Instantaneous assessment of hedonic olfactory perception using heartbeat nonlinear dynamics: A preliminary study

Emotional perception to hedonic olfactory stimuli is under direct control of the limbic system, whose dynamics is known to affect autonomic nervous system activity on cardiovascular control. Mainly due to methodological limitations, previous investigations failed to uncover specific trends in heartbeat dynamics between ultra short-time (i.e., lasting < 10s), pleasant and unpleasant olfactory stimuli. To this extent, in this study we computed instantaneous estimates from heartbeat series gathered from 32 healthy subjects (age: 26±2; 16M) undergoing hedonic olfactory elicitation. Each subject exhibited a similar olfactory perception threshold, and scored five 5s stimuli in terms of arousal and valence level using the self-assessment manikin test. We analyzed the heartbeat series using our recently proposed inhomogeneous point-process nonlinear framework, obtaining instantaneous estimates defined in the time (mean and standard deviation), frequency (power in the LF and HF bands, as well as LF/HF ratio), and nonlinear/complexity (bispectra, sample and approximate entropy, and Lyapunov exponent) domains. A feature set comprising average estimates within the 5s windows was taken as an input of a K-Nearest Neighborhood classification algorithm, whose cross-validation relied on a leave-one-subject-out procedure. Results demonstrate that our framework allows to finely characterize affective olfactory elicitation with an average recognition accuracy of 71.88%. Feature selection highlighted that the most discriminating power was contributed by instantaneous LF power, instantaneous Lyapunov exponents, and instantaneous approximate entropy.