Music and the Brain
Music and the BrainWhat is the secret of music's strange power?
Seeking an answer, scientists are piecing together a picture of
what happens in the brains of listeners and musicians
By Norman M. Weinberger
Music surrounds usand we wouldn't have it any other way. An
exhilarating orchestral crescendo can bring tears to our eyes and
send shivers down our spines. Background swells add emotive punch
to movies and TV shows. Organists at ballgames bring us together,
cheering, to our feet. Parents croon soothingly to infants.
And our fondness has deep roots: we have been making music since
the dawn of culture. More than 30,000 years ago early humans were
already playing bone flutes, percussive instruments and jaw
harps--and all known societies throughout the world have had music.
Indeed, our appreciation appears to be innate. Infants as young as
two months will turn toward consonant, or pleasant, sounds and away
from dissonant ones. And when a symphony's denouement gives
delicious chills, the same kinds of pleasure centers of the brain
light up as they do when eating chocolate, having sex or taking
cocaine.
Therein lies an intriguing biological mystery: Why is
music--universally beloved and uniquely powerful in its ability to
wring emotions--so pervasive and important to us? Could its
emergence have enhanced human survival somehow, such as by aiding
courtship, as Geoffrey F. Miller of the University of New Mexico
has proposed? Or did it originally help us by promoting social
cohesion in groups that had grown too large for grooming, as
suggested by Robin M. Dunbar of the University of Liverpool? On the
other hand, to use the words of Harvard University's Steven Pinker,
is music just "auditory cheesecake"--a happy accident of evolution
that happens to tickle the brain's fancy?
Why is music--universally beloved and uniquely powerful in its
ability to wring emotions--so pervasive and important to us?
Neuroscientists don't yet have the ultimate answers. But in
recent years we have begun to gain a firmer understanding of where
and how music is processed in the brain, which should lay a
foundation for answering evolutionary questions. Collectively,
studies of patients with brain injuries and imaging of healthy
individuals have unexpectedly uncovered no specialized brain
"center" for music. Rather music engages many areas distributed
throughout the brain, including those that are normally involved in
other kinds of cognition. The active areas vary with the person's
individual experiences and musical training. The ear has the fewest
sensory cells of any sensory organ--3,500 inner hair cells occupy
the ear versus 100 million photoreceptors in the eye. Yet our
mental response to music is remarkably adaptable; even a little
study can "retune" the way the brain handles musical inputs.
Inner Songs Until the advent of modern imaging techniques,
scientists gleaned insights about the brain's inner musical
workings mainly by studying patients--including famous
composers--who had experienced brain deficits as a result of
injury, stroke or other ailments. For example, in 1933 French
composer Maurice Ravel began to exhibit symptoms of what might have
been focal cerebral degeneration, a disorder in which discrete
areas of brain tissue atrophy. His conceptual abilities remained
intact--he could still hear and remember his old compositions and
play scales. But he could not write music. Speaking of his proposed
opera Jeanne d'Arc, Ravel confided to a friend, "...this opera is
here, in my head. I hear it, but I will never write it. It's over.
I can no longer write my music." Ravel died four years later,
following an unsuccessful neurosurgical procedure. The case lent
credence to the idea that the brain might not have a specific
center for music.
The experience of another composer additionally suggested that
music and speech were processed independently. After suffering a
stroke in 1953, Vissarion Shebalin, a Russian composer, could no
longer talk or understand speech, yet he retained the ability to
write music until his death 10 years later. Thus, the supposition
of independent processing appears to be true, although more recent
work has yielded a more nuanced understanding, relating to two of
the features that music and language share: both are a means of
communication, and each has a syntax, a set of rules that govern
the proper combination of elements (notes and words, respectively).
According to Aniruddh D. Patel of the Neurosciences Institute in
San Diego, imaging findings suggest that a region in the frontal
lobe enables proper construction of the syntax of both music and
language, whereas other parts of the brain handle related aspects
of language and music processing. Imaging studies have also given
us a fairly fine-grained picture of the brain's responses to music.
