Musical Illusions
philomel.com
via RadioLab: http://www.wnyc.org/shows/radiolab/episodes/2006/04/21 (I'm catching up on the last 4 years).
philomel.com
Be sure to use headphones and prepare to be surprised!
http://philomel.com/musical_illusions/example_octave_illusio...
The mechanisms that give rise to the illusion are not well understood, but it seems to have something to do with hemispheric asymmetry. Individual differences in how the illusion is heard often correlate with handedness (ie whether you're left or right-handed)
Radiolab interviewed Diana Deutsch on her research a few years back, and it's one of my favorite episodes.
http://www.wnyc.org/shows/radiolab/episodes/2006/04/21/segme...