The mysterious phenomenon of ball lightning may be caused by the triggering of currents in the brain, say scientists.
People have been seeing these glowing spheres of electricity for hundreds of years and associating it with everything from thunderstorms and earthquakes to flying saucers.
Reporting on the prepress website arxiv.org Dr Joseph Peer and Dr Alexander Kendl from the University of Innsbruck in Austria say Transcranial Magnetic Stimulation (TMS) has provided them with new clues as to a possible cause for the phenomenon.
TMS is used to explore the workings of the brain by subjecting people to a rapidly changing magnetic field, powerful enough to induce currents in brain neurones.
The fields can be tightly focused to very specific areas of the brain to see what happens.
Scientists found when the magnetic fields are focused on the visual cortex, the induced eddies cause people to think they're seeing lights that look like discs and lines.
And when the field is moved within the cortex, the subject sees the lights move too.
Peer and Kendl claim rapidly changing fields associated with repeated lightning strikes are powerful enough to cause a similar phenomenon in humans within 200 metres.
"The strike has to be of a particular type with multiple return strokes at the same point over a period of several seconds," say the researchers.
As a conservative estimate, Peer and Kendl say "roughly 1% of close lightning experiencers are likely to perceive transcranially induced above-threshold cortical stimuli in this way."
"The observers don't need to be outside but could instead be safely inside a building or aircraft," they add.
"And someone experiencing one of these events may well have hallucinations similar to those induced by the lab tests, in other words they see luminous lines and balls that appear to float in space."
Can't explain everything
Australian ball lightning expert Emeritus Professor Robert Crompton, a visiting fellow with the Research School of Physics and Engineering at the Australian National University, says "the paper explains some but not all documented cases".
He sites a case in which a man was standing next to his garage door when lightning struck nearby.
"Shortly afterwards he saw an orange yellow ball of rotating light slowly floating towards him. It entered his metal garage through the open door and bounced twice on a board of wood before disappearing in a loud bang like a rifle shot."
Crompton says "the object left marks on the board which were later analysed by the Australian Federal Police forensics lab and found to be titanium oxide, a heavy metal of the type used in paints. But there wasn't paint splash or spill marks as you could expect to find in a garage".
Crompton thinks ball lightning is an ionized electrical discharge, and in this case, lightning struck a painted pole with some of the paint then being incorporated into the ball lightning.
He concludes "if the marks were made by ball lightning as claimed, then this event can't be explained by Peer and Kendl's work".