The difference in timbre between the 2 sounds in this piece causes our brain to separated the music into 2 streams. This effect is pronounced as the music becomes faster, and also bolstered by the melodies I chose following Gestalt's "continuity' principle (arpeggiated patterns in one stream, simple repeating melody in the other). We can see how important this continuity principle is when the auditory illusion is shattered at the end of the piece...