Previous modeling work has focused on the effect of neuromodulatory neurotransmitters such as catecholamines on information processing. The present proposal seeks to build on this work. The candidate will be mentored in the design and conduct of a set of related empirical studies all using the Eriksen conflict choice-reaction time to focus on the mechanisms of processing highlighted by the modeling work. Specifically, all studies will be conducted with normal subjects and will consist of: (1) behavioral studies of different variants of the Eriksen task that test the mechanisms suggested by the model; (2) "Cognitive subtraction" PET studies using the same variants of the task and that will link mechanisms in the model to specific areas of activation in the brain; (3) pharmacological challenge studies with D-amphetamine to assess neuromodulatory effects on these mechanisms; and (4) a combined study using PKF and Pharmacological challenge (D-amphetamine) to establish a link between modulation of specific mechanisms to changes in activation of particular brain regions. The overall purpose of these combined studies is to develop a new research approach -- based on computer modeling, drug-challenges and functional brain imaging -- to study how neuromodulary neurotransmitters implicated in psychiatric disorders affect specific mechanisms of information processing and the activation of anatomical structures in the human brain.