These results make the most sense when placed in the context of how
the ear conveys sounds in general to the brain. Like other sensory
systems, the one for hearing is arranged hierarchically, consisting
of a string of neural processing stations from the ear to the
highest level, the auditory cortex. The processing of sounds, such
as musical tones, begins with the inner ear (cochlea), which sorts
complex sounds produced by, say, a violin, into their constituent
elementary frequencies. The cochlea then transmits this information
along separately tuned fibers of the auditory nerve as trains of
neural discharges. Eventually these trains reach the auditory
cortex in the temporal lobe. Different cells in the auditory system
of the brain respond best to certain frequencies; neighboring cells
have overlapping tuning curves so that there are no gaps. Indeed,
because neighboring cells are tuned to similar frequencies, the
auditory cortex forms a "frequency map" across its surface.
The response to music per se, though, is more complicated. Music
consists of a sequence of tones, and perception of it depends on
grasping the relationships between sounds. Many areas of the brain
are involved in processing the various components of music.
Consider tone, which encompasses both the frequencies and loudness
of a sound. At one time, investigators suspected that cells tuned
to a specific frequency always responded the same way when that
frequency was detected.
But in the late 1980s Thomas M. McKenna and I, working in my
laboratory at the University of California at Irvine, raised doubts
about that notion when we studied contour, which is the pattern of
rising and falling pitches that is the basis for all melodies. We
constructed melodies consisting of different contours using the
same five tones and then recorded the responses of single neurons
in the auditory cortices of cats. We found that cell responses (the
number of discharges) varied with the contour. Responses depended
on the location of a given tone within a melody; cells may fire
more vigorously when that tone is preceded by other tones rather
than when it is the first. Moreover, cells react differently to the
same tone when it is part of an ascending contour (low to high
tones) than when it is part of a descending or more complex one.
These findings show that the pattern of a melody matters:
processing in the auditory system is not like the simple relaying
of sound in a telephone or stereo system.
Although most research has focused on melody, rhythm (the
relative lengths and spacing of notes), harmony (the relation of
two or more simultaneous tones) and timbre (the characteristic
difference in sound between two instruments playing the same tone)
are also of interest. Studies of rhythm have concluded that one
hemisphere is more involved, although they disagree on which
hemisphere. The problem is that different tasks and even different
rhythmic stimuli can demand different processing capacities. For
example, the left temporal lobe seems to process briefer stimuli
than the right temporal lobe and so would be more involved when the
listener is trying to discern rhythm while hearing briefer musical
sounds.
The situation is clearer for harmony. Imaging studies of the
cerebral cortex find greater activation in the auditory regions of
the right temporal lobe when subjects are focusing on aspects of
harmony. Timbre also has been "assigned" a right temporal lobe
preference. Patients whose temporal lobe has been removed (such as
to eliminate seizures) show deficits in discriminating timbre if
tissue from the right, but not the left, hemisphere is excised. In
addition, the right temporal lobe becomes active in normal subjects
when they discriminate between different timbres.
Brain responses also depend on the experiences and training of
the listener. Even a little training can quickly alter the brain's
reactions. For instance, until about 10 years ago, scientists
believed that tuning was "fixed" for each cell in the auditory
cortex. Our studies on contour, however, made us suspect that cell
tuning might be altered during learning so that certain cells
become extra sensitive to sounds that attract attention and are
stored in memory.
Learning retunes the brain, so that more cells respond best to
behaviorally important sounds.
To find out, Jon S. Bakin, Jean-Marc Edeline and I conducted a
series of experiments during the 1990s in which we asked whether
the basic organization of the auditory cortex changes when a
subject learns that a certain tone is somehow important. Our group
first presented guinea pigs with many different tones and recorded
the responses of various cells in the auditory cortex to determine
which tones produced the greatest responses. Next, we taught the
subjects that a specific, nonpreferred tone was important by making
it a signal for a mild foot shock. The guinea pigs learned this
association within a few minutes. We then determined the cells'
responses again, immediately after the training and at various
times up to two months later. The neurons' tuning preferences had
shifted from their original frequencies to that of the signal tone.
