Wednesday, October 31, 2018

Musical illusions

Here's the English version of my column on music cognition for the current issue of the Italian science magazine Sapere.

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“In studying [optical] illusions”, writes Kathryn Schulz in her book Being Wrong, “scientists aren’t learning how our visual system fails. They are learning how it works.” What Schulz means is that normal visual processing is typically a matter of integrating confusing information into a plausible story that lets us navigate the world. Colour constancy is a good example: the brain “corrects” for variations in brightness so that objects don’t appear to change hue as the lighting conditions alter. The famous “checkerboard shadow” illusion devised by vision scientist Edward Adelson fools this automatic recalibration of perception.


Adelson’s checkerboard illusion. The squares A and B are the same shade of grey.

In this regard as in many others, auditory perception mirrors the visual. The brain often rearranges what we hear to create something that “makes more sense” – with the same potential for creating illusions. Psychologist of music Diana Deutsch has delved deeply into the subject of musical illusions, some of which are presented on a CD released by Philomel in 1995. Several of these have to be heard through stereo headphones: they deliver different pitches to the left and right ears, which the brain reassigns to create coherence. For example, in the “scale illusion” the notes of two simultaneous scales – one ascending, one descending – are sent alternately to each ear. But what one hears is a much simpler pattern: an ascending scale in one ear, descending in the other. Here the brain is choosing to perceive the more likely pattern, even though it’s wrong.

Another example reveals the limitations of pitch perception. A familiar tune (Yankee Doodle) is played with each note assigned a random octave. It sounds incomprehensible. The test shows that, in deciphering melody, we attend not so much to absolute pitch class (a C or D, say) as to relative pitch: how big pitch jumps are between successive notes. Arnold Schoenberg’s twelve-tone serialism ignored this, which is why the persistence of his “tone rows” is often inaudible.

Perhaps the strangest thing about optical illusions is that we enjoy them, even if – indeed, because – we find them perplexing. Instead of being upset by the brain’s inability to “get it right”, we are apt to laugh – not a common response to wrongness, although it’s actually how a lot of comedy works. You might, then, expect to find musical illusions put to pleasurable use in music, especially by jokers like Mozart. But they are rather rare, maybe because we simply won’t notice them unless we see the score. Something like the scale illusion is used, however, in the second movement of Rachmaninov’s Second Suite for Two Pianos, where two sets of seesawing notes on each piano are heard as two sets of single repeated notes. It seems likely that Rachmaninov (not noted for jocularity) wasn’t just having fun – it’s merely easier to play these rapid quavers using pitch jumps rather than on the same note.