Revolution Void

I learned about this while attending audio engineering classes at Shoreline Community College, but whenever I told anyone, they didn't believe me. Finally, thanks to a post on Boing Boing (and the magic of the Internet), I have located scientific evidence to back it up!

Figure 1 shows the pattern that produces this illusion. Two tones which are spaced an octave apart are alternated repeatedly. The identical sequence is played to both ears simultaneously, except that when the right ear receives the high tone the left ear receives the low tone, and vice versa.

Despite its simplicity, this pattern is almost never heard correctly, and instead produces a number of illusions. Many people hear a single tone which switches from ear to ear, while its pitch simultaneously shifts back and forth between high and low. So it seems as though one ear is receiving the pattern 'high tone - silence - high tone - silence' while at the same time the other ear is receiving the pattern 'silence - low tone - silence - low tone'. Even more strangely, when the earphone positions are reversed many people hear the same thing: The tone that had appeared in the right ear still appears in the right ear, and the tone that had appeared in the left ear still appears in the left ear.

The octave illusion has another surprising property - righthanders and lefthanders tend to hear this pattern in different ways. Righthanders tend to hear the high tone on the right and the low tone on the left, regardless of how the earphones are positioned. Yet lefthanders vary considerably in terms of where the high and low tones appear to be coming from. Moreover, the tendency to hear the high tone on the right and the low tone on the left is stronger among people with only righthanders in their family than among those with a lefthanded parent or sibling.