Thus, learning retunes the brain so that more cells respond best to
behaviorally important sounds. This cellular adjustment process
extends across the cortex, "editing" the frequency map so that a
greater area of the cortex processes important tones. One can tell
which frequencies are important to an animal simply by determining
the frequency organization of its auditory cortex.
The retuning was remarkably durable: it became stronger over
time without additional training and lasted for months. These
findings initiated a growing body of research indicating that one
way the brain stores the learned importance of a stimulus is by
devoting more brain cells to the processing of that stimulus.
Although it is not possible to record from single neurons in humans
during learning, brain-imaging studies can detect changes in the
average magnitude of responses of thousands of cells in various
parts of the cortex. In 1998 Ray Dolan and his colleagues at
University College London trained human subjects in a similar type
of task by teaching them that a particular tone was significant.
The group found that learning produces the same type of tuning
shifts seen in animals. The long-term effects of learning by
retuning may help explain why we can quickly recognize a familiar
melody in a noisy room and also why people suffering memory loss
from neurodegenerative diseases such as Alzheimer's can still
recall music that they learned in the past.
Even when incoming sound is absent, we all can "listen" by
recalling a piece of music. Think of any piece you know and "play"
it in your head. Where in the brain is this music playing? In 1999
Andrea R. Halpern of Bucknell University and Robert J. Zatorre of
the Montreal Neurological Institute at McGill University conducted
a study in which they scanned the brains of nonmusicians who either
listened to music or imagined hearing the same piece of music. Many
of the same areas in the temporal lobes that were involved in
listening to the melodies were also activated when those melodies
were merely imagined.
Well-Developed Brains Studies of musicians have extended many of
the findings noted above, dramatically confirming the brain's
ability to revise its wiring in support of musical activities. Just
as some training increases the number of cells that respond to a
sound when it becomes important, prolonged learning produces more
marked responses and physical changes in the brain. Musicians, who
usually practice many hours a day for years, show such
effects--their responses to music differ from those of
nonmusicians; they also exhibit hyperdevelopment of certain areas
in their brains.
Christo Pantev, then at the University of Mnster in Germany, led
one such study in 1998. He found that when musicians listen to a
piano playing, about 25 percent more of their left-hemisphere
auditory regions respond than do so in nonmusicians. This effect is
specific to musical tones and does not occur with similar but
nonmusical sounds. Moreover, the authors found that this expansion
of response area is greater the younger the age at which lessons
began. Studies of children suggest that early musical experience
may facilitate development. In 2004 Antoine Shahin, Larry E.
Roberts and Laurel J. Trainor of McMaster University in Ontario
recorded brain responses to piano, violin and pure tones in four-
and five-year-old children. Youngsters who had received greater
exposure to music in their homes showed enhanced brain auditory
activity, comparable to that of unexposed kids about three years
older.
Musicians may display greater responses to sounds, in part
because their auditory cortex is more extensive. Peter Schneider
and his co-workers at the University of Heidelberg in Germany
reported in 2002 that the volume of this cortex in musicians was
130 percent larger. The percentages of volume increase were linked
to levels of musical training, suggesting that learning music
proportionally increases the number of neurons that process it.
In addition, musicians' brains devote more area toward motor
control of the fingers used to play an instrument. In 1995 Thomas
Elbert of the University of Konstanz in Germany and his colleagues
reported that the brain regions that receive sensory inputs from
the second to fifth (index to pinkie) fingers of the left hand were
significantly larger in violinists; these are precisely the fingers
used to make rapid and complex movements in violin playing. In
contrast, they observed no enlargement of the areas of the cortex
that handle inputs from the right hand, which controls the bow and
requires no special finger movements. Nonmusicians do not exhibit
these differences. Further, Pantev, now at the Rotman Research
Institute at the University of Toronto, reported in 2001 that the
brains of professional trumpet players react in such an intensified
manner only to the sound of a trumpet--not, for example, to that of
a violin.
