From Samatha.org, four meditators were monitored with EEG while practicing rupa jhanas (i.e. the first 4 jhanas) and arupa or formless jhanas (the next 4 - or more - jhanas) as well as the practice of energy or "psychic power."
The results strike me as being all over the map, but as they put it, this is very provisional and they need a bigger sample. Many of the scans appear to differ from Leigh Brasington's brain on jhana, the research that motivated me to start this blog. EEG was limited to 30 Hz, thus missing some of the important gamma effects.