In the 1960s and 70s, groups like The Beatles and Black Sabbath were accused of recording satanic messages into songs, which could only be heard when the music was played backward.

Has Aphex Twin done the modern day equivalent?

The image found within the audio spectrum analysis is the aural equivalent of steganography: the practice of hiding secret messages and watermarks in images.

The track in question is DeltaMi-1=aSigman=1Di(n)(SigmajEC(i)Fij(n-1)+Fexti(n-1)) - more simply referred to as [Equation]. Simply replace the greek alphabet where applicable.