Touch in infancy and adolescence teaches our brain networks what we value

Brain networks have patternmatching layers that make predictions and sense prediction errors. The anatomy shown here for vision has a counterpart for touch.

…the hierarchical neuronal message passing that underlies predictive coding.

…neuronal activity encodes expectations about the causes of sensory input, where these expectations minimize prediction error. Prediction error is the difference between (ascending) sensory input and (descending) predictions of that input.

On the left: this schematic shows a simple cortical hierarchy with ascending prediction errors and descending predictions.

On the right: this provides a schematic example in the visual system.[1]

Touch is crucial to emotion

Affective touch may… convey information about available social resources…[2]

interoception… refers to the perception and integration of autonomic, hormonal, visceral and immunological signals…—or more informally as the sense of the body ‘from within’.

…we propose that emotional content is determined by beliefs (i.e. posterior expectations) about the causes of interoceptive signals across multiple hierarchical levels.

Emotion produces conscious experience

It is tempting to speculate that deep expectations at higher levels of the neuronal hierarchy are candidates for—or correlates of—conscious experience, largely because their predictions are domain general and can therefore be articulated (through autonomic or motor reflexes).

…interoceptive predictions can perform physiological homoeostasis by enlisting autonomic reflexes… More specifically, descending predictions provide a homoeostatic set-point against which primary (interoceptive) afferents can be compared. The resulting prediction error then drives sympathetic or parasympathetic effector systems to ensure homoeostasis or allostasis, for example, sympathetic smooth-muscle vasodilatation as a reflexive response to the predicted interoceptive consequences of ‘blushing with embarrassment’.

Touch is central to selfhood and boundaries

…experiences of selfhood unfold across many partially independent and partially overlapping levels of description… A simple classification, from ‘low’ to ‘high’ levels, would range

  • from experiences of being and having a body…,
  • through to the experience of perceiving the world from a particular point of view (a first person perspective, …),
  • to experiences of intention and agency…,
  • and at higher levels the experience of being a continuous self over time (a ‘narrative’ self or ‘I’ that depends on episodic autobiographical memory,… )
  • and finally, a social self, in which my experience of being ‘me’ is shaped by how I perceive others’ perceptions of me…

In this putative classification, interoception plays a key role in structuring experiences of ‘being and having a body’ (i.e. embodied selfhood) and may also shape selfhood at other, hierarchically higher levels.[1]

The emotions we feel are largely predictions based on past experiences

…interoceptive inference involves hierarchically cascading top-down interoceptive predictions that counterflow with bottom-up interoceptive prediction errors. Subjective feeling states – experienced emotions – are hypothesized to depend on the integrated content of these predictive representations across multiple levels…[3]

Intuition suggests that perception follows sensation and therefore bodily feelings originate in the body. However, recent evidence goes against this logic: interoceptive experience may largely reflect limbic predictions about the expected state of the body that are constrained by ascending visceral sensations.[4]

Reward and motivation are predicted, based on emotions that are predicted, based on touch that was experienced previously

Reward is a complex construct comprised of a feeling and an action. Components of reward include the hedonic aspects, i.e. the degree to which a stimulus is associated with pleasure, and the incentive motivational aspects, i.e. the degree to which a stimulus induces an action towards obtaining it… Typically, the feeling is described as “pleasurable” or “positive” and the actions comprise behavior aimed to approach the stimulus that is associated with reward.[5]

…the representation of self is constructed from early development through continuous integrative representation of biological data from the body, to form the basis for those aspects of conscious awareness grounded on the subjective sense of being a unique individual.

Interoception refers to the sensing of the internal state of one’s body. …interoception… is proposed to be fundamental to motivation, emotion (affective feelings and behaviours), social cognition and self-awareness.[6]

  1. Seth, Anil K., and Karl J. Friston. “Active interoceptive inference and the emotional brain.”  Trans. R. Soc. B 371.1708 (2016): 20160007.
  2. Krahé, Charlotte, et al. “Affective touch and attachment style modulate pain: a laser-evoked potentials study.”  Trans. R. Soc. B 371.1708 (2016): 20160009.
  3. Seth, Anil K. “Interoceptive inference, emotion, and the embodied self.” Trends in cognitive sciences 17.11 (2013): 565-573.
  4. Barrett, Lisa Feldman, and W. Kyle Simmons. “Interoceptive predictions in the brain.” Nature Reviews Neuroscience 16.7 (2015): 419-429.
  5. Paulus, Martin P., and Jennifer L. Stewart. “Interoception and drug addiction.” Neuropharmacology 76 (2014): 342-350.
  6. Tsakiris, Manos, and Hugo Critchley. “Interoception beyond homeostasis: affect, cognition and mental health.Philosophical Transactions B: Biological Sciences 371.1708 (2016): 20160002.

Pleasant touch is best when it’s warm, close, soft, and slow

A pleasant touch to his head makes a dog squint and smile.

CT afferents sense pleasant touch

Unmyelinated afferents responding to light touch were first described in furry animals in 1939. In humans, a similar type of afferents was identified about 50 years later (CT, C tactile).

…the role of CT afferents… is to boost the feeling of pleasantness when touched by a friendly human being.

CT afferents are substantially more responsive to an initial touch stimulus than to succeeding stimuli. It may be speculated that the system is designed to make us more inclined to appreciate a friendly touch when first perceived.

The system would be of significance in supporting feelings of pleasure (reward), confidence, comfort, and security as you are close to your parents, lover, kin, or friends. Moreover, it may have a role in hormonal responses as well as in bonding individuals emotionally together.

Pleasant touch feels best at body temperature 

… afterdischarge in CT afferents is highly dependent on temperature, as it is more frequent and more prominent at lower than neutral temperatures. In a sample of 15 CT units, afterdischarge was seen in 80 % at 15 °C, but in only 13 % at… 32 and 42 °C…

… subjects’ feelings of pleasantness were optimal when the temperature of the moving object was neutral…

Pleasant touch is stronger when it’s closer in

CTs are abundant in the hairy skin of the human body, scarcer in the distal parts of the extremities and seem to be lacking altogether in the glabrous skin.

The pad skin of furry animals which is the homologue of human glabrous skin was primarily designed to take the wear and tear of walking and running.

Pleasant touch feels best when it’s soft

For slow to medium-velocity touch in the left graph, soft touch (open circles) and harder touch (filled circles) produced nearly the same CT response impulse rate. Even so, for slow to medium-velocity touch in the center graph and in the right graph, soft touch was rated as much more pleasant.

The most pleasant touch to CT afferents is relatively slow speed and is soft.

Pleasant touch feels best when it’s slow

… human CT afferents are tuned to a relatively slow speed of movements across the skin…

…in CT afferents maximal rate occurs at a fairly low speed (about 1–3 cm s −1 [0.4 to 1 inch per second]) which corresponds fairly well to human caressing movements.

… subjects’ estimates of pleasantness… are similarly tuned to the speed of movement…

… a prominent and sometimes long-lasting afterdischarge is seen in CTs…[2]

  1. Minette. “Not all Dogs Like Being Petted.” 12 Nov. 2012, Accessed on 5 Nov. 2016.
  2. Vallbo, Åke, Line Löken, and Johan Wessberg. “Sensual Touch: A Slow Touch System Revealed with Microneurography.” Affective Touch and the Neurophysiology of CT Afferents. Springer New York, 2016. 1-30.