Our brains play all kinds of tricks on us when we listen to music, and cognitive psychologist Diana Deutsch has spent a fair portion of her career uncovering those tricks. One of the coolest ones is the "scale illusion," which shows how our brains rearrange the music we hear into the patterns that make the most sense to us.
Here's Deutsch demonstrating the illusion on an old episode of Nova. (Heads up: the audio is wonky. Scroll down further for another video that gives a better demonstration of how the illusion actually works.) The really cool bit comes at the end of the video, where an orchestra demonstrates how the illusion factors into Tchaikovsky's sixth symphony; at one point, you hear a melody that nobody's playing.
Quick note: Deutsch discovered the scale illusion in 1973. Tchaikovsky wrote it into his sixth symphony in 1893. Just saying.
Here's another video that demonstrates how the scale illusion works, in proper stereo. Headphones are an asset.
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