling (1960), which operate fairly automatically and decay autonomously within several hundred milliseconds, and a variety of more central memories, including so-called "working memories," that are used to carry out all the operations of cognition (e.g., Baddeley, 1986, 1990). Short-term memories allow control processes to be carried out because they hold information for temporary periods of, say, up to a minute or more. The long-term memory system can be divided by function or other properties into separate systems, but this is a matter of current research. The operations associated with memory include those of rehearsal and coding used to store new information in long-term memory; operations that govern the activities of the cognitive system, such as decisions; and operations used to retrieve information from long-term memory. Almost all simulations incorporate such structural features.

Different aspects of memory and learning are important for simulations aimed at different purposes. For example, a simulation of a tank gunner might rely heavily on short-term vision, short-term working memory, and whatever task procedures have been learned previously and stored in long-term memory. Retrieval from visual short-term memory may be important, for example, when the positions of recent targets and terrain features must be retained during a series of maneuvers that move these targets and features out of direct vision. For a gunner in a short firefight, learning during the task may be relatively unimportant since the gunner will rely primarily on previously trained procedures and strategies or decisions made in working memory. In addition, the process of accessing learned procedures in long-term memory may not need to be modeled, only assumed (since this process may be well learned and automatic). On the other hand, strategic decision making by a battle commander will involve working memory and the processes of long-term retrieval. Sensory memories and the retention role of short-term memories may be relatively unimportant. Finally, in the course of a war lasting several years, learning will probably be critical for models of personnel at all levels of command.

It should be obvious that the process of learning itself is critical whenever training and the development of skill and expertise are at issue. The nature of storage, the best means of retrieval, ways to utilize error correction, and best modes of practice are all essential components of a simulation of the learner. It is well known that the largest effects on performance observed in the literature are associated with the development of expertise (see also Chapter 4). The development of skills can proceed quite quickly in short periods of time, but typically continues for long periods. Given motivation and the right kinds of practice, development of the highest levels of skill in a task may require 3 to 4 hours a day of rigorous practice for a period of 10 years (e.g., Ericcson et al., 1993; Ericcson, 1996). Although the nature of the learning process in such situations is not yet understood in detail, there is much empirical evidence (starting with Chase and Simon, 1973) demonstrating that a critical component of the learning of a skill is the storage of massive amounts of information about particular situations and

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