ASMR - a strange tingle in the head caused by certain sights and sounds - is a growing YouTube subculture. Now scientists are starting to investigate what the possible causes might be.
Just over a year ago I wrote a piece about ASMR, the nice warm buzzing or tingling feeling in the head that some people experience, and about the extraordinary online subculture it has spawned - in particular, the thousands of YouTube videos designed to trigger the sensation.
The videos include the sounds of whispering, crinkling, tapping or scratching, people painstakingly folding towels or taking things out of boxes, people giving each other head massages, and others pretending to be doctors, therapists, hairdressers or beauticians as they "give you an examination" or "apply make-up to your face".
Many people find them profoundly relaxing and say they help them to relieve stress and to sleep at night.
I wrote at the time that no-one was yet able to explain the mechanism that produces such a distinctive physical reaction from such a diverse range of stimuli, because no-one had yet researched the question.
One year on, and there's still no answer to what exactly happens in our bodies to trigger such a distinctive response - so pleasurable that some people call it a "braingasm" - from such mundane activities. But there are signs that serious scientists are now interested in finding out, braving the puzzled looks and occasional ridicule of those who don't themselves experience ASMR and think the whole business is slightly loopy. (Full disclosure: I experience ASMR but my wife, who doesn't, thinks I'm crackers.)
Earlier this year I travelled to Sheffield to meet four PhD students who are studying (in their own time) what happens to people who say their ASMR is triggered when they watch certain videos. Giulia Poerio, Tom Hostler and Emma Blakey all experience ASMR themselves (it stands for Autonomous Sensory Meridian Response - a scientific-sounding term coined by laymen). Theresa Veltri doesn't.
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In a bare conference room in Sheffield University's Department of Psychology I was wired up to a set of monitors designed to measure my heart rate, breathing, temperature and skin conductance. Then I watched three videos, while my body's responses were measured.
One video I'd chosen myself because it had in the past triggered my own ASMR - it was by Emma WhispersRed, the "ASMR artist" I'd interviewed for my earlier piece.
A second video was by Maria Gentle Whispering, the US-based woman and towel-folding expert who is the world's most popular ASMR video maker. The third showed a man teaching you how to make pasta - it was brusque and noisy, the very opposite of the quiet tone and slow pace of videos designed to induce ASMR.
I'm not sure I did very well in the test. Personally, I find many ASMR videos unconvincing and they don't work as triggers for me.
When we looked back at the results, the strongest reaction was to the pasta-maker, the anti-ASMR video - the other two left me rather cold. But I did get a very distinct buzz before watching the videos, as Theresa Veltri wired me up for the session, methodically attaching sensors to my fingertips and quietly talking me through the process as she did so.
That kind of "close personal attention" is a widely-reported ASMR trigger, and it's what the doctor/therapist/beautician "roleplay" videos try to replicate.
The students told me the study is meant to establish, firstly, that ASMR really does exist. "Anecdotal reports seem to suggest that ASMR is something that induces relaxation in people," Emma Blakey says. "So based on the physiological study that we're doing we would expect to see markers of relaxation in the physiology, like decreased heart rate and slower breathing rate" as people watch the videos.
The team, in other words, are looking for evidence that it's not all just in people's heads. "A lot of people, especially those who don't get it, find it really weird," says Tom Hostler. "And because a lot of the triggers are so mundane there is this scepticism."
The team don't have to look far for a sceptic. Veltri finds many of the videos frankly rather creepy - and rather sexual. "They're moving their hands slowly, and touching things, and they have these whispery voices," she says. "I can understand how that is relaxing, but when someone's touching phallic-like objects..."
Gloria Poerio says she can see why people might think the videos' appeal has something to do with sex - many involve attractive young women getting right up close to the viewer, often seeming to whisper in the listener's ear. But she insists sex has nothing to do with it. "People who have ASMR know that it isn't sexual - they don't get turned on," she says.
Once they've established that experiencing ASMR triggered by the videos really does have a measurable effect on the body, the team hope their work will lead to wider use of ASMR as an aid to health.
"A lot of people who've done our test and who comment on YouTube videos online say it helps them to feel very relaxed, and in particular that it helps them to get to sleep," says Hostler. "So I think there's definitely a therapeutic aspect to it. What we need to do is investigate what it is about it that makes it therapeutic, so we can fine tune it."
This kind of talk rings alarm bells with some professionals, like Francis McGlone, a neuroscientist at Liverpool John Moores University who I spoke to a year ago. He worried then that people with serious conditions like depression might try to treat themselves by watching videos. He called it "snake oil".
But other scientists are now embracing this aspect of ASMR. What's reputed to be the first peer-reviewed scientific paper to be published on ASMR was written by two psychologists at Swansea University, Dr Nick Davis and Emma Barratt. They interviewed nearly 500 people about their experience of ASMR.
They concluded that ASMR "provides temporary relief in mood for those suffering from depression, with many individuals consciously using it for this purpose... Many reported that even in the absence of tingling sensations, they felt that their mood and symptoms of pain had been improved."
Watching something that brings on your ASMR may well be beneficial, Nick Davis says. "If you feel like you're experiencing the symptoms of depression, or persistently low mood, then absolutely you need to find professional help," he says.
"But we all go through periods of stress and periods of low mood and just feeling a bit down, and this is something that could be used to treat yourself to a little 'me time', like having a hot bath or going for a run, and nothing wrong with that I think."
There's still plenty we don't know about ASMR. Establishing that people who say their heads are tingling are also relaxed is just a first step. We still have no idea why their state of relaxation manifests in this particular and distinctive way.
We don't know if there's a link with comparable phenomena, like "musical chills" or frisson, the goosebumps that some people get when listening to music. We don't know what proportion of the population experiences it. And we can't say if there's a downside (it seems unlikely, but you never know).
But we do know that the number of ASMR videos just keeps on growing. And with any luck they're doing us good, however weird they may seem.
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When I was a child I sometimes experienced a pleasant physical sensation in my scalp when I was especially intrigued or fascinated by something, or when being touched or stroked in certain ways.
The sensation continued into adulthood, but when I tried to explain what I was feeling to other people I was met with baffled looks and a quick change of subject.