Open any college neuroscience book to the chapter on sensory perception and you’ll quickly learn that you can’t tickle yourself, no matter how hard you try (and chances are, upon reading this, you will try). As this phenomenon suggests, the brain makes a distinction between self-touch and touch from other people (other-touch) or from objects.
Now, recent work from Håkan Olausson, Linköping University, Sweden, and colleagues enhances the understanding of what happens in the nervous system during these different forms of touch sensation.
Using a brain imaging method called functional magnetic resonance imaging (fMRI) to scan brain activity in healthy human volunteers, the researchers saw that areas of the brain involved with processing sensations from the body, social information and emotional context became strongly activated in response to touch from another person. But those same regions showed a profound pattern of deactivation when study subjects touched themselves.
Meanwhile, experiments using electrodes to examine electrical activity in the nervous system in response to touch also revealed this self- vs. other-touch distinction, both at the level of the spinal cord and the brain.
While the study looked only at touch, the researchers believe their findings may also have implications for the study of pain.
“It is a very nice collection of experiments that show a quite dramatic difference in brain response between self-touch and other-touch,” said Joel Greenspan, a pain researcher at the University of Maryland, US, who did not take part in the study. “They captured the neural mechanisms behind the two in a really meaningful way,” he said, referring to how the nervous system works to produce such a difference.
The research was published February 5, 2019 in the journal Proceedings of the National Academy of Sciences.
Different touch experiences, different brain patterns
First author Rebecca Böhme, Linköping University, Sweden, said that she and her colleagues set out to understand what takes place in the nervous system during different kinds of touch, a sensation that is crucial from an early age.
“We know that touch is very important to development. Newborn babies learn about the world and their bodies when they are touched by their parents, and these social touches help them develop a concept of self,” Böhme explained. “But we also know that there is a big difference between when we touch ourselves and when others touch us. Even if the stimulus is the same—a hand stroking the same place with the same pressure—our brains know that it is different. And we haven’t investigated the neural mechanisms behind that.”
To learn more, Böhme and colleagues recruited healthy human volunteers to undergo fMRI while they stroked their own forearm with two fingers (self-touch), as if stroking someone they liked, or while they had their forearm stroked by the experimenter (other-touch).
The investigators found significant differences in fMRI activity between the other-touch and self-touch conditions. In the former, they saw activation in areas known to play a role in bodily sensation and in regions that play a role in how the brain manages interactions with people and social situations (this is known as social cognition).
In contrast, when participants touched themselves, there was a widespread pattern of deactivation, both in the social cognition areas but also in regions implicated in processing bodily sensations like touch.
“I was surprised that the deactivation pattern was so distinct. Often, in fMRI studies, you get small differences, but the magnitude here was quite strong,” said Böhme.
Effects of brain deactivation on perception
In a second study, the researchers wanted to learn whether the deactivation seen in the self-touch condition had any impact on participant’s perceptual abilities.
Of particular note, they used von Frey fibers, which are thin filaments commonly used in pain studies, to stimulate the forearm during each of the touch conditions. They discovered that when participants were engaged in self-touch, it required 100 times the force to detect even the weakest filament. This increase in the so-called detection threshold was higher than the force needed to activate pain-sensing neurons.
Finally, in a third study, the researchers stimulated the base of the thumb, to target a nerve in the arm called the radial nerve, using an electrode that delivered non-painful pulses during the touch conditions. While doing so, they recorded electrical activity using electrodes at the level of the spinal cord and on the scalp.
The researchers saw lower readings from the recording electrodes during self-touch at the brain level, meaning there was less brain activity in response to that form of touch, compared to the other-touch condition. The researchers say this is in line with the widespread brain deactivation they saw in their fMRI experiments.
“Taken together, all of these results suggest that, when someone else touches us, we need to understand the context and the brain has to do a lot of work to do that,” she said. “But the brain has no need of this information when we touch ourselves so we see this kind of deactivation across the brain.”
Self-touch as a form of anesthetization?
Greenspan said the study was “well thought out” and looks forward to seeing follow-up work.
“The perceptual findings were really intriguing,” he said. “But the most striking thing is just how different the fMRI results are across the whole brain between self-touch and other-touch. Even though these touches may not seem all that different to us, it’s clear that the nervous system certainly sees them as something very distinct from one another.”
Böhme believes, given the distinct brain deactivation pattern during self-touch, along with the fact that it takes 100 times more pressure to notice a von Frey filament while engaged in self-touch, her findings may have relevance to pain.
“When we bump our arm against a table, we instinctively go and rub on that spot,” she said. “With this difference in self-touch and other-touch, it’s possible that when we rub on that painful spot, the brain deactivation created by the self-touch helps to anesthetize it, lowering the feeling of pain. There are many questions we can try to answer after this study—there is still a great deal to understand.”
Kayt Sukel is a freelance writer based outside Houston, Texas.
This story first appeared on the Pain Research Forum and has been adapted for RELIEF.