At www.physorg.com/print 192952150.html according to physicists at the University of Innsbruck in Austria (see http://arxiv.org/abi/1005.1153 ) magnetic fields associated with lightning are powerful enough to produce hallucinations of hovering balls of light in nearby observers and these ‘visions’ are interpreted as ball lightning. The researchers based their hypothesis on magnetic fields used in clinical transcranial magnetic stimulation (TMS). Their endeavours showed that variable magnetic fields produced by lightning are of the same magnitude and frequency of TMS – and these produce hallucinations such as balls of light known as cranial phosphenes. Hence, reports of ball lightning that occur during storms with lightning could also be hallucinations.
However, Paul Devereux, in Earth Lights reports balls of light hovering over a chapel in mid Wales that was witnessed over a period of days, by a number of different people. It provoked a religious revival in the town. This does not appear to conform to the above research.