Being gently caressed by another person
is both a physical and an emotional experience. But the way we are
touched and the reaction this elicits in the brain are a science of
their own.
Researchers from the Institute of
Neuroscience and Physiology at the University of Gothenburg’s
Sahlgrenska Academy have studied how the brain reacts to caresses.
Volunteers were given MRI scans to measure blood flows in the brain
while being stroked either slowly or quickly with a soft brush.
Our perception of a caress, whether felt
or seen, may thus be constrained by specific, affectively relevant
features of touch.
One candidate is the speed at which a touch travels over the skin: a
typical caress involves light, dynamic stroking that neither moves
too slowly nor too quickly. This is supported by the recent
discovery that human CT (tactile C) afferent fibers are sensitive to
a caress-like range of stroking speeds associated with positively
hedonic subjective responses.
CT afferents are slow-conducting, unmyelinated, low-threshold
mechanoreceptive nerve fibers that carry signals from the receptive
fields in the epidermis of mammalian hairy skin.
When forearm skin is gently stroked at
speeds consistent with a social caress (1–10 cm/s), the CT afferents
of human volunteers fire vigorously, decreasing when the skin is
stroked at faster or slower speeds. This velocity-sensitive CT
response is tightly correlated with subjective ratings of how
pleasant the touch feels. Human CT pathways ultimately target the
insular cortex, a region associated with emotion and homeostatic
balance.
Not unexpectedly, the brain reacted most
strongly to the slow strokes. More surprising results emerged when
the volunteers instead watched videos of another person being
caressed.
"The aim was to understand how the brain
processes information from sensual contact, and it turned out that
the brain was activated just as quickly when the volunteers got to
watch someone else being caressed as when they were being caressed
themselves," says India Morrison, one of the researchers behind the
study. “Even when we are only watching sensual skin contact, we can
experience its emotional meaning without actually feeling the touch
directly."
As a comparison, the volunteers also got
to watch a video where a hand caresses an inanimate object, and in
this case the brain was not activated anywhere near as strongly.
“These results indicate that our brain
is wired in such a way that we can feel and process other people’s
sensations, which could open up new ways of studying how we create
empathy," says Morrison. “It's important for us as people to
understand the significance of different types of touch – to know
whether two people are in a relationship or are about to start a
fight.”
The study has been published in the
Journal of Neuroscience.
For more information
The Journal of Neuroscience
"Vicarious Responses to Social Touch in Posterior Insular Cortex Are
Tuned to Pleasant Caressing Speeds"
Authors: India Morrison, Malin Björnsdotter, and Håkan Olausson.
http://www.jneurosci.org/content/31/26/9554.abstract
(MDN) |