Successfully navigating our social world requires the ability to understand the facial expressions of others. Indeed, facial expressions provide information about the emotions and intentions of others. While some expressions provide clear predictive information that something good (e.g., happy) or bad (e.g., angry) will happen, other expressions, like surprise, can be interpreted negatively by some and positively by others. In this way, surprised expressions offer a means to assess individual differences in one's positivity-negativity bias.;The objective of this thesis was to integrate our understanding of the behavioral, psychophysiological, and neural concomitants of this bias when processing facial expressions with an ambiguous valence (e.g., surprise), as compared to those with a clear valence (e.g., happy, angry). Specifically, we used psychophysiological and functional neuroimaging (fMRI) methods to: (a) document individual differences in bias when resolving ambiguity, (b) generalize ambiguity resolution of surprised expressions to that of other ambiguous emotional stimuli, and (c) provide support for our "initial negativity" hypothesis that posits a primacy for negative interpretations in response to ambiguity, and conversely, that positivity requires regulation.;The present results demonstrated that activity in the corrugator supercilii region, an objective measure of perceived valence, in response to surprised faces reflected the participants' bias, rather than matching their valence ratings on any given trial. Moreover, the behavioral and neural underpinnings of positivity-negativity bias generalized beyond facial expressions to non-face emotional stimuli (e.g., scenes). Finally, negative interpretations were first and fast when resolving the associated valence ambiguity.;Taken together, these findings suggest that individual differences in positivity-negativity bias are indeed manifested in behavioral, psychophysiological, and neural responses, and these biases can be used to predict one's propensity to ascribe negativity to an ambiguous facial expression of emotion. The results of these studies in psychiatrically healthy participants may ultimately provide tools and insights for the future study of psychopathological groups where a pathological negativity bias impedes social interactions. These would include the mood and anxiety disorders, as well as social communication disorders (e.g., autism), where patients display clear limitations in comprehending social situations, particularly with regards to nonverbal facial communication.
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