[Paleopsych] Nature: Music, the Food of Neuroscience?

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Music, the food of neuroscience?

Nature 434, 312 - 315 (17 March 2005); doi:10.1038/434312a

ROBERT ZATORRE AND JAMES MCGILL

Robert Zatorre is a cognitive neuroscientist and James McGill professor
of neuroscience at the Montreal Neurological Institute.


Playing, listening to and creating music involves practically every
cognitive function. Robert Zatorre explains how music can teach us about
speech, brain plasticity and even the origins of emotion.

We tend to consider art and culture from a humanistic or historical
perspective rather than a biological one. Yet these products of human
cognition must have their origin in the function and structure of the
human nervous system. As such, they should be able to yield valuable
scientific insights. This line of reasoning is nowhere more evident than
in the contemporary interest in the neuroscience of music.

Music provides a tool to study numerous aspects of neuroscience, from
motor-skill learning to emotion. Indeed, from a psychologist's point of
view, listening to and producing music involves a tantalizing mix of
practically every human cognitive function. Even a seemingly simple
activity, such as humming a familiar tune, necessitates complex auditory
pattern-processing mechanisms, attention, memory storage and retrieval,
motor programming, sensory-motor integration, and so forth ( Fig. 1
<http://www.nature.com/nature/journal/v434/n7031/fig_tab/434312a_F1.html
>).


<http://www.nature.com/nature/journal/v434/n7031/images/434312a-f1.0.jpg
>	Figure 1 The processing of sound waves from a musical
instrument.    Full legend
<http://www.nature.com/nature/journal/v434/n7031/fig_tab/434312a_F1_lgnd
.html>

High resolution image and legend
<http://www.nature.com/nature/journal/v434/n7031/fig_tab/434312a_F1.html
> (90k)


Likewise, the musician does not consider music to be monolithic, but
recognizes within it multiple features including melodies, chords,
themes, riffs, rhythms and tempos. This complexity - both psychological
and musicological - makes music a challenging topic for a scientific
research programme. Increasing numbers of investigators are convinced
that music can yield valuable information about how the brain works:
they believe that the study of the brain and the study of music can be
mutually revealing.

How does one go about studying this intricate thing called music? Few
scientists would accept that such a complex function could be studied,
let alone understood, without first identifying and describing its
various components. But this raises the thorny problem of deciding which
components of music are pertinent, and how these components are shared
or distributed among different cognitive functions. Some cognitive
functions, such as figuring out pitch interval ratios, may be unique to
music, whereas others, such as memory, may be general systems that are
used in many different domains.

The oldest scientific technique for understanding brain functions is to
study the consequences of brain lesions. We have long known that severe
damage to the auditory cortex - where information coming from the ear is
first analysed and interpreted - disturbs the ability to make sense of
sounds in general. But occasionally, lesions of certain auditory
cortical regions result in an unusual phenomenon: a highly selective
problem with perceiving and interpreting music, termed 'amusia'
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B1>.

People with this type of damage have no problem speaking or
understanding speech, or making sense of everyday sounds. But they
cannot notice wrong notes inserted into tunes, or recognize even the
most familiar melody. Even more surprising is that a minority of
otherwise normal individuals appear to be born with the same inability
to recognize tunes. In some cases, the deficit seems to run in families,
suggesting a genetic component
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B2>.

This extraordinarily selective problem in processing music, whether
acquired or inborn, could result from very selective damage or
dysfunction in an area of the auditory cortex where fine-grained pitch
differences and sound frequency ratios (musical intervals) are processed
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B3> . Such a specific
deficit at one of the earliest steps of music processing could propagate
through the perceptual system, resulting in a global disability. The
ability to compute pitch relations is critical to music processing, and
if the brain is unable to represent pitch, the entire music perception
mechanism could easily be destabilized.

The study of people with amusia has shown us that music depends on
certain types of neural process. Such people provide living examples of
what results when these neural processes are disrupted. And they have
shown us that music can indeed lend itself to scientific study.

Music and speech
Scientists would also like to understand why we have evolved a sense for
music in the first place, and, in particular, whether musical ability is
somehow an extension of speech: many have argued for this on the
reasonable grounds that music and speech share several formal
similarities. So researchers have tried, using various techniques, to
determine the extent to which the processing of music and that of speech
share neural resources. The results so far are somewhat conflicting, but
also intriguing.

One of the striking things about the neurobiological processing of
speech is that it mostly takes place in the left half of the brain. It
has therefore been natural to ask whether this asymmetry is mirrored in
a right-hemisphere predominance for music. There are also many case
reports of individuals who have lost their speech functions after
extensive damage to speech regions in the left cerebral hemisphere, yet
continue to show intact high-level musical function (for example, the
Russian composer Vissarion Shebalin
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B4>).

