Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, February 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 97-102).Successful interaction with the external world requires choosing appropriate actions in the context of available choices. Such decisions require the evaluation of the reward magnitude, or value, associated with each potential action. Delineating the neural circuits involved in this process remains an important goal in systems neuroscience. However, little is known about the neural circuits that compute, or represent, low level primary reward signals. We have combined quantitative psychophysical measures of subjective reward magnitude elicited by rewarding electrical brain stimulation, fMRI as a readout of whole-brain neural activity, and local inactivation of brain structures, to identify the neural representation of subjective reward magnitude. We find that multiple brain regions are activated by rewarding brain stimulation, but only two brain regions, the nucleus accumbens and the central and basolateral nucleus of the amygdala, exhibit patterns of activity levels that track the reward magnitude measured psychophysically, suggesting a role in the neural representation of reward magnitude. Furthermore, pharmacological silencing of the ventral tegmental area (VTA) disrupts reward-tracking behavior and increases stimulus-dependent activity in the nucleus accumbens and amygdala. Together these data suggest that ascending and descending pathways combine to produce a signal that ultimately guides behavior and is subject to modulation by VTA inputs.by Ana Marcia Fiallos.Ph.D.