Physiological investigation on tactually perceived objects

Abstract

Touching objects not only allows us to perceive and discriminate their physical properties (e.g., roughness and softness), it may also evoke affective feelings such as that of pleasantness. There is considerable interest in the commonalities and differences of mechanisms between affective and discriminative aspects of touch. Previous psychophysical studies have demonstrated that physical characteristics of object properties similarly affect affective and discriminative touch, highlighting the common mechanisms underlying the two aspects of touch. However, because explicit measures such as subjective ratings can involve self-report biases, it is unclear about the nature of implicit processing underlying these types of touch. The present study investigated the psychophysical relationship between pleasantness and perceived softness through both implicit and explicit measures. Polyurethane rubber stimuli of different compliances were pressed onto participants’ fingertips at two different forces. Two groups of participants made numerical estimations of the perceived magnitude of either pleasantness or softness while target muscle activity was recorded through non-invasive facial electromyography (EMG). Perceived magnitudes of both pleasantness and softness monotonically increased as a function of increasing object compliance. However, significant changes in implicit facial EMG responses were only found when participants were instructed to evaluate pleasantness associated with the contact with stimuli. These results indicate that contrary to explicit measures, the implicit processing of stimuli is different between pleasantness and perceived softness and that there might be partially distinct mechanisms between affective and discriminative touch. Possible explanations for the findings are discussed.