These data suggest that music and speech processing do not use
completely overlapping neural substrates. But neuroimaging studies
indicate that some functions, such as syntax, may require common neural
resources for both speech and music
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B5> . In other words, the
ability to organize a set of words into a meaningful sentence and the
ability to organize a set of notes into a well-structured melody might
engage brain mechanisms in a similar way.

But the data from which we have drawn these conclusions have
limitations. On the one hand, many of the case reports were studied in a
descriptive, anecdotal manner. On the other hand, neuroimaging can be
notoriously difficult to interpret: similar patterns of brain activity
do not necessarily mean that similar neural substrates are involved,
because many complexities of neural patterning are beyond our present
technology's ability to measure.

The key to resolving these questions comes from a more systematic
understanding of the different cognitive components involved, and the
specific neural circuits associated with them. Fine-grained pitch
processing - a highly critical component of music perception - has
proven particularly valuable in dissecting the differences between how
the brain handles speech and music.

Recent evidence from functional brain activation, magnetic recording and
lesion studies, suggests that a particular region of the auditory cortex
in the right hemisphere is much more specialized for representing
detailed pitch information than its counterpart on the left side of the
brain. Tones that are close together in pitch seem to be better resolved
by neurons on the right.

Why should this functional segregation have emerged? It could be related
to the requirement to sample sound information from the environment in
different ways, according to need: either quickly and roughly, or if
time allows, accurately
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B6> . If the sound energy
is changing very rapidly, for example, a quick snapshot may be needed.
The perceptual system needs to track these changes online, and hence
must sacrifice detail to achieve speed. Such may be the case for speech
recognition where detailed temporal information is essential to recover
the sounds produced by the rapidly moving articulatory muscles of the
lips and tongue. Conversely, some aspects of sounds that are important
for perceiving music evolve much more slowly, so the nervous system can
take a more detailed look at the structure of the sound. This takes more
time, of course, but yields a finer-grained internal representation.
Naturally occurring periodic sounds (many vibrating objects, voices or
animal calls) contain pitch information that is important to process.
Pitch is also a good cue to distinguish one sound from another in a
noisy environment. So the postulated pitch processing mechanisms need
not have evolved for music per se, but could be part of a general system
for using natural sounds from the environment.

Thus, the different specializations of the auditory cortex on the two
sides of the brain can be seen as different parameter settings on what
are essentially two parallel systems. This approach shows us that it is
perhaps less interesting to ask, "on which side of the brain is music
processing located?" than to set about systematically studying the
various subcomponents that contribute to various aspects of musical
function.

Music and development
Another reason music has caught the attention of scientists trying to
understand the brain is that the ability to perceive music seems to be
present from very early in development. Of course, we learn the
specifics of our musical culture from the environment. But the human
infant seems to come into the world with a brain already well prepared
to figure out its musical world.

Any mother can attest to the way an infant will respond to the pitch and
rhythm of her voice. But babies are surprisingly sophisticated
mini-musicians: they are able to distinguish different scales and
chords, and show preferences for consonant over dissonant combinations,
for example
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B7> . They can recognize
tunes played to them over periods of days or weeks, and are capable of
remarkable feats of statistical learning, being sensitive to
regularities in sounds
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B8> . In other words,
babies' nervous systems seem to be equipped with a capacity to sort out
the different musical sounds reaching their ears in order to construct a
grammar, or system of rules.

It could be argued that this is part of a general capacity to make sense
of the world - to be able to predict what is coming up next. In some
sense this is certainly true. But the notable thing is that this ability
endows infants with the capacity expressed later in life to respond to
and enjoy music. All this evidence supports the general idea that the
ability to perceive and process music is not some recent add-on to our
cognition, but that it has been around long enough to be expressed from
the earliest stages of our neural development.

Music involves not only listening, but also playing and creating, where
individual differences are much more evident. Although nearly everyone
seems to have sophisticated neural systems that allow them to perceive
music, and to reproduce musical patterns by singing, not everyone is
able to play the piano like Vladimir Horowitz.

This leads to two very interesting scientific questions, which are the
subject of active research. How can we explain individual differences in
'native' ability? And what effects does training have on brain function
and structure? Little progress has been made on the first question,
except in the very specific domain of 'absolute pitch', where
interactions between genetic and environmental factors are beginning to
be unraveled
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B9> . It is now clear that
absolute pitch cannot develop without some musical training, but
critically, the exposure must happen during childhood: past the age of
12 to 15, it is essentially impossible to learn it. From this one can
conclude that the brain must be particularly sensitive during a certain
time in development. But not all children given music lessons develop
this skill, so other factors must also be at play. New evidence suggests
that genetics has a role 10
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B10>. This is a field to
watch in the near future.

