addresses: MRC Center for Neuropsychiatric Genetics and Genomics, Institute of Psychological Medicine and Clinical Neurosciences, Cardiff University, UK. caserasx@cardiff.ac.uk ; notes: PMCID: PMC3640293 ; types: Journal Article; Research Support, Non-U.S. Gov't ; This is an open-access article distributed under the terms of the Creative Commons Attribution Non-commercial License, which permits use, distribution, and reproduction in any medium, provided the original work is properly cited, the use is non commercial and is otherwise in compliance with the license. See: http://creativecommons.org/licenses/by-nc/3.0/ and http://creativecommons.org/licenses/by-nc/3.0/legalcode. ; Copyright © 2013 by the American Psychiatric Association ; The official published article is available online at http://ajp.psychiatryonline.org/article.aspx?articleid=1676088 ; Little is known about the neurobiology of bipolar II disorder. While bipolar I disorder is associated with abnormally elevated activity in response to reward in the ventral striatum, a key component of reward circuitry, no studies have compared reward circuitry function in bipolar I and bipolar II disorders. Furthermore, associations among reward circuitry activity, reward sensitivity, and striatal volume remain underexplored in bipolar and healthy individuals. The authors examined reward activity in the ventral striatum in participants with bipolar I and II disorders and healthy individuals, the relationships between ventral striatal activity and reward sensitivity across all participants, and between-group differences in striatal gray matter volume and relationships with ventral striatal activity across all participants.