with distinct processing time characteristics, and separate motor processors for vocal, manual, and oculomotor (eye) movements.

EPIC assumes that all capacity limitations are a result of limited structural resources, rather than a limited cognitive processor. Thus the parsimonious production system can fire any number of rules simultaneously, but since the peripheral sense organs and effectors are structurally limited, the overall system is sharply limited in capacity. For example, the eyes can fixate on only one place at a time, and the two hands are assumed to be bottlenecked through a single processor.

Like the ACT-class architectures (see the earlier discussion of ACT-R) EPIC assumes a declarative/procedural knowledge distinction that is represented in the form of separate permanent memories. Task procedures and control strategies (procedural knowledge) are represented in production rules. Declarative knowledge is represented in a preloaded database of long-term memory elements that cannot be deleted or modified. At this time, EPIC does not completely specify the properties of working memory because clarification of the types of working memory systems used in multiple-task performance is one of the project's research goals. Currently, working memory is assumed to contain all the temporary information tested and manipulated by the cognitive processor's production rules, including task goals, sequencing information, and representations of sensory inputs (Kieras et al., 1998).

Unlike many other information processing architectures, EPIC does not assume an inherent central-processing bottleneck. Rather, EPIC explains performance decrements in multiple-task situations in terms of the strategic effects of the task instructions and perceptual-motor constraints. Executive processes—those that regulate the priority of multiple tasks—are represented explicitly as additional production rules. For instance, if task instructions say a stimulus-response task should have priority over a continuous tracking task, a hand-crafted production will explicitly encode that priority and execute the hand movements of the former task before those of the latter. These are critical theoretical distinctions between EPIC and other architectures (although they may not make a practical difference in the modeling of military tasks; see Lallement and John, 1998).

At this time, there is no learning in the EPIC cognitive processor—it is currently a system for modeling task performance.

Architecture and Functionality

EPIC's cognitive, perceptual, and motor processors work together as follows. EPIC's perceptual processors are "pipelines" in that an input produces an output at a certain time later, independent of what particular time the input arrives. A single stimulus input to a perceptual processor can produce multiple outputs in working memory at different times. The first output is a representation

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