Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, 2008.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 178-186).The ability to switch between tasks reflects a fundamental part of our intelligence. A foundation of this ability lies in perceiving and processing information pertinent to the situation at hand. It is our capacity to attend to specific objects and, more importantly, our ability to switch our attention from object to object, that supports complex cognitive behavior. Therefore, by understanding the neural mechanisms involved in directing attention we hope to better understand cognition. Previous work investigating the ability to control attention has suggested that attention is influenced from two sources -- attention can either be driven from external sources in an bottom-up, exogenous manner or directed internally in an top-down, endogenous manner.This project will utilize two different forms of visual search in order to emphasize these two different types of attentional control. Both the prefrontal and parietal regions are implicated as the source of this control. In order to investigate their relative roles we recorded simultaneously from both parietal cortex (specifically, the lateral intraparietal cortex) and prefrontal cortex (specifically, the frontal eye fields and dorsolateral prefrontal cortex). We address four main questions. First, we contrast the respective roles of frontal and parietal cortex in the direction of attention when it is under either top-down or bottom-up control. We use the timing of attention signals between frontal and parietal cortex to establish that frontal cortex directs top-down attention back into parietal cortex, while bottom-up attention is reflected first in parietal cortex, flowing forward to frontal cortex. Secondly, we investigated the role of synchrony and the inter-areal relationships underlying top-down and bottom-up control of attention. Our results suggest synchrony between areas shifts as the task shifts, likely aiding in the selection of the network best suited to the current task. Third, we compare the neural mechanisms between internal and external control of attention.(cont) Finally, we investigate the neural correlates of the putative parallel and serial mechanisms underlying visual search, finding support for the existence of a serial search and for the role of the frontal eye fields in the direction of spatial attention.by Timothy J. Buschman.Ph.D.