Music special: The illusion of music - life - 23 February 2008 - New Scie... http://www.newscientist.com/article/mg19726441.500-music-special-the-...
1 of 5
My New Scientist
Home | Life | In-Depth Articles | Back to article
Music special: The illusion of music
23 February 2008 by Daniel Levitin
Magazine issue 2644. Subscribe and save
For similar stories, visit the The Human Brain and Human Evolution Topic Guides
ADVERTISEMENT
Hear five of the most striking auditory illusions
IMAGINE that you stretch a pillowcase tightly
across the opening of a bucket, and different
people throw ping-pong balls at it from
different distances. They can each throw as
many balls as they like, and as often as they
like. Your job is to figure out, just by looking at
how the pillowcase moves up and down, how
many people there are, who they are and
whether they are walking towards you, away
from you or standing still. This is essentially
the problem your auditory system has to
contend with when it uses the eardrum as the
gateway to hearing.
1 more image
Sound is transmitted through the air by molecules vibrating at certain frequencies. These bombard the
eardrum, causing it to wiggle in and out depending on how hard they hit it (related to the volume, or
amplitude, of the sound) and how fast they are vibrating (related to what we call pitch). But there is
nothing in the molecules that tells the eardrum where they came from, or which ones are associated
with which object. Voices may be mixed in with other voices, or the sounds of machines, wind and
footsteps. Most of the time the input is incomplete or ambiguous. So how does the brain figure out,
from this disorganised mixture of molecules beating against a membrane, what is out there in the
world?
Most people assume that the world is just as they perceive it to be. Yet experiments have forced
researchers, including myself, to confront the reality that this is not the case. What we actually hear is
the end of a long chain of mental events that give rise to an impression - a mental image - of the
physical world. Nowhere is this more striking than in the perceptual illusion in which our brain imposes
structure and order on a sequence of sounds to create what we call music.
The chain of mental events begins with a process called feature extraction. The brain extracts basic,
low-level features from the music, using specialised neural networks that decompose the signal into
information about pitch, timbre, spatial location, loudness, reverberant environment, tone durations and
the onset times for different notes (and for different components of tones). This bottom-up processing
of basic elements occurs in the peripheral and phylogenetically older parts of our brains. Next comes
a process called integration. Parts of the higher brain - mostly in the frontal cortex - receive the basic
features from lower brain regions and work top-down to integrate them into a perceptual whole.
The brain faces three difficulties in feature extraction and integration. First, the information arriving at
the sensory receptors is undifferentiated in terms of location, source and identity. Second, the
information is ambiguous: different sounds can give rise to similar or identical patterns of activation on
the eardrum. Third, the information is seldom complete. Parts of the sound may be masked by other
sounds, or lost. The brain has to make a calculated guess about what is really out there. So, auditory
3/27/2011 2:06 PM
Music special: The illusion of music - life - 23 February 2008 - New Scie... http://www.newscientist.com/article/mg19726441.500-music-special-the-...
2 of 5
perception is a process of inference. And when the sensory input is music, these inferences include
several factors over and above the sounds themselves: what has come before in the piece of music
we are hearing; what we remember will come next if the music is familiar; what we expect will come
next if the genre or style is familiar; and any additional information we may have, such as a summary of
the music that we have read, a sudden movement by a performer or a nudge by the person sitting next
to us.
The brain thus constructs a representation of reality, based on both the component features of what
we actually hear and our expectations of what we think we should be hearing. There are good
evolutionary reasons for this - a perceptual system that can restore missing information can help us
make quick decisions in threatening situations - but it is not without drawbacks. The top-down
expectations can cause us to misperceive things by resetting some of the circuitry in the bottom-up
processors. This is partly the neural basis for perceptual illusions such as the one demonstrated by
cognitive psychologist Richard Warren from the University of Wisconsin. He recorded a sentence,
"The bill was passed by both houses of the legislature", cut out part of it from the recording tape and
then replaced the missing piece with a burst of white noise (static) of the same duration. Nearly
everyone who heard the altered recording reported that they heard both a sentence and static. Yet a
large proportion of people couldn't tell when the static occurred because the auditory system had filled
in the missing speech information, so that the sentence seemed to be uninterrupted.
This filling-in phenomenon is not just a laboratory curiosity. Composers exploit the same principle,
knowing that our perception of a melodic line will continue, even if part of it is obscured by other
instruments. It also happens whenever we hear the lowest notes on the piano or double bass. We are
not actually hearing 27.5 or 35 hertz, because those instruments are typically incapable of producing
much energy at these ultra-low frequencies. Instead, our ears are filling in the information and giving us
the illusion that the pitch is that low.
