Affective function of touch and the neurophysiological mechanism

Abstract

<p id="p00005">Touch is an important sensory channel for individuals to explore the external world in everyday life. The sense of touch helps us to discriminate the location of a stimulus on the skin surface, to identify the shape, size and texture of objects. However, touch can also be affective. The motivational-affective dimension of touch is involved in coding its valence and motivational relevance. This aspect of tactile sensation plays an important role in maintaining social bonding and promoting interpersonal communication. <p id="p00006">The affective function of interpersonal touch can be achieved through unimodal presentations or multimodal information integration processing. That is, tactile action itself, either a strong handshake or a tender hug, can directly convey emotions. Although the accuracy of decoding six basic emotions (anger, fear, happiness, sadness, disgust, and surprise) from interpersonal touch are slightly less than from the facial expression and vocal information, interpersonal touch has advantages in conveying social emotions such as "love", "gratitude" and "sympathy". Such advantages reflect the important significance of affective touch in establishing social bonding and promoting cooperative relationships. Further more, the affective meaning and social attributes carried by interpersonal touch can provide social background for emotion of other modal （e.g., visual or auditory） information processing, so as to enhance participants’ attention and to sharpen their social evaluation of emotional cues. <p id="p00007">Compared with sensory-discriminative subsystem, the motivational-affective subsystem has a special neurophysiological mechanism. For example, C-tactile (CT) afferents are strongly implicated as the neurobiological substrate underlying the affective property of touch. CTs, a class of slowly conducting mechanoreceptors, are specially tuned to the properties of human physical contact. They fire maximally to skin-temperature, light-pressure stroking at a rate of 1-10 cm per s. C-tactile-mediated affective tactile stimulation project in spinothalamic tract (STT) pathway (the spinal signaling of orofacial C-fiber mediated affective touch is still unclear), bypass the primary somatosensory cortex, directly project to the insular cortex, and then process in the amygdala, medial prefrontal cortex (mPFC), posterior superior temporal sulcus (pSTS) and other core areas of the “social brain” neural network. In addition, evidence from EEG research shows that, beta oscillation at parietal scalp sites, may be related to the affective representation of tactile stimuli, and theta oscillation at frontal, parietal, temporal, occipital scalp sites, may reflect an attentional-emotional regulatory mechanism of affective touch. <p id="p00008">Another complex facet of touch is that, affective touch would be complicated by its inextricable links to context, gender and sexuality, culture, and other individual, interpersonal, and societal factors. Moreover, learned from the previous research on facial emotion perception, a considerable body of work about affective touch are also around the topic of coding-decoding of basic emotions, so that the results fail to fully reveal the specificity of tactile channel in transmitting social emotions. Therefore, we should pay more attention to top-down contextual factors in the future researches, such as personal relationship, cultural difference, and social context, that may influence how to define and interpret the emotion and motivation of interpersonal touch. Meanwhile, although the time course of emotional touch perception was explored with electrophysiological measures, however no clear index has been identified till now. In addition, in order to reveal the relevance and independence between the sensory-discriminative and motivational-affective subsystems, such as activation likelihood estimate (ALE) meta-analysis and meta-analytic connectivity modeling (MACM) analysis can be used to reveal the cortical functional neuroanatomy supporting a distinction between affective and discriminative touch.