Application of Chaotic Time Series Analyses on Psychophysiological Responses to Odors

It is well-known that human biological systems produce chaotic behaviors such as rhythms of heart beat and plethysmograph. Odors are known to influence the psychophysiological state of a person, and produce fluctuations in plethysmographs regulated by autonomic system. In this study, we attempted to establish an index of psychophysiological responses induced by odors, using a feature of the chaotic behavior in plethysmograph. Such index could be extremely valuable in a research to characterize odors according to their responses in relation to human emotion.We first confirmed the chaotic nature of the time-series data of plethysmographs. Correlation dimension, which reflects the chaotic characteristic of self-similarity in the plethysmograph, was estimated by the Grassberger-Procaccia method. Also investigated were relationships among correlation dimensions, physiological and psychological parameters including: 1) effects on correlation dimension of plethysmograph caused by changes in mental states when a pleasant or unpleasant odor is introduced, 2) relationships among psychological index and physiological parameters such as galvanic skin resistance and power spectrum of plethysmograph, 3) effects of physiological parameters on correlation dimension.The results showed that: 1) correlation dimensions increased when unpleasant odor was introduced, 2) pleasant and unpleasant odors produced differences in galvanic skin resistance and power spectrum, 3) correlation dimension correlated with low-frequency component and ratio of power spectral components related to pleasure, displeasure and anxiety involved in emotion. It is concluded that the correlation dimension is sensitive to different odors and may be used as a reliable physiological index of pleasure and displeasure.