Most contemporary recordings contain another type of auditory illusion. Our brains use cues about the
spectrum of the sound and the types of echoes to tell us about the auditory world around us, much as
a mouse uses its whiskers to learn about the physical world around it.
Recording engineers have learned to mimic those cues to imbue recordings with a real-world, lifelike
quality even when they are made in sterile recording studios. Artificial reverberation makes vocalists
and lead guitars sound as if they are coming from the back of a concert hall, even when we are
listening on headphones and the sound is an inch away from our ears. The same principles can also
generate auditory tricks, such as making a guitar sound as if it is 10 feet wide and your ears are right
where the soundhole should be.
Special effects
Recorded music allows us to experience other sensory impressions that we never actually have in the
real world. Recording engineers and musicians create special effects that tickle our brains by
stimulating neural circuits that evolved to discern important features of our auditory environment. For
example, our brains can estimate the size of an enclosed space on the basis of the reverberation and
echo present in the signal that hits our ears. Even though few of us understand the equations
necessary to describe how one room differs from another, we can all tell whether we are standing in a
small tiled bathroom, a medium-sized concert hall or a large church with high ceilings. And we can tell
what size room the singer or speaker is in when we hear recordings of voices. Recording engineers
exploit this ability to create what I call "hyper-realities", playing with our perceptions of space in the
auditory equivalent of the cinematographer's trick of mounting a camera on the bumper of a speeding
car.
Another illusion involves timing. Our brains are exquisitely sensitive to timing information. We are able
to localise objects in the world based on differences of only a few milliseconds between the time of
arrival of a sound at one of our ears versus the other. Many of the special effects we love to hear in
recorded music are based on this sensitivity. The sounds of jazz guitarist Pat Metheny or that of David
Gilmour of Pink Floyd use multiple delays of the signal to give an otherworldly, haunting effect that
triggers parts of our brains in ways that humans had never experienced before, simulating the sound
of an enclosed cave with multiple echoes such as would never actually occur in the real world - the
3/27/2011 2:06 PM
Music special: The illusion of music - life - 23 February 2008 - New Scie... http://www.newscientist.com/article/mg19726441.500-music-special-the-...
3 of 5
auditory equivalent of the barbershop mirrors that repeat infinitely.
Perhaps the ultimate illusion in music, however, is the illusion of structure and form. There is nothing in
a sequence of notes themselves that creates the rich emotional associations we have with music,
nothing about a scale, a chord or a chord sequence that intrinsically causes us to expect a resolution.
Our ability to make sense of music depends on experience and on neural structures that learn and can
modify themselves with each new song or piece of music we hear, and with each new listen to music
we are already familiar with. Our brains learn a kind of musical grammar that is specific to the music of
our culture, just as we learn to speak the language of our culture. This becomes the basis for our
understanding of music, and ultimately the basis for what we like in music, what music moves us, and
how it moves us.
Read more about music in our special issue
Top five musical illusions
• In piano works such as Chopin's Fantasy-Impromptu in C-sharp Minor, opus 66, or
Sinding's The Rustle of Spring, the notes go by so quickly that an illusory melody emerges.
When the notes are close enough together in time, the melody "pops out" because the
perceptual system binds them together, giving an emergent impression of tunefulness. Play
the tune slowly and this disappears.• In a Sardinian style of a cappella singing studied by
Bernard Lortat-Jacob at the Musée de l'Homme in Paris, a fifth female voice called the
quintina (literally "fifth one" in Sardinian) emerges from four male voices when their harmony
and timbres are just right. The voice is said to be that of the Virgin Mary coming to reward the
singers for their piety, but in fact it is simply a misperception of the chord and its harmonics.•
The Eagles' song, One of These Nights, opens with a pattern played by bass and guitar that
sounds like one instrument. The bass plays a single note, and the guitar adds a glissando,
but the perceptual effect is of the bass sliding due to the gestalt principle of good
continuation, which binds together two objects when the trajectory of one implies the
continued trajectory of another.• Jazz pianist George Shearing created a new timbral effect
by having a guitar (or in some cases, vibraphone) precisely match what he was playing on the
piano. Listeners come away wondering, "What is that new instrument?", when in reality it is
two separate instruments whose sounds have perceptually fused.• In Lady Madonna, the
Beatles sing into their cupped hands during an instrumental break and we could swear that
there are saxophones playing. This perception is based on the unusual timbre they achieve,
coupled with our expectation that saxophones should be playing in a song of this genre. (This
is not to be confused with the actual saxophone solo that occurs in the song.)
