Abstract

This functional MRI experiment uses the alliesthesia phenomenon to map the brain correlates of hedonic experience and sensation. Alliesthesia refers to the dependence of the hedonic impact of a stimulus on the ‘milieu interne’. Thus, a thermal stimulus applied to the skin during hyperthermia or hypothermia should be perceived as having the same temperature but opposite hedonic impact. Twenty participants took part in two successive scanning sessions during which mild states of hyperthermia and hypothermia were induced. Participants wore a water-perfused suit consisting of a long-sleeved shirt and trousers. In two of the stimulus conditions, either cold or hot water perfused the suit; in the third ‘neutral’ condition, water flow was interrupted. At intervals of 9 seconds, the participants rated, on two Likert scales, the thermal sensation perceived (from hot to cold) and the corresponding hedonic experience (from unpleasant to pleasant).Three different regressors were built from the ratings: hedonic, homeostatic, and sensory. The hedonic regressor was derived from the pleasantness ratings, whereas the sensory regressor was derived from the temperature ratings. The homeostatic regressor was also derived from the temperature ratings, but the sign was adjusted to conform to the value that would oppose the deviation of core temperature. Application of the hedonic and homeostatic regressors revealed conjoint activations in the more lateral areas of the orbitofrontal cortex. Application of the sensory regressor revealed clusters of activation, mainly in the middle frontal gyrus, the superior frontal gyrus, the precentral gyrus, and the right orbitofrontal cortex.