Affective touch (C-tactile afferents)
Why the wiki cares: it is an interoceptive channel you can reach
The wiki’s recurring structural complaint — recorded on the roadmap, the validity debate, interoceptive-psychopathology and now Quigley et al. (2021) — is that human interoception research is correlational because its afferents cannot be stimulated. They are thin, unmyelinated, and diffusely arrayed; standard peripheral nerve stimulation does not work on them.
CT afferents have all three of those properties and are in the skin. That is the whole proposal: a fibre population with interoceptive anatomy that sits somewhere accessible, so it can be stimulated with a brush and recorded with microneurography in awake humans.
If the identification holds, the wiki’s correlational literature acquires a manipulation.
The evidence for calling it interoceptive
- Tuning. CT fibres are preferentially activated by stroking at a force, velocity and temperature “similar to that typically used when primates stroke one another” (McGlone et al. 2014; Ackerley et al. 2014 for the temperature tuning — they are tuned to skin-temperature caress). Stroking that activates them is rated affectively pleasant.
- Fibre morphology. Thin, unmyelinated, diffusely arrayed — “many features consistent with those of interoceptive afferents.”
- Central projection, and this is the load-bearing one. In a patient with selective loss of large-diameter myelinated afferents in hairy skin, activating the intact CT fibres activated dorsal posterior insula — like other interoceptive afferents — and did not activate S1 or S2, as exteroceptive input would (Olausson et al. 2002). See insular-cortex.
That third item is a single patient. The wiki should hold it as such: the anatomical warrant for the entire proposal is an n = 1 dissociation from 2002, and the review resting on it is by the authors who proposed the application (Burleson & Quigley 2019; Cascio et al. 2019).
It also breaks the wiki’s central dichotomy
Hairy skin co-mingles myelinated and unmyelinated afferents. So a caress is one stimulus delivering discriminative (exteroceptive) and affective (interoceptive) signals simultaneously, from the same patch of skin. Quigley et al. use this as their clearest illustration of multisensory-integration across the boundary, and note that it means touch “can easily blur the classical distinction between interoceptive and exteroceptive processes.”
This is not new to the wiki so much as newly mechanized. Craig (2002) already put sensual touch inside interoception when he widened Sherrington’s taxonomy — the wiki’s interoception page has carried it in the list since the first ingest. Craig’s grounds were the lamina I pathway and the insular destination; CT afferents are the peripheral half of the same argument, with the fibre population named.
What it connects to
- social-origins-of-interoception: a fibre system tuned to grooming parameters is a fibre system for being touched by another mammal. If interoception is assembled through caregiver interaction, a dedicated afferent channel for caregiver contact is the kind of thing that account needs — and it is anatomical, not developmental, so it sits awkwardly with the strong “social not biological” reading. See social-vs-biological-origins-of-interoception.
- core-affect: pleasantness here is reported as a property of the afferent channel, not of a conceptualization. Whether that is a problem for the constructionist reading depends on whether “rated pleasant” is being read as valence in the core-affect sense; the wiki should not resolve it from this source, which asserts the rating and does not theorize it.
Status
A proposal, not a programme. Quigley et al. present affective touch as a “potentially promising way” to study interoceptive signalling in humans — future work, in the same breath as the Outstanding Questions. The wiki holds no study that has actually used CT stimulation as an interoceptive manipulation. Until one is ingested, this page records an unusually well-motivated idea and the single patient it rests on.