20 April 1976  US 3,951,134 A

Apparatus for and method of sensing brain waves at a position remote from a subject whereby electromagnetic signals of different frequencies are simultaneously transmitted to the brain of the subject in which the signals interfere with one another to yield a waveform which is modulated by the subject’s brain waves. 

Link To Apparatus and method for remotely monitoring and altering brain waves – US 3,951,134