Dottore Alberto Gallace of the Università di Milano-Bicocca is a very clever guy. He spoke at the first PainAdelaide meeting (I recall he spoke at length about drinking beer with Lorimer Moseley), he’s published a bunch of papers with other very clever people and is the multisensory processing section editor at bodyinmind.org.
The OUPblog has a nice interview with Dottore Gallace talking about his research into human touch.
“To my students I often say, where our touch begins, we are. I wanted to understand more of these topics. I wanted to compare touch with other sensory modalities. In doing that I was convinced that research on touch had to get away from the fingertips or hands and extend to the whole body surface. The more I studied this sense, the more I became interested in it. For every question answered there were many more without responses. I like touch a lot because there are many things that still need to be understood about it, and I am a rather curious person, particularly when it comes to science.
I am not sure if it’s the most important development, but what I certainly consider important is the recent study of certain neural fibres specialized in transmitting socially-relevant information via the sense of touch. That is, the C tactile afferents in humans, that are strongly activated by ‘caress like’ stimuli, might play an important role in many of our most pleasant social experiences.”
“C Fibres” are often seen as the bad guys, and while we know that they are not “pain fibres”, they are frequently part of a discussion of pain in relation to their nociceptive properties.
But the C Tactile afferents that Dr Gallace mentions are a different kind of C fibre altogether. From a nice little, open access paper (however this one’s not authored by Dr Gallace):
“Human C-tactile (CT) afferents respond vigorously to gentle skin stroking and have gained attention for their importance in social touch. Pharmacogenetic activation of the mouse CT equivalent has positively reinforcing, anxiolytic effects, suggesting a role in grooming and affiliative behavior.
We show that CTs are unique among mechanoreceptive afferents: they discharged preferentially to slowly moving stimuli at a neutral (typical skin) temperature, rather than at the cooler or warmer stimulus temperatures
Furthermore, the CT firing frequency correlated with hedonic ratings to the same mechano-thermal stimulus only at the neutral stimulus temperature, where the stimuli were felt as pleasant at higher firing rates. We conclude that CT afferents are tuned to respond to tactile stimuli with the specific characteristics of a gentle caress delivered at typical skin temperature. This provides a peripheral mechanism for signaling pleasant skin-to-skin contact in humans, which promotes interpersonal touch and affiliative behaviour.”
I like the word hedonic. Defined as “relating to, characterized by, or considered in terms of pleasant sensations”, its one of those words that for me captures complex, multiple, sensual, intimate, tangible experiences like no other word can. It’s remarkably consistent across languages – the Germans make it their own with “hedonischen”, the French make it all fancy like with “hédonique” while the Estonians give it a nice ring with “hedoonilises”. Not to be confused with hedonism- “the pursuit of pleasure; sensual self-indulgence.” or this, a hedonic experience might be in many ways the very opposite of a pain experience.
I have had a number of psychologist friends that point out how lucky I am as a physiotherapist that I am allowed to touch people. Perhaps having this modality restricted in their interactions with people in trouble they have realised its power? I wonder if physio-/physical therapists take this a bit for granted? With so many physical therapy techniques being quite brisk, strong and robust, are we missing the opportunities to provide clients with a bit of a hedonic experience with some lighter, gentler touch? Activating those C Tactile afferents that are “tuned to respond to tactile stimuli with the specific characteristics of a gentle caress”? If you’re in the business of touching people for a living, is this worth considering? Do you/ how do you give clients a nice little hedonic buzz?