The microwave auditory effect, also known as the microwave hearing effect or the Frey effect, consists of the human perception of audible clicks, or even speech, induced by pulsed or modulated radio frequencies. The communications are generated directly inside the human head without the need of any receiving electronic device. The effect was first reported by persons working in the vicinity of radar transponders during World War II. In 1961, the American neuroscientist Allan H. Frey studied this phenomenon and was the first to publish information on the nature of the microwave auditory effect. The cause is thought to be thermoelastic expansion of portions of the auditory apparatus, although competing theories explain the results of holographic interferometry tests differently.