Musicians also must develop greater ability to use both hands,
particularly for keyboard playing. Thus, one might expect that this
increased coordination between the motor regions of the two
hemispheres has an anatomical substrate. That seems to be the case.
The anterior corpus callosum, which contains the band of fibers
that interconnects the two motor areas, is larger in musicians than
in nonmusicians. Again, the extent of increase is greater the
earlier the music lessons began. Other studies suggest that the
actual size of the motor cortex, as well as that of the
cerebellum--a region at the back of the brain involved in motor
coordination--is greater in musicians.
Ode to Joy--or Sorrow beyond examining how the brain processes
the auditory aspects of music, investigators are exploring how it
evokes strong emotional reactions. Pioneering work in 1991 by John
A. Sloboda of Keele University in England revealed that more than
80 percent of sampled adults reported physical responses to music,
including thrills, laughter or tears. In a 1995 study by Jaak
Panksepp of Bowling Green State University, 70 percent of several
hundred young men and woman polled said that they enjoyed music
"because it elicits emotions and feelings." Underscoring those
surveys was the result of a 1997 study by Carol L. Krumhansl of
Cornell University. She and her co-workers recorded heart rate,
blood pressure, respiration and other physiological measures during
the presentation of various pieces that were considered to express
happiness, sadness, fear or tension. Each type of music elicited a
different but consistent pattern of physiological change across
subjects.
Until recently, scientists knew little about the brain
mechanisms involved. One clue, though, comes from a woman known as
I. R. (initials are used to maintain privacy), who suffered
bilateral damage to her temporal lobes, including auditory cortical
regions. Her intelligence and general memory are normal, and she
has no language difficulties. Yet she can make no sense of nor
recognize any music, whether it is a previously known piece or a
new piece that she has heard repeatedly. She cannot distinguish
between two melodies no matter how different they are.
Nevertheless, she has normal emotional reactions to different types
of music; her ability to identify an emotion with a particular
musical selection is completely normal! From this case we learn
that the temporal lobe is needed to comprehend melody but not to
produce an emotional reaction, which is both subcortical and
involves aspects of the frontal lobes.
An imaging experiment in 2001 by Anne Blood and Zatorre of
McGill sought to better specify the brain regions involved in
emotional reactions to music. This study used mild emotional
stimuli, those associated with people's reactions to musical
consonance versus dissonance. Consonant musical intervals are
generally those for which a simple ratio of frequencies exists
between two tones. An example is middle C (about 260 hertz, or Hz)
and middle G (about 390 Hz). Their ratio is 2:3, forming a
pleasant-sounding "perfect fifth" interval when they are played
simultaneously. In contrast, middle C and C sharp (about 277 Hz)
have a "complex" ratio of about 8:9 and are considered unpleasant,
having a "rough" sound.
What are the underlying brain mechanisms of that experience? PET
(positron emission tomography) imaging conducted while subjects
listened to consonant or dissonant chords showed that different
localized brain regions were involved in the emotional reactions.
Consonant chords activated the orbitofrontal area (part of the
reward system) of the right hemisphere and also part of an area
below the corpus callosum. In contrast, dissonant chords activated
the right parahippocampal gyrus. Thus, at least two systems, each
dealing with a different type of emotion, are at work when the
brain processes emotions related to music. How the different
patterns of activity in the auditory system might be specifically
linked to these differentially reactive regions of the hemispheres
remains to be discovered.
In the same year, Blood and Zatorre added a further clue to how
music evokes pleasure. When they scanned the brains of musicians
who had chills of euphoria when listening to music, they found that
music activated some of the same reward systems that are stimulated
by food, sex and addictive drugs.
Overall, findings to date indicate that music has a biological
basis and that the brain has a functional organization for music.
