On the pleasantness of a haptic stimulation: how different textures can be recognized through heart rate variability nonlinear analysis.

The hedonic attributes of cutaneous elicitation play a crucial role in everyday life, influencing our behavior and psychophysical state. However, the correlation between such a hedonic aspect of touch and the Autonomic Nervous System (ANS)-related physiological response, which is intimately connected to emotions, still needs to be deeply investigated. This study reports on caress-like stimuli conveyed to the forearm of 32 healthy subjects through different fabrics actuated by a haptic device, which can control both the strength (i.e. the normal force exerted by the fabric) and the velocity of the elicitation. The mimicked caresses were elicited with a fixed force of 6 N, two levels of velocity of 9.4 mm/sec and 65 mm/sec, and four different fabrics with different textures: burlap, hemp, velvet and silk. Participants were asked to score the caress-like stimuli in terms of arousal and valence through a self-assessment questionnaire. Heartbeat data related to the perceived most pleasant (silk) and unpleasant (burlap) fabrics were used as an input to an automatic pattern recognition procedure. Accordingly, considering gender differences, support vector machines using features extracted from linear and nonlinear heartbeat dynamics showed a recognition accuracy of 84.38% (men) and 78.13% (women) while discerning between burlap and silk elicitations at the higher velocity. Results suggest that the fabrics used for the caress-like stimulation significantly affect the nonlinear cardiovascular dynamics, also showing differences according to gender.