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Transcranial magnetic stimulation (TMS) is an extraordinary technique pioneered by neuroscientists to explore the workings of the brain. The idea is to place a human in a rapidly changing magnetic field that is powerful enough to induce currents in neurons in the brain. Then sit back and see what happens.

Since TMS was invented in the 1980s, it has become a powerful way of investigating how the brain works. Because the fields can be tightly focused, it is possible to generate currents in very specific areas of the brain to see what they do.

Focus the field in the visual cortex, for example, and the induced eddys cause the subject to ‘see’ lights that appear as discs and lines. Move the the field within the cortex and the subject sees the lights move too.

All that much is repeatable in the lab using giant superconducting magnets capable of creating fields of as much as 0.5 Tesla inside the brain.

But if this happens in the lab, then why not in the real world too, say Joseph Peer and Alexander Kendl at the University of Innsbruck in Austria. They calculate that the rapidly changing fields associated with repeated lightning strikes are powerful enough to cause a similar phenomenon in humans within 200 metres.

To be sure, this is a rare event. The strike has to be of a particular type in which there are multiple return strokes at the same point over a period of a few seconds, a phenomenon that occurs in about 1-5 per cent of strikes, say Peer and Kendl.

And the observer has to be capable of properly experiencing the phenomenon; in other words uninjured. “As a conservative estimate, roughly 1% of (otherwise unharmed) close lightning experiencers are likely to perceive transcranially induced above-threshold cortical stimuli,” say Peer and Kendl. They add that these observers need not be outside but could be otherwise safely inside buildings or even sitting in aircraft.

So what would this kind of lightning-induced transcranial stimulation look like to anybody unlucky enough to experience it? Peer and Kendl say it may well look like the type of hallucinations induced by lab-based tests, in other words luminous lines and balls that appear to float in space in front of the subject’s eyes.

It turns out, of course, that there are numerous reports of these types of observations during thunder storms. “An observer reporting this experience is likely to classify the event under the preconcepted term of “ball lightning”,” say Kendl and Peer.

That’s an interesting idea: that a large class of well-reported phenomenon may be the result of hallucinations induced by transcranial magnetic stimulation.

A difficult idea to test, to be sure, but no less interesting for it. And it raises an important question: in what other circumstances are ambient fields large enough to trigger hallucinations of one kind or another?

Ref: arxiv.org/abs/1005.1153: Transcranial Stimulability Of Phosphenes By Long Lightning Electromagnetic Pulses

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