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Sensing and Feeling: Part 1

Updated: Jul 1

By Laura Rathbone

This month we have split one paper into two. This blog is in reference to the first half the paper:

Specifically exploring: pages 737 - 744

The Complex Nature of Touch: Sensing, Feeling, and Social Significance

This month’s them has rather beautifully increased my awareness of all the gentle, caring touch i give and receive throughout a usual day. Hundreds of moments of touch happen for me, some days less and some days more, with everyone from the gentle stroking of my kid’s back as they sit next to me reading, the exquisite warm, softness of my cat’s fur as I stroke her back, or she glides her side against my leg, the gentle hug of a friend, the warm and firm handshake of a stranger (no longer) and the time-stopping moment-savouring way my youngest rests her hand on my leg when she sits next to me on the couch….

Touch is hard to discuss and define - a bit like pain really - it’s easily reified, and if we aren’t intentional, the discussions can quickly become dehumanised, or non-scientific.

The first part of this paper is focusing on getting us up to speed on the authors’ interpretation of the available literature. The interpretive framework of biomedicalism and reductionism can be quite challenging to navigate as clinicians who deal with human experiences, not sensory mechanisms…so hold it lightly.

The term "reification" refers to the process of treating abstract concepts or ideas as if they were concrete and tangible entities. It involves giving physical form or existence to something that is abstract or conceptual.

The perception of touch is a complex aspect of experience for all sentient beings and indeed we see the importance of affective touch across the animal species. It encompasses both discriminative and affective (affective means - arising from, or influencing feelings or emotions) components, allowing us to perceive and most importantly interpret various tactile stimuli.

The paper that we are reading this month takes a traditional input-output approach to describing some of the causally relevant physiological events involved in the perception of affective touch, but I am aware that many of our Pain Geeks have already updated their physiological modelling to a Predictive Processing model, a quick note I try to remember is that within the PP model generally the direction of processing is reversed to prediction weighed against error - so the post-synaptic terminal would be holding the prediction and what is measured against that prediction is what binds to it, if it is as expected, then no further activation is required. (Watch out for little notes in brackets beginning with PP).

When reading a paper like this, I try to hold both narratives alongside each other and remember that:

Sensation: relates to the process of nerve conduction, transmission and modulation

Perception: is the consciously experienced phenomenon that includes sensation, but is not equal to sensation.

As with all things, ‘Touch’ is not just one thing. Touch has been broken down into many categories and sub-catagories mostly relating the psychophysical property (the connection between stimulus and the perception of the stimulus) which allows for narratives around biomedical reasoning and input-output modelling to be easier to define. For example: discriminative touch enables us to identify and localise sensory input. Discriminative touch is thought to be more closely linked to sensory information (impulses) carried via the Ab neuronal system (large diameter, myelinated mechanosensitive).

(PP: Sensory Input could be considered error weighed against the prediction)

Affective touch on the other hand is the kind of touch that gives rise to (usually) pleasant experiences and typically used to describe slowly moving, low-force mechanical stimulation which is often reported as pleasant and should be distinct from pain or itch.

C-Tactile afferents are a type of sensory nerve that are activated by low-force touch and conduct impulses along the fibre, they are typically associated with affective touch because of their phenotype.

(C tells us that they are a type of C-fibre so they are small diameter and un-myelinated axons, afferent means that the information is travelling up the nerve to the spinal cord) *check out Table 1 exported from the article. 

The perception of affective touch varies not only between individuals but also across contexts. The term affective touch is typically used to describe slowly moving, low-force mechanical stimulation which is often reported as pleasant and should be distinct from pain or itch. Much like defining pain though, defining pleasant touch is a tricky business.

*Why not try it for yourself? Close your eyes and gently the back of your arm. Try to get a sense of what it feels like and how you would define it. How would it be if it was carried out by someone you love, versus someone you are afraid of? Affective touch can be pleasant of unpleasant - but in usual circumstances would not be painful (we would call it Allodynia if it was painful).

The role of CT afferents extends beyond perception and into the realm of autonomic regulation. Although individuals with neuropathy may struggle to provide concise descriptions of CT-aimed stimuli, they still exhibit a sympathetic skin response when exposed to it. This suggests that the CT system can be causally relevant in local autonomic events despite its seemingly weak perceptual quality.

In terms of spinal processing, CT afferents project to inner lamina II of the spinal dorsal horn. Here, they interact with spinal neurons that are phenotypically specialised to the CT-activation. Lamina II receives inputs solely from un-myelinated afferents (but not solely from C-Tactile fibres! Check out the other un-myelinated nerves hanging out there! Remember the Pain gate Theory by Melzack and Wall?). These neurons transmit tactile information carried by C fiber mechanoreceptors to the brain.

Taken from the Article - a quick reminder of your fibre types!

From the spinal cord, CT-mediated signals involved in the perception of touch reach various cortical regions thought be causally relevant in affective processing. Soft brush stroking on hairy skin can correlate (under the right circumstances) to a relative-increase in BOLD signal within the areas S1 and S2 traditionally termed the somatosensory processing areas, as well as the posterior contralateral insular cortex. The insular cortex, can also demonstrate a relative increase in BOLD signal which has led some researchers to reify this area as a gateway from sensory systems to emotional systems. Additionally, the posterior superior temporal sulcus and the medial prefrontal cortex have also been implicated in CT-targeted touch processing. These cortical areas, may play a contributing role in the emotional and hedonic aspects of touch perception.

fMRI studies are limited and it is not yet possible to establish causation between increased ‘relative’ blood flow to one region and a perceptual experience. However, this is often what it is stated in the literature.

And yet the story is of course much much more complex. CT-fibre afferents fire at low thresholds but are still slower in conduction speed than Ab fibres due to their morphology. When a CT-fibres is activated, so to are other fibres of somatosensory processing, including the Ab fibres. This complex relationship in activation and information carrying is still not fully understood in terms of perception, and much work within neuroscience, cognitive neuroscience and neurophilosophy is to be done.

It is emerging, however, that CT-mediated experiences of touch have significant implications for social interactions. Gentle touch has been shown to lower stress levels and increase pain thresholds. It plays a crucial role in development, counselling sessions, intimacy and cognitive processes. How and why and to what extent these fibres are causally relevant or implicated in the perception of touch is yet to be explored in part 2, and the wider research field in general.

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