The Shepard Scale Auditory Illusion

[YouTube - Link]

Here's a cool auditory illusion that may or may not cause you to question your sanity. According to Wikipedia:

A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the base pitch of the tone moving upwards or downwards, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower.

The way to really get the experience is to play it again, again, again, and again. And it'll just seem like the sound keeps getting higher in pitch and tone. Best way to keep the illusion from breaking is to keep your mouse button on the very beginning of the YouTube play bar and when the video reaches the end click and you'll be able to continue listening. [via - Boing Boing]

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Huh. I still don't understand how this works. I followed the seemingly ascending pitch with my voice (and yes, I am a classically trained singer), and the pitch did indeed go up on the musical scale as far as my vocal mimicking was concerned. I have a three and a half octave range, but eventually I couldn't follow it anymore! So that, to me, is the really interesting question. If it's not actually ascending, then why is it that a reproduction of that scale, and one which at least SEEMS to be accurate to the ear, definitely and unmistakably DOES ascend in pitch?