Ali Cook

Diana Deutsch is one of the most important audio illusion researchers – I've blogged about her before. (Isn't it interesting how illusions presented one way are 'magic' and another are strictly 'scientific research'?) 

Watch this video: 

Scale Illusion from meara o'reilly on Vimeo.

You can see it's two different melodies played one after the other, then both played at once. Close your eyes and listen again. What do you hear when the two play simultaneously?

Strangely, when played together, the melodies suddenly sound like two scales ascending or descending in unison.

The reason is that our brain groups sound according to similar characteristics. If there are lots of ways the brain can decide to organise some sound, a system kicks in. Because the tones are close in pitch, the brain chooses the do something with the proximity of the pitches, ignoring the original melodies, and creating the illusion of a third melodic trajectory.

The illusion works over headphones as well, this video is ace and explains it really well – and features Diana herself: