Discriminative touch and emotional touch.

Abstract

Somatic sensation comprises four main modalities, each relaying tactile, thermal, painful, or pruritic (itch) information to the central nervous system. These input channels can be further classified as subserving a sensory function of spatial and temporal localization, discrimination, and provision of essential information for controlling and guiding exploratory tactile behaviours, and an affective function that is widely recognized as providing the afferent neural input driving the subjective experience of pain, but not so widely recognized as also providing the subjective experience of affiliative or emotional somatic pleasure of touch. The discriminative properties of tactile sensation are mediated by a class of fast-conducting myelinated peripheral nerve fibres--A-beta fibres--whereas the rewarding, emotional properties of touch are hypothesized to be mediated by a class of unmyelinated peripheral nerve fibres--CT afferents (C tactile)--that have biophysical, electrophysiological, neurobiological, and anatomical properties that drive the temporally delayed emotional somatic system. CT afferents have not been found in the glabrous skin of the hand in spite of numerous electrophysiological explorations of this area. Hence, it seems reasonable to conclude that they are lacking in the glabrous skin. A full understanding of the behavioural and affective consequences of the differential innervation of CT afferents awaits a fuller understanding of their function.

The development and validation of sensory and emotional scales of touch perception

“…Such a result is further consistent with a hypothesis that low-threshold Ctactile (CT) mechanoreceptors are important in affective touch (Liu et al, 2007;Löken et al, 2009;McGlone et al, 2007;Reinisch & Tschachler, 2005;Vallbo et al, 1999;Vallbo et al, 1993;Wessberg et al, 2003), given that hairy skin (e.g., of the forearm) is innervated by such fibers, but glabrous skin (e.g., of the fingerpad) is not (Liu et al, 2007;Reinisch & Tschachler, 2005). However, stimulation of the glabrous fingerpad led to a greater emotional response than stimulation of either of the axillary sites, which suggests that the presence of CT afferents per se is not sufficient for stimulation of that site to elicit an especially high affective response.…”

“…In contrast, the forearm is less sensitive in this regard, but stimulation can evoke a large affective response (Essick et al, 2010;Guest et al, 2009;Löken, Wessberg, Morrison, McGlone, & Olausson, 2009). Hairy skin, such as that of the forearm, is innervated by low-threshold C-tactile (CT) mechanoreceptors that are important in conveying affective touch (Liu et al, 2007;Löken, et al, 2009;McGlone, Vallbo, Olausson, Loken, & Wessberg, 2007;Vallbo, Olausson, & Wessberg, 1999;Vallbo, Olausson, Wessberg, & Norrsell, 1993;Wessberg, Olausson, Fernström, & Vallbo, 2003). In contrast, glabrous skin (e.g., of the fingerpad) lacks CT afferents (Liu et al, 2007;Reinisch & Tschachler, 2005).…”

Primary somatosensory cortex discriminates affective significance in social touch

“…However, the SI modulation is foreshadowed by a change in activity pattern, detectable by using multivoxel pattern classification in the 4 s preceding the likely onset of the caress. The visually triggered modulation of touch showed trial-by-trial correlation with pleasantness ratings and was significant within SI but not in the insula or ACC, areas previously thought to mediate this effect (14,15,63), and it seems to be independent of the modulation in the OFC, as suggested by a lack of functional connectivity between OFC and SI in our experiment. By showing that SI, a region classically associated with the processing of sensory properties of touch, has activity correlated with pleasantness, our data suggest that sensory and affective properties of touch are processed by partially overlapping neural systems.…”

“…Differences in brain activation in the insula, anterior cingulate cortex (ACC), and orbitofrontal cortex (OFC) have been found according to stimulus type (e.g., a wooden rod vs. a velvet cloth) (11) and subjective ratings of emotion (12,13). Thus, previous work has shown that SI primarily discriminates sensory properties (e.g., location, pressure, texture), whereas the insula, together with the ACC and the OFC, primarily discriminate affective/emotional properties (i.e., perceived pleasantness) (7,11,14,15). Although some models propose an interaction between these systems, they propose that SI's role in affective processing is probably indirect: (i) SI is modulated by sensory properties of touch, such as the velocity [e.g., 3 cm/s vs. 30cm/s (16)] or location [e.g., arm vs. palm (17)], that happen also to modulate the pleasantness of the caress by differentially recruiting CT fibers; or (ii) affective properties are first processed in the insula, which in turn modulates responses in SI/secondary somatosensory cortex (SII) (7).…”

Haptic perception: A tutorial

“…In keeping with recent trends in other fields, the science of touch has also begun to focus on understanding affective aspects of this modality, such as pleasantness and emotional expression. For example, neuroscientists and psychophysicists have recently hypothesized that the rewarding, emotional aspects of touch may be subserved by a class of unmyelinated peripheral nerve fibers known as CT (or C tactile) afferents that are found in hairy, but not glabrous (hairless), skin (Löken, Wessberg, Morrison, McGlone, & Olausson, 2009;McGlone, Vallbo, Olausson, Löken, & Wessberg, 2007;Olausson et al, 2002).…”