The sensation of gentle touch is conveyed by a specific type of neuron – tactile C (CT) afferents – found only in hairy skin. These neurons respond most strongly to soft touches and “caress-like” speeds, and send signals to a brain region called the posterior insula, which helps interpret bodily sensations like pain, warmth or cold, heartbeat, a full bladder or stomach, and balance – and also, interestingly enough, the physical feelings associated with music, laughter, and empathy.
What this new study explores is the insula’s response when volunteers observe another person being caressed. A team led by India Morrison of the Institute of Neuroscience and Physiology at the University of Gothenburg’s Sahlgrenska Academy began by searching for an optimal stroking speed for triggering CT and insula activity – they found that stroking the subjects’ arms at a speed of about three cm/s provoked the strongest response in the insula:
A speed optimal for eliciting CT discharge (3 cm/s) also gives rise to higher BOLD responses in posterior insula than a nonoptimal speed (30 cm/s).
The next step was to study fMRI scans of the volunteers’ brains as the subjects watched videos of another person’s arm being stroked at various speeds. Again, videos showing a stroking speed of about three cm/s provoked the strongest insular response:
When participants viewed videos of others’ arms being stroked at CT-optimal versus -nonoptimal speeds, the posterior insula showed a similar response as to directly felt touch.
Interestingly, the insula seems to respond most strongly to videos depicting social touch, as opposed to “nonsocial dynamic-touch videos.” (No word yet on what the insula thinks of Facebook pokes.) The authors use this observation about social touch to bring up an intriguing point about why our insula might respond more strongly to some caresses than to others:
Such selective tuning for CT-optimal signals in insula may allow recognition of the hedonic relevance of a merely observed caress.
In short, our neurons and brains may be finely tuned to recognize exactly which caresses “mean” what.
On the whole, these results aren’t particularly shocking – after all, we all know that erotica is popular because (at least on some level) it works. Going by that example, it seems that even reading about a caress, or imagining one, might trigger similar insular responses.
What makes this especially interesting, though, is that even when we’re not physically feeling a sensation of touch, our brains are still tuned to respond most strongly to a specific pressure and stroking speed on certain body parts – or to the idea of that specific pressure and speed on those parts. Writers of erotic fiction, take note.
This discussion also awakens the dragon of the ongoing and fiercely fought mirror neuron debate. Without going too tangential here, the basic idea is that some studies seem to suggest the existence of “mirror neuron” groups, which are activated not only when we perform an action, but also when we see it being performed, or even when we hear it being performed in another room. While the system has been invoked to explain everything from task-learning to language acquisition to empathy, mirror neurons (if they actually exist in human brains at all) don’t seem to be essential for any of those tasks.
It may be that, rather than using a specific “mirror neuron” system to model perceived actions, our brains generally respond to perceived and experienced events in roughly the same way, and make the distinction of “self/other” at some point along that process. This seems, to me to offer a sort of Occam’s razor to explain why movies can seem so real, songs can feel like they’re about us, and stories of another person’s pain or pleasure can give us vicarious sensations – which, nonetheless, never feel quite as real as actual physical ones.
I think it’ll be fun to try some experiments with my friends – they can watch while I gently stoke my own arm, then compare that feeling to the one they experience when I touch theirs. Strokin’ for Science!