The cognitive strategies, and rules, which man devises to accommodate task requirements substantially control behavior. While much research has aimed at elucidating characteristics of such information processing structures once instituted, little is known about the processes whereby humans select and alter "psychological software." In particular, the time and attention (or capacity) demands of such dynamic processes are unknown at present. The goal of this proposal is an understanding of processes involved in rapid changes of cognitive strategies and rules used by humans to accomplish task goals. Specifically, the proposed research aims at establishing: 1) the temporal and attentional, or capacity, requirements for strategy and rule changes, 2) whether these requirements interfere with overall task performance by diverting limited psychological resources. An important aspect of the overall research will involve development and validation of measures of momentary attentional demands. In the main experiments proposed, subjects will be required to change speed-accuracy tradeoff strategies or stimulus-response mapping rules within discrete double- stimulation trials. This allows close examination of the micro- structure of strategy and rule changes. In contrast to the modal double-stimulation paradigm in which the interval between successive stimuli is varied, the proposed research varies the interval between first response and second stimulus in order to control the time available for cognitive shifts. Probe signal methodology will be used to assess attention demands of such changes. An interactive minicomputer system will allow adjustment of probe task difficulty to individual subject abilities.