It seems fairly clear, even at this early stage of inquiry, that
many brain regions participate in specific aspects of music
processing, whether supporting perception (such as apprehending a
melody) or evoking emotional reactions. Musicians appear to have
additional specializations, particularly hyperdevelopment of some
brain structures. These effects demonstrate that learning retunes
the brain, increasing both the responses of individual cells and
the number of cells that react strongly to sounds that become
important to an individual. As research on music and the brain
continues, we can anticipate a greater understanding not only about
music and its reasons for existence but also about how multifaceted
it really is.Do You Hear What I Hear?
By Paul D. Lehrman
LEARNING TO LISTEN IN A MEDIATED WORLD
There's a priceless moment on the Firesign Theatre's third album
when an authority figure (a prosecutor who is somehow also an
auctioneer) bellows, What do I hear? and a stoned voice from the
back of the room responds, That's metaphysically absurd, man. How
can I know what you hear?
This brings to mind two questions. First of all, as we're
professionals who depend on our hearing to produce sounds that will
appeal to other people's ears, how do we know what our audience is
actually hearing? And second, for that matter, how do we know what
we're hearing? These two questions are becoming even more prevalent
today, as most music listeners are enjoying sounds on low-fi
playback systems or headphones far from the quality of studio
monitors.
When it comes to our audience, you might as well ask, What do
you mean by green? Physicists can agree on a range of wavelengths
for green, while everyone else can point to different objects and
get a general consensus from those around them that said objects
are or are not the color in question. But no one can possibly put
themselves into someone else's mind to see exactly how they
experience green. As conscious beings, our perceptions are ours
alone. Lily Tomlin's character Trudy the Bag Lady, in The Search
for Signs of Intelligent Life in the Universe, put it perfectly
when she said, Reality is nothing but a collective hunch.
Similarly with sound, we can measure its volume, look at its
spectrum, see how it changes over time and analyze the impulse
response of the space in which it's produced. But there's that
subjective response to the sound that's within our heads that can't
be measured at least not without a sensor on every brain cell and
synapse involved.
Because we're in the business of shaping the reality of sounds,
it's fairly important that our hunches be correct. And it's our
ears that we trust. No amount of visual or data analysis will allow
us to decide that a sound is right without hearing it.
How do we make that decision? A crucial part of the act of
hearing is making comparisons between what our ears are telling us
at the moment and the models that live in our memory of what we've
heard before. From the moment our auditory faculties first kick in,
those memories are established and baselines are formed. The first
sounds all humans hear are their mothers, and then they hear other
family members, then domestic sounds and gradually they take in the
larger world outside. I imagine it's a safe bet to say that for
most of us in this business, among those earliest aural experiences
were the sounds of singing and musical instruments. Not only did
these sounds intrigue and inspire us, but they provided us with the
context in which we would listen and judge the sounds we would work
with in our professional lives.
So we know what things are supposed to sound like. As
professionals, we learn something else: What we're hearing through
the studio monitors isn't the same as what we hear when there's a
direct acoustic path from the sound source to our ears. Ideally,
speakers would be totally flat with no distortion or phase error
and with perfect dispersion, but even the best monitors are still
far from being totally transparent. In addition, every indoor space
that's not an anechoic chamber has its peculiar colorations, which
are different from any other space. We need to be able to
compensate for these distortions, consciously or unconsciously, and
block out the sound of the speakers and the room as we listen. Our
experience and training as professionals teach us how to eliminate
the medium and concentrate on the source.
But this weird thing has happened in the past hundred or so
years, and the trend is accelerating: The proportion of musical
sounds that people are exposed to throughout their lives that are
produced by organic means has been decreasing and is quickly
approaching zero. This means that the baselines that we, and our
audiences, need to determine what sounds real and what doesn't are
disappearing.
Before the end of the 19th century, the only music anyone heard
was performed live. The sound that reached an audience member's
ears was that of the instruments and the singers, with nothing
mediating between the mechanism of production whether it was a
stick hitting a dried goatskin, the plucking of a taut piece of
feline intestine or the vibrations of a set of vocal cords and the
mechanism of perception.
But with the invention of the radio and the phonograph, all of
that has changed. People could now listen to music 24 hours a day
every day if they wanted and be nowhere near actual musicians.
