Pulsating heads and expert skull feeling (Member Post)

The outer limits of tactile perception

Let’s begin with a dorky digression for important context: what’s the smallest thing you can feel, period? What are the limits of tactile perception? Impressively, in the right conditions, our palpative powers can extend to the nanoscale — about 10 nanometres, roughly the size of larger proteins or the smallest transistors. Amazing!2

But don’t get too excited about your microscope fingers!

Real world conditions make it effectively impossible to come anywhere close to feeling anything that small. The 10nm limit was found in highly specialized testing conditions.3 You cannot feel a giant protein between your finger tips: it would fall into the crevices of your fingerprint like a basketball in the grand canyon.

The limits of more practical tactile perception are a lot less impressive … and messy. It’s impossible in principle to answer the question “what’s the smallest thing we can feel,” because it depends on many variables. What’s the smallest isolated hard particle, say, that you can feel on polished metal? There’s no one citation for this, but loosely based on a variety of sources, I conservatively estimate it’s roughly 100 μm, or a tenth of a millimetre — roughly the average thickness of human hair.

And that’s still in relatively ideal conditions.

And now back to the pulsating skulls. 💀💀💀

A sensitive robot

Rasmussen et al made a touch robot: an extremely sensitive gadget for measuring microscopic changes in skull movement. They put the touchbot on fifty relaxed, healthy people.

The robot’s mission? To detect any rhythm in the movements that was not related to breathing or pulse.

Such a rhythm might correspond to the classic controversial claim of the founder of cranial osteopathy, William Sutherland, and a key conceptual pillar of craniosacral therapy. If such a rhythm could be confidently identified, it would be a Very Big Deal for craniosacral therapy. They could put it in all their brochures!

The third rhythm: circulation, respiration, and … something else?

The authors claim to have succeeded. “I am Jack’s complete lack of surprise.” They report that their methods successfully detected both pulse and respiration, plus a third unknown rhythm: a change in skull size of about a tenth of a millimetre, 4-6 times per minute.

Mind blown? Mind expanding? Probably not. There are two major issues here:

1. Did they detect something real?
2. So what if they did?

The reliability of the signal: is that your skull swelling, or just a bird flying by? In the stratosphere?

Famously, the exotic new gravity wave detectors are so sensitive that their big challenge is separating the signal from the “noise” of vibrations from things like a car driving by … a kilometre away.

This experiment had a similar challenge. They may well have detected a signal, but there are legitimate concerns. The authors claim that their device can detect as little a micron of movement — much smaller than a hair — which means that it will produce very noisy data, because it can also pick up a butterfly fart.

In data like that, the signal is a needle in a haystack of noise, and it can only be extracted with clever software… which is a perfect storm for p-hacking. The more complicated your experimental protocol and analysis, the more ways there are to cook the books to find the result you want.

None of this means that they did not detect a “third rhythm,” but it does cast a lot of doubt on the whole exercise. There’s really no way to clear this up without replication by independent researchers.

The significance of the signal: does it matter if your head swells microscopically 5 times per minute?

Finding a signal does not mean that it’s important information, and the authors graciously acknowledge that “the physiological and clinical significance of the third rhythm identified on the living human head remains to be investigated”… and yet they proceed to speculate that it might “point to central importance in human health.” 🙄

Sure, it could! But probably not. Not every measurable thing is meaningful … and not everything measurable and meaningful is clinically useful! For instance, we know that the human heart rate is a readily measurable and meaningful signal… but it has exactly zero clinical significance to an osteopath trying help someone with chronic headaches.

It’s also possible that the signal does correlate with physiology, but not specific physiology. It could be a spin-off effect from other physiological rhythms, more like an echo than an original sound.

No one’s feelin’ it

Clinical significance also depends on the ability of mortal human healthcare professionals to detect that rhythm — amidst the substantial noise. A tenth of a millimetre is small: 100 micrometres is the width of a typical human hair, a dust particle, or the thickness of a coat of paint. It is the size of a paramecium, a human ovum, or a Demodex mite from your hair follicles. (Wikipedia has a nice list.)

And we can’t reliably palpate a hair on a perfectly smooth surface, let alone a rough or “noisy” one.

A tenth of a millimetre change in the relative position of a soft surface over ten seconds — very “low contrast” — is absurdly below the threshold of reliable human detection … even if it was the only signal. But this signal co-exists with multiple others in the patient and the therapist, and even the hardest parts of the skull are still easily compressible by a millimetre — ten times the size of the effect!

Feeling such a minuscule pulsation is not possible even for an exceptional individual, let alone for the “average” therapist. It’s like trying to hear a slowly recurring whisper standing under a running jet engine at a firing range.

The salamander’s conclusion

This is a poorly written paper, obviously biased in favour of osteopathy, claiming to have dug a signal out of very noisy data, identifying an unknown microscopic rhythm in skull movement that may or may not actually exist, but which in any case has no known physiological significance at all, let alone therapeutic value in the context of musculoskeletal and pain medicine, and even if it is does exist it’s extremely implausible that anyone could actually detect it.

But it does conveniently resemble something that Sutherland claims to have found by touch, so that was the take-home message of this paper: there is an extremely subtle signal, and it might matter, and it might be what some old-timey quack claims to have felt almost a century ago. So it’s got that going for it!

Notes

1. Rasmussen TR, Meulengracht KC. Direct measurement of the rhythmic motions of the human head identifies a third rhythm. J Bodyw Mov Ther. 2021 Apr;26:24–29. PubMed 33992252 ❐
2. Skedung L, Arvidsson M, Chung JY, et al. Feeling small: exploring the tactile perception limits. Sci Rep. 2013;3:2617. PubMed 24030568 ❐ PainSci Bibliography 51380 ❐
3. Specifically it was a 10 nm contrast repeated many times in the ripples on an *exotically smooth and regular surface*, a much smoother surface than anything we deal with in the real world. Stroking across such a spooky surface, repeating that 10 nm difference *thousands of times per stroke* — nanoscale features not just in the aggregate, but perfectly repeated — made it possible to feel the difference. Just barely.