They just don't get it
History is littered with figures noted for their hopeless unmusicality. Ulysses S. Grant, the 18th
president of the United States, had a tin ear and found music profoundly irritating; Che
Guevara famously couldn't distinguish one piece of music from another. Once, such people
would have been described as "tone deaf"; today they are seen as much more interesting
than that.In the past few years it has become clear that the inability to hold a tune can
sometimes be caused by a neurological condition called congenital amusia, which
completely robs people of what is normally an instinctive and spontaneous appreciation of
music. No wonder the condition has become a major research topic in the bid to understand
the mysteries of how the brain handles music.The first case report of "note deafness"
appeared in 1878, and the literature is full of anecdotal accounts of people with a lifelong
failure of music perception. It wasn't until 2002, however, that the first proper study of
congenital amusia was published. A team led by Isabelle Peretz of the University of Montreal
in Canada reported the case of Monica, a woman in her early 40s who had always lacked
even the most basic of musical abilities (Neuron, vol 33, p 185).Peretz concluded that
Monica's problem was a failure to detect pitch changes in melodies. Played two notes in
sequence, she could rarely tell whether the second was higher or lower than the first or had
the same pitch. Most people can easily distinguish small differences in pitch - half a
semitone, say - but for amusics, even a leap of an octave, equivalent to the first two notes of
3/27/2011 2:06 PM
Music special: The illusion of music - life - 23 February 2008 - New Scie... http://www.newscientist.com/article/mg19726441.500-music-special-the-...
4 of 5
Somewhere Over The Rainbow, can be barely perceptible. Tones and semitones are the
building blocks of melody, so no wonder amusics find music monotonous in more than one
sense of the word.Peretz and others have since documented dozens of similar cases. These
people all have normal hearing, intelligence and memory, but absolutely no grasp of melody.
For them, one tune sounds very much like another, familiar songs are unrecognisable without
lyrics, and dissonant chords that cause most of us to wince elicit no response. Amusics
cannot sing, though they often don't recognise this. The condition is unusual but not
particularly rare - the accepted figure is 4 per cent of the population - and it runs in families.So
what causes congenital amusia? According to Peretz, the best explanation is that the human
brain is equipped with a specialised "module" for processing melody, which occasionally fails
to develop properly. That would explain why amusia appears to affect musical perception
alone. If correct, music, like language, is an innate human adaptation that was hard-wired into
our brains by evolution.AUDITORY CHEESECAKE?Not everyone agrees with this view,
however. Steven Pinker once famously described music as "auditory cheesecake" pleasurable but with no adaptive function. What's more, there is some evidence that amusia
is not a purely musical deficit but is linked to problems with language or spatial processing.
So perhaps amusia (and by extension, normal music perception) is rooted in the brain circuits
that handle intonation in language, or that look after the concepts of "highness" and "lowness"
central to our mental representations of melody.Peretz's group and others are now scanning
the brains of amusics in search of anatomical anomalies that might lead them to the
underlying problem. So far they have found some minor differences in the thickness of white
matter in a brain area called the right inferior frontal gyrus - a region linked with musical pitch
perception and melodic memory (Brain, vol 129, p 2562). They are also searching for the
genes that make amusia heritable, in the hope of gaining new insight into abnormal brain
development in amusia (The American Journal of Human Genetics, vol 81, p 582).Another
key question is whether congenital amusia is one condition or several. Some amusics like
listening to music because they enjoy the rhythms, but Peretz's team has found that around
half their subjects have a problem with rhythm perception. This suggests there may be a
related condition that wipes out timing as well as melody. There's also the problem of
"clatterers" - amusics to whom music sounds like a drainpipe being hit with a wrench. "Only a
very few amusics hear clattering," says Peretz. "For the majority, music is just confusing."
That has led some researchers to propose a separate disorder of music perception called
dystimbria, which prevents people from perceiving musical "colour", or timbre.Whether
amusia is one condition or many, the hope is that understanding it better will benefit those
unfortunates excluded from the profound pleasure of music. Peretz thinks that with early
intervention it might be possible to tap into the natural plasticity of the brain and stem some of
the damage. "There's no chance of helping adults," she says. "We've tried. But with children,
maybe."Graham LawtonHave you got amusia? Test yourself here: http://www.delosis.com
/listening/home.html
Daniel Levitin is at the department of psychology at McGill University in Montreal, Canada. This
article is an extract from his new book This Is Your Brain On Music: Understanding a human
obsession, published by Atlantic Books, £17.99
From issue 2644 of New Scientist magazine,
page 34-37.
As a subscriber, you have unlimited access to our
online archive.
Why not browse past issues of New Scientist
magazine?
3/27/2011 2:06 PM
Music special: The illusion of music - life - 23 February 2008 - New Scie... http://www.newscientist.com/article/mg19726441.500-music-special-the-...
5 of 5
If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication
department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options
available for use of articles and graphics we own the copyright to.
Back to article
ADVERTISEMENT
3/27/2011 2:06 PM