Theta-wave emissions were emissions on the high end of the mu spectrum. They were capable of "synthesising and transmitting thought patterns".
The Javaman built a machine, called the Dorganator, that emitted theta-waves. He used it to enslave the Volnexians and to draw the Sixth Doctor's TARDIS to Volnex VI. According to the Doctor, the theta-waves from the Dorganator created a "feedback loop in the telepathic circuits", which caused the Doctor and Peri Brown to switch their minds into each others' bodies, a process known as corporelectroscopy. (PROSE: Turnabout is Fair Play)