Abstract This article provides a selective review of the literature and current theories regarding the role of prefrontal cortex, along with some other critical brain regions, in emotion and motivation. Seemingly contradictory findings have often appeared in this literature. Research attempting to resolve these contradictions has been the basis of new areas of growth and has led to more sophisticated understandings of emotional and motivational processes as well as neural networks associated with these processes. Progress has, in part, depended on methodological advances that allow for increased resolution in brain imaging. A number of issues are currently in play, among them the role of prefrontal cortex in emotional or motivational processes. This debate fosters research that will likely lead to further refinement of conceptualizations of emotion, motivation, and the neural processes associated with them.

Human emotion has not always been a popular topic for brain researchers interested in the cortex. Twenty-five years ago, the vast majority of studies focused on cognition, with relatively little research addressing emotion. Interest in emotion has increased exponentially, however, with the term `affective neuroscience' (Davidson & Sutton, 1995) now competing with the term `cognitive neuroscience'. In recent years, studies have moved beyond investigating emotion and cognition as separate phenomena toward acknowledging their complex coupling under many ordinary circumstances, developing experimental paradigms that examine their relationships, and even wondering whether they are fundamentally distinguishable (Blair et al., 2007; Gray, 2004; Kalisch, Wiech, Critchley, & Dolan, 2006; Miller, 1996; Mohanty et al., 2007; Pessoa, 2005). Emotion is now not merely one of many facets of brain structure and function to be better understood but a core domain for understanding brain mechanisms more broadly. For example, the study of emotion has become an important target of research attempting to uncover neural processes associated with psychopathology.

Conclusion Research on neural correlates of emotion and motivation has led to an increasingly complex understanding of the brain systems that implement these psychological functions. Technological advances such as dense-array EEG and fMRI have allowed this investigation to more precisely identify the neural circuits that are involved in the experience of, and processing related to, emotion and motivation (for review, see Miller, Elbert, Sutton, & Heller, 2007). These advances have moved our understanding of human brain organization from assuming simple, large-scale, hemisphere-level phenomena to distinguishing differences between activity in relatively small and sometimes adjacent regions, such as differential responses to appetitive and aversive stimuli in different regions of the OFC (O'Doherty et al., 2001, 2003). The research discussed in the present paper has provided a number of important lessons and directions for future researchers. First, it will be important for researchers to continue to distinguish between neural phenomena related to different aspects of emotional phenomena. Phenomena commonly called `emotional' sometimes involve stimulus properties, types of processing, effect on putatively non-emotional processing, diverse central and peripheral physiological phenomena, and diverse overt verbal and motor behaviors. Relevant neural mechanisms are surely equally diverse. Second, it will be important to attempt to differentiate correlates of emotional and motivational processes to the extent possible. Research conducted on animals by Berridge et al. may provide further direction (for review, see Berridge & Robinson, 2003). This research has differentiated two processes involved in interactions with rewarding stimuli, specifically distinguishing pleasure and approach. This research suggests that emotional and motivational systems are separable on a fundamental level, although there may still be considerable overlap, and healthy function may depend on their interaction or balance. This view is very much in line with the perspective of Wacker et al. (2003) discussed above, wherein valence/arousal and approach/withdrawal concepts fit different aspects of the same data set. Third, the research discussed in the present paper emphasizes the importance of being alert to unforeseen interactive effects involving the concepts being investigated, the type of task employed in the service of those concepts, and the neural mechanisms foregrounded by their combination. For example, when investigating asymmetry in brain activity related to motivational or emotional valence, it will be important to employ both spatial and verbal tasks. Otherwise, differential effects that are attributed solely to emotion or motivation may be an artifact, in part, of task demands. Furthermore, effects that would be evident during other tasks might be overlooked. Finally, it can be assumed that the many dichotomous organizing principles and models currently prominent in the literature that address functional brain organization will continue to evolve. There is considerable overlap among them and a long history behind some of them. Much supporting data are already available for each; choices among them would be premature and will probably prove unnecessary. Attempts at conceptual integration (e.g., Coombes et al., 2007; Elliot & Thrash, 2002; Wacker et al., 2003) are needed. The structural and functional organization of the brain and the diversity of our paradigms offer enough degrees of freedom that each dichotomy may receive considerable support, but as data accrue the models are likely to become more complex and less distinct. Choosing among them will be less important than helping them mature.

Acknowledgement This work was supported by the National Institute of Drug Abuse (R21 DA14111), the National Institute of Mental Health (R01 MH61358, T32 MH19554), and the University of Illinois Beckman Institute, Department of Psychology, and Intercampus Research Initiative in Biotechnology.

Short Biographies Jeffrey M. Spielberg is a doctoral student in the Clinical/Community division at the University of Illinois at Urbana-Champaign. His research interests include motivational and emotional processes and their associated neural correlates, focusing specifically on how these factors are involved in the etiology and maintenance of depression and anxiety. He holds a BA in Psychology from the George Washington University. • Jennifer L. Stewart is a doctoral student in the Clinical/Community Division at the University of Illinois at Urbana-Champaign. Her primary research interest is investigating the neural correlates of motivation and emotion regulation in angry and depressed individuals. She holds a BS in Psychology from the University of California, San Diego and a MA in Clinical/Community Psychology from the University of Illinois, Urbana-Champaign. • Rebecca L. Levin is a doctoral student in the Clinical/Community division at the University of Illinois at Urbana-Champaign. Her research investigates the neural correlates of executive function impairments in depression and anxiety. She holds a BA is psychology from Macalaster College. • Gregory A. Miller has appointments as Professor of Psychology, Adjunct Professor of Psychiatry, Beckman Institute Cognitive Neuroscience Group Leader, and Richard and Margaret Romano Professorial Scholar at the University of Illinois at Urbana-Champaign and as Adjunct Professor of Psychiatry at the University of New Mexico. He has been a Guest Professor at the Universities of Tuebingen and Konstanz. He teaches courses on clinical neuroscience and psychopathology, clinical assessment and intervention, and psychophysiology. He publishes laboratory studies on brain mechanisms in psychopathology and conceptual review papers on relationships between psychological and biological phenomena in cognition, emotion, and psychopathology. He has been president of the Society for Psychophysiological Research and editor of Psychophysiology. He has an AB in Psychology and Social Relations from Harvard University and an MS and PhD in Psychology from the University of Wisconsin-Madison. • Wendy Heller is Professor of Psychology, director of Clinical Training, and associate department head in the Psychology Department at the University of Illinois. Her research investigates the role of the brain in emotion, personality, and psychopathology, particularly anxiety and depression. She uses psychological theories to determine how fundamental emotion and personality constructs can be mapped onto brain systems to clarify both the neural mechanisms of emotion and models of emotion themselves. Her work has shown that emotion and cognition interact to influence both feelings and task performance and that different patterns of brain activity are associated with different patterns of cognitive functions in types of anxiety and depression. Her more recent work has examined individual differences in personality (e.g., schizotypy), motivation (e.g., approach/avoidance) and executive function and their relationship to brain function and psychopathology. She teaches neuropsychological assessment of children, adolescents, and adults, and is founder and director of the Neuropsychological Assessment Service at the Psychological Services Center at the University of Illinois. Heller holds a BA in Psychology from the University of Pennsylvania and an MA and PhD in Biopsychology from the University of Chicago.