Hedonic representations in primary somatosensory cortices; Pain but not pleasure?
Emotion is typically understood to be an internal subjective experience originating in the brain. Yet in the somatosensory system hedonic information is coded by mechanoreceptors at the point of sensory contact before it reaches the central nervous system. It remains unknown, however, how these distinct peripheral channels for tactile hedonic information contribute to representations of interoceptive states relative to exteroceptive experience. In this fMRI study we applied representational similarity analyses with pattern component modeling, a technique that deconstructs representational states into a weighted set of distinct predefined constructs, to dissociate how discriminatory vs. hedonic tactile information, carried by A- and C-/CT-fibers respectively, contributes to population code representations in the human brain. Results demonstrated that information about appetitive and aversive tactile sensation is represented separately from non-hedonic tactile information across cortical structures. Specifically, although hedonic touch originates as a peripheral signal, labeled at the point of contact, representations in somatosensory cortices are guided by experiences of non-hedonic touch, By contrast, representations in regions associated with interoception and affect encode signals of hedonic touch. This provides evidence of complex tactile encoding that involves both external-exteroceptive and internal-interoceptive dimensions. Importantly, hedonic touch contributes to representations of internal state as well as those of externally generated stimulation.