Compared to real instruments, wax cylinders and crystal sets
sounded dreadful, but the convenience of hearing a huge variety of
music at any time without leaving home more than made up for the
loss in quality for most people.
The hi-fi boom that started in the 1950s improved things, as
listeners began to appreciate better sound reproduction and the
price of decent-sounding equipment fell to where even college
students who soon became the music industry's most important market
could afford it. Today's high-end and even medium-priced home audio
equipment sounds better than ever.
But as the media for music delivery have blossomed from wax
cylinders to XM Radio fewer people experience hearing acoustic
music. Symphony orchestras are cutting back seasons or going out of
business altogether all over America, and school music programs,
which traditionally have given students the precious opportunity to
hear what real instruments sound like from both a player's and a
listener's perspective, are in the toilet. While there are
certainly parts of the live music scene that are still healthy,
they depend on sound systems that, as they get bigger and more
complex to project to the farthest reaches of a large venue, serve
to isolate the audiences even more from what's happening onstage
acoustically.
And, as electronic sources of music have become more prolific,
another thing has happened: Because it is now so easy to listen to
music, people actually listen to it less, and it has become more of
an environmental element like aural wallpaper. Because audiences
aren't focusing so much on the music, the quality of the systems
that many listen to has been allowed to slip backward. Personal
stereos have been a major factor in this: From the Sony Walkman to
the iPod, people are listening to crummy sound reproduction at top
volume, screening out any kind of sonic reality and replacing it
with a lo-fi sound. Everyone can now have their own private
soundtrack, as if they were perpetually walking alone through a
theme park, without any other aural distractions, with a 15dB
dynamic range and nothing below 100 Hz.
I remember this hitting me like a ton of bricks one day in the
summer of 1979. I had been out of the country for a few months, and
soon after I returned to the U.S., I was walking in New York City's
Central Park and came upon an amazing picture: On a patch of
blacktop were several dozen gyrating disco-dancing roller skaters,
but the only sound I could hear was that of the skate wheels on the
pavement. Each of the dancers was sporting a pair of headphones
with little antennae coming out of them. Inside each of the
headphones, I soon realized, was an FM radio, and they were all
dancing to music that I couldn't hear. But it became obvious after
I watched them for a few minutes that they weren't all dancing to
the same music; each was tuned to a different station.
The multimedia speaker systems that people now plug into their
computers so they can listen to MP3 streams have taken us further
down the same road. Companies that decades ago revolutionized
speaker designs such as Advent, KLH and Altec Lansing have had
their brands swallowed up by multinational electronics foundries
that slap those once-revered names on tinny little underpowered
speakers connected to subwoofers that produce a huge hump at 120 Hz
so that consumers think they're getting something for their
money.
More recently, the tools of personal audio wallpaper have
entered the production chain. Again, one incident sticks out in my
mind that showed me clearly where this was going: A couple of years
ago, I went into a hip coffeehouse where the blaring post-punk
music makes it impossible to hold a normal conversation and sat
down at a table near a young man wearing earbuds and peering
intently into a PowerBook. I glanced over, and to my amazement, I
realized he was working on something in Digital Performer.
How many composers live in apartment buildings where they work
late into the night and, for fear of disturbing their neighbors,
never turn on their monitors but only mix on headphones? How many
of your colleagues, or even you, boast of doing some of your best
audio editing on a transcontinental plane flight?
A pessimist looking at this might conclude we were approaching a
kind of perfect storm in which we lose complete control over what
our audience hears. No one ever finds out what a real instrument
sounds like; the systems that we use to reproduce and disseminate
music are getting worse. And because most people don't even listen
closely to music anymore, they don't care.
In my own teaching, I've seen how the lack of proper aural
context results in an inability to discriminate between good and
bad, real and not-real sound. In one of my school's music labs, I
use a 14-year-old synth that, although I really like it as a
teaching tool, I'll be the first to admit has a factory program set
that is a little dated. But one of my students recently said, The
sounds are so realistic, why would anyone need to use anything
else?
