DESCRIPTION (Applicant's Abstract): Laboratory studies disclose that when people try to perform different cognitive, perceptual or motor activities at the same time, interference often arises, even with seemingly trivial sensorimotor tasks. The limitations that cause this interference, and the precise forms that dual-task performance takes, illuminate basic questions about the functional organization of the mind/brain. Our research addresses these questions by analyzing dual-task interference and dual-task interactions in fine-grained detail. The studies proposed here examine four issues. The first is the control of processing order and task priority. Previous research indicates a stubborn "bottleneck" in action planning and memory retrieval; proposed studies will examine what determines the order in which these operations are queued, exploring the limits of voluntary control over scheduling of very rapid mental events. Similarity of two tasks affects the ease with which some cognitive tasks can be performed together, but this has not been observed with relatively elementary tasks; we will attempt to understand the limits and causes of similarity effects. Everyday observations suggests that when tasks are sufficiently practiced, dual-task interference is lessened (tasks become "automatized"). This has not, however, been demonstrated rigorously for mental operations that are subject to the most stubborn dual-task interference (e.g., action planning). Proposed studies will train subjects in serial reaction-time tasks within the lab and test their automatization in various ways; we will also examine professional typists' ability to type while performing other concurrent tasks. Attentional limitations in mental timing will also be examined. Studies will asses when timed rhythmic responding can function independently of concurrent tasks, and test hypotheses about beat-based versus interval-based timing mechanisms. Finally, by combining memory span tasks with concurrent serial reaction time, we will explore the degree to which information can be stored in short-term memory independently of the main processing limitations, following up on results implying an unexpected degree of independence.