Olfaction in affective disorders: an investigation of psychophysiological, behavioral and social response

Contrary to common sense, where olfaction has always been disregarded compared to the other senses, experimental and observational evidence highlighted the prominent role the sense of smell plays in human lives. Among olfactory stimuli, recently particular attention has been directed at odors that convey social information, which are substantially odors produced by humans (body odors). Chapters 1 and 2 provide a review of the olfactory system and the communicative functions of body odors. Across five studies, to better characterize olfaction in affective disorders, a comprehensive approach has been adopted, starting from the perception of common odors and the role of metacognitive abilities in shaping the olfactory experience, to the use of emotional body odors as a medium for transferring both negative (i.e., fear) and positive (i.e., happiness) emotions in order to investigate their psychophysiological and subjective effects and, finally, their use as a catalyst for treatment. A meta-analysis was conducted to unravel the neural correlates involved in the processing of olfactory social signals (Chapter 3), confirming the role of body odors in social communication and in the communication of emotions. Chapters 4 and 5 were dedicated at the investigation of the olfactory metacognitive and perceptual abilities in individuals with depressive, anxiety and social anxiety symptoms. Findings revealed that anxiety is associated with higher odor awareness, while social anxiety and depression are related to social odor awareness. In addition, higher depressive symptoms resulted to be characterized by reduced olfactory abilities, but only in individuals with low odor awareness. These results provide new insight into the understanding of the underlying factors implicated in the development and maintenance of olfactory dysfunction, possibly using odor awareness as a specific treatment target in clinical settings. Chapters 6 and 7 focused on the use of emotional body odors (both happiness and fear body odor) to investigate the processing of these odors using a psychophysiological approach (Chapter 6) as well as their potential role in a mindfulness treatment (Chapter 7). In general, body odors confirmed their role as powerful emotional cues, especially in healthy individuals. Individuals with social anxiety resulted to be particularly sensitive to fear social signals, as stated by an increased delta power in response to the fear body odor compared to the control group. Individuals with depressive symptoms, at a subjective level, evaluated the neutral faces as more arousing when presented with the fear odor, however, at a neural level, both the fear and the happiness body odors seemed to help these individuals to extract the social meaning of the situation. Indeed, when presented without the odors, individuals with depressive symptoms reported a reduced affective disposition as well as reduced cognitive processing of neutral facial expressions compared to the control group. However, these differences disappeared with the odors. Finally, in Chapter 7 an attempt to use emotional body odors during a mindfulness treatment in individuals with social anxiety has been provided. Both participants performing the training with happiness and with fear body odor reported a significant reduction of anxiety after the training. However, only the fear odor resulted in a reduced heart rate variability compared to the clean air condition. These results confirm body odors’ role in modulating subjective and physiological responses and give potential insight into how body odors may be utilized to support positive outcomes of psychological therapy. The last chapter of this thesis, Chapter 8, constitutes a general discussion of the empirical studies that composed the present dissertation, providing a unique framework in the study of olfaction and affective disorders.