There are nine workstations in that lab, which means the
students have to work on headphones. We use pretty decent
closed-ear models and the students generally don't have any
complaints. That is until we play back their assignments on the
room's powered speakers. Why does it sound so incredibly different?
one will invariably ask. I take this as a splendid opportunity to
teach them something about acoustics: how reflections and room
modes affect bass response, the role of head effects in stereo
imaging and so on. They dutifully take it in, but then they say,
Yes, but why does it sound so incredibly different? The idea of the
music and the medium being separate from each other sometimes just
doesn't sink in.
If you're looking for an answer or even a conclusion here, I
haven't got one. But I do know that the next generation of audio
engineers and mixers if there's going to be one will have a hard
time if they don't have more exposure than the average young person
to natural, unamplified and unprocessed sound. If every sound we
ever hear comes through a medium (and most of them suck), then how
are we ever going to agree on what we hear?
Which means that our ears and our judgment are still all we
have. Try to take care of both of them. And keep listening and keep
learning.
How We Localize Sound Relying on a variety of cues, including
intensity, timing, and spectrum, our brains recreate a
three-dimensional image of the acoustic landscape from the sounds
we hear. -- William M. HartmannFor as long as we humans have lived
on Earth, we have been able to use our ears to localize the sources
of sounds. Our ability to localize warns us of danger and helps us
sort out individual sounds from the usual cacophony of our
acoustical world. Characterizing this ability in humans and other
animals makes an intriguing physical, physiological, and
psychological study (see figure 1). John William Strutt (Lord
Rayleigh) understood at least part of the localization process more
than 120 years ago.1 He observed that if a sound source is to the
right of the listeners forward direction, then the left ear is in
the shadow cast by the listeners head. Therefore, the signal in the
right ear should be more intense than the signal in the left one,
and this difference is likely to be an important clue that the
sound source is located on the right.
Interaural level differenceThe standard comparison between
intensities in the left and right ears is known as the interaural
level difference (ILD). In the spirit of the spherical cow, a
physicist can estimate the size of the effect by calculating the
acoustical intensity at opposite poles on the surface of a sphere,
given an incident plane wave, and then taking the ratio. The level
difference is that ratio expressed in decibels. As shown in figure
2, the ILD is a strong function of frequency over much of the
audible spectrum (canonically quoted as 2020 000 Hz). That is
because sound waves are effectively diffracted when their
wavelength is longer than the diameter of the head. At a frequency
of 500 Hz, the wavelength of sound is 69 cm -- four times the
diameter of the average human head. The ILD is therefore small for
frequencies below 500 Hz, as long as the source is more than a
meter away. But the scattering by the head increases rapidly with
increasing frequency, and at 4000 Hz the head casts a significant
shadow.
Ultimately, the use of an ILD, small or large, depends on the
sensitivity of the central nervous system to such differences. In
evolutionary terms, it would make sense if the sensitivity of the
central nervous system would somehow reflect the ILD values that
are actually physically present. In fact, that does not appear to
be the case. Psychoacoustical experiments find that the central
nervous system is about equally sensitive at all frequencies. The
smallest detectable change in ILD is approximately 0.5 dB, no
matter what the frequency.2 Therefore the ILD is a potential
localization cue at any frequency where it is physically greater
than a decibel. It is as though Mother Nature knew in advance that
her offspring would walk around the planet listening to portable
music through headphones. The spherical-head model is obviously a
simplification. Human heads include a variety of secondary
scatterers that can be expected to lead to structure in the
higher-frequency dependence of the ILD. Conceivably, this structure
can serve as an additional cue for sound localization. As it turns
out, that is exactly what happens, but that is another story for
later in this article.