In contrast, a number of very clear findings are now emerging that help
us to understand how the brain is sculpted by musical experience. Most
of this work shows that training in music enhances the activity of
certain neural systems. For example, areas of the motor cortex
corresponding specifically to the fingers of the left hand show an
enhanced electrical response among violin players 11
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B11> . These changes are
directly related to the age at which training is begun: those who began
studying music in early childhood show the most extensive modification
to brain response, whereas those who waited until after puberty show
much less. Similar effects have been described for the auditory cortex's
response to sounds produced by specific instruments 11
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B11>.

Moreover, anatomical changes accompany these enhancements in
responsivity. Several studies have reported greater tissue density, or
enlargement of motor- and auditory-related structures among musicians,
indicating that years of training actually change the underlying
structure of the nervous system 12
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B12> . These findings
should not be taken as evidence that music makes a person's brain bigger
and therefore better. The changes are very specific, and it could be
that they come at the expense of other functions. But such findings of
brain plasticity have very general implications for our understanding of
the interplay between the environment and the brain, particularly in the
context of development, as the age at which training takes place is so
critical.

Music and emotion
One of the questions that most frequently comes up in discussions of
music, and yet has received relatively little attention in the
neuroscience community, concerns emotion. Indeed, non-scientists are
often puzzled that this aspect has been relatively neglected in favour
of more esoteric concerns, given that, for most people, music exists
solely to express or communicate emotion. There are some sophisticated
treatises in the musicological tradition on this question (for example,
the classic volume by Leonard Meyer 13
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B13>), but only recently
has the topic begun to attract serious attention from neuroscientists 14
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B14>.

One thing we do know is that music can elicit not only psychological
mood changes, but also physiological changes in heart rate, respiration
and so forth, that mirror the changes in mood. Indeed, music's
anxiolytic effect is known not only to the specialist, but to anyone who
listens to a favourite piece of music to relax after a trying day.

What brain responses can explain these effects? At the moment we simply
don't know. But plausible hypotheses are guiding research. One notion is
that music results in physical entrainment of motor and physiological
functions: music drives the body. So, loud, rhythmic, fast music tends
to make you feel lively - or even want to dance - whereas slow, soft
music leads to calmness, and even sadness. A possible explanation is
that these effects could be mediated through sensory-motor feedback
circuits, which have been much discussed in neurophysiology; that is,
through the so-called mirror-neuron system 15
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B15> . Although there is
no direct evidence for this idea, it is plausible in that this system is
thought to mediate imitative behaviour by linking perception directly to
action. A similar mechanism might explain some of the effects of music
on physical movement, and so mood induction.

But music's emotional undercurrents run deeper than such an analysis
might suggest. Studying the very complex and idiosyncratic responses to
music is challenging because it depends on so many difficult-to-control
factors, not least individual preferences. What is 'music' to one
person's ears is often offensive to another's (consider teenagers and
their parents as a typical example). So cultural and social factors
clearly have important roles in modulating our emotional response to
music. Yet there are still likely to be common neural pathways that
mediate responses, such as pleasure, to music.

One intriguing and very specific emotional response is the 'chills down
the spine' effect. Anyone who has experienced this knows exactly what I
refer to: for the minority who haven't, it won't do much good to try to
explain it. But we are beginning to understand some of the neural
mechanisms that underlie these kinds of response. When listeners
experience the chills, neuro-imaging shows that the brain areas
recruited include regions thought to be involved in mechanisms of reward
and motivation. Examples are the basal forebrain and certain brainstem
nuclei, along with cortical areas involved in emotional evaluation, such
as the orbitofrontal and insular regions 16
<http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v434/n7
031/full/434312a_r.html&filetype=&dynoptions=#B16> . These circuits are
similar to those involved in mediating responses to biologically
rewarding stimuli, such as food or sexual stimuli.

But why should music, an abstract pattern of sound, have any commonality
at all with such survival-related systems? It is a stretch to suggest
that music is essential for life or reproduction. However, perhaps this
research is beginning to illuminate the complex relation between
cognitive-perceptual systems that analyse and represent the outside
world, and evolutionarily ancient neural systems involved in assessing
the value of a stimulus relative to survival and deciding what action to
take. Maybe music, and all art in a way, manages to transcend mere
perception precisely because it contacts our more primordial
neurobiology.

To caricature the idea, we can think of the neocortex as being able to
analyse relations and notice patterns, but then this processed
information interacts with the emotion/evaluation system, which in turn
leads to pleasure (or sadness, fear, excitement and so forth). The
vagueness of these concepts indicates how far we are from having
anything like a model of the processes going on - although an optimist
might point out that even being able to talk about it, albeit in unclear
terms, shows how far we have come.

  ------------------ <http://www.nature.com/nature/images/orange.gif>

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