Strange experience today – had Steve Reich’s long, hypnotising ‘Drumming’ playing in a room that I was dipping in and out of. While in the room, I could rarely hear the shifting rhythms but every time I came back in after a short absence I heard a different main beat. Is that just because the music moves slowly, or am I locking into a particular pulse as you do when looking at a view of a Necker cube? But for other optical illusions the retina at a low level becomes desensitised, ‘tired’ of a particular location or colour which is why you see read afterimages after looking at a green spot. Can this also happen in acoustics and music, I wonder?