I'm surprised that no one has mentioned that Brain Waves (eg Beta, Alpha, Theta, Delta) were discovered at the beginning of the 20th century.
Different brain wave frequencies have long been associated with different mental states.
Building Brain Wave Detectors was all the rage amongst hobbyists many years ago.
Brain Waves are normally detected using electrodes on the scalp, but they also generate very weak fields which can be picked up by non-contact methods in a screened room.
Surely this is simply an extension of that research?