Deutsch's scale illusion

Discovered by Diana Deutsch in 1973, Deutsch's scale illusion is an auditory illusion produced by simultaneous ascending and descending major scales beginning in separate stereo channels with each successive note being switched to the opposite channel. With the left channel: C'-D-A-F--A-D-C'; and the right: C-B-E-G-E-B-C; the ear hears both: C'-B-A-G--A-B-C'; and: C-D-E-F--E-D-C. The tones are equal-amplitude sine waves, and the sequence is played repeatedly without pause at a rate of four tones per second. (Listen to the Stereo Sound Example linked below.)

When listening to the illusion over headphones, most right-handers hear a melody corresponding to the higher tones as on the right and a melody corresponding to the lower tones as on the left. When the earphone positions are reversed, the higher tones continue to appear to be coming from the right and the lower tones from the left. Other people experience different illusions, such as the higher tones on the left and the lower tones on the right, or a pattern in which the sounds appear to be localized in different and changing ways. Right-handers and left-handers differ statistically in how the scale illusion is perceived.

As with all (most?) other sensory illusions, they are the result of the actions of the brain to "clean" the data it's fed in order to make sense of it. In this case, it perceives 2 logical signals each made of half of the notes, as opposed to a string of random occurences for each note. Whenever there is a correlation, the brain tries to make a pattern out of it. The pattern may not be 100% accurate or what the original sender intended it to be, but it usually is accurate enough for a species to evolve training its natural neural net. Any crude signal processor can be fooled. Any complex signal processor may be too complex, big, expensive, slower than realtime etc. to be of any practical use. Practical, in our case, meaning balancing costs/efficency to assure survival/evolution.