In the long-wavelength limit, the spherical-head model correctly
predicts that the ILD should become uselessly small. If sounds are
localized on the basis of ILD alone, it should be very difficult to
localize a sound with a frequency content that is entirely below
500 Hz. It therefore came as a considerable surprise to Rayleigh to
discover that he could easily localize a steady-state low-frequency
pure tone such as 256 or 128 Hz. Because he knew that localization
could not be based on ILD, he finally concluded in 1907 that the
ear must be able to detect the difference in waveform phases
between the two ears.3Interaural time differenceFor a pure tone
like Rayleigh used, a difference in phases is equivalent to a
difference in arrival times of waveform features (such as peaks and
positive-going zero crossings) at the two ears. A phase difference
corresponds to an interaural time difference (ITD) of t = /(2f) for
a tone with frequency f. In the long-wavelength limit, the formula
for diffraction by a sphere4 gives the interaural time difference t
as a function of the azimuthal (leftright) angle :
where a is the radius of the head (approximately 8.75 cm) and c
is the speed of sound (34 400 cm/s). Therefore, 3a/c = 763 s.
Psychoacoustical experiments show that human listeners can
localize a 500 Hz sine tone with considerable accuracy. Near the
forward direction ( near zero), listeners are sensitive to
differences as small as 12. The idea that this sensitivity is
obtained from an ITD initially seems rather outrageous. A 1
difference in azimuth corresponds to an ITD of only 13 s. It hardly
seems possible that a neural system, with synaptic delays on the
order of a millisecond, could successfully encode such small time
differences. However, the auditory system, unaware of such
mathematical niceties, goes ahead and does it anyway. This ability
can be proved in headphone experiments, in which the ITD can be
presented independently of the ILD. The key to the brains success
in this case is parallel processing. The binaural system apparently
beats the unfavorable timing dilemma by transmitting timing
information through many neurons. Estimates of the number of
neurons required, based on statistical decision theory, have ranged
from 6 to 40 for each one-third-octave frequency band.
There remains the logical problem of just how the auditory
system manages to use ITDs. There is now good evidence that the
superior olivea processing center, or nucleus, in the midbrainis
able to perform a cross-correlation operation on the signals in the
two ears, as described in the box below. The headphone experiments
with an ITD give the listener a peculiar experience. The position
of the image is located to the left or right as expected, depending
on the sign of the ITD, but the image seems to be within the
listeners headit is not perceived to be in the real external world.
Such an image is said to be lateralized and not localized. Although
the lateralized headphone sensation is quite different from the
sensation of a localized source, experiments show that
lateralization is intimately connected to localization.
Figure 1. The sound localization facility at Wright Patterson
Air Force Base in Dayton, Ohio, is a geodesic sphere, nearly 5 m in
diameter, housing an array of 277 loudspeakers. Each speaker has a
dedicated power amplifier, and the switching logic allows the
simultaneous use of as many as 15 sources. The array is enclosed in
a 6 m cubical anechoic room: Foam wedges 1.2 m long on the walls of
the room make the room strongly absorbing for wavelengths longer
than 5 m, or frequencies above 70 Hz. Listeners in localization
experiments indicate perceived source directions by placing an
electromagnetic stylus on a small globe. (Courtesy of Mark Ericson
and Richard McKinley.)
Using headphones, one can measure the smallest detectable change
in ITD as a function of the ITD itself. These ITD data can be used
with equation 1 to predict the smallest detectable change in
azimuth for a real source as a function of . When the actual
localization experiment is done with a real source, the results
agree with the predictions, as is to be expected if the brain
relies on ITDs to make decisions about source location.
Like any phase-sensitive system, the binaural phase detector
that makes possible the use of ITDs suffers from phase ambiguity
when the wavelength is comparable to the distance between the two
measurements. This problem is illustrated in figure 3. The
equivalent temporal viewpoint is that, to avoid ambiguity, a half
period of the wave must be longer than the delay between the ears.
When the delay is exactly half a period, the signals at the two
ears are exactly out of phase and the ambiguity is complete. For
shorter periods, between twice the delay and the delay itself, the
ITD leads to an apparent source location that is on the opposite
side of the head compared to the true location. It would be better
to have no ITD sensitivity at all than to have a process that gives
such misleading answers. In fact, the binaural system solves this
problem in what appears to be the best possible way: The binaural
system rapidly loses sensitivity to any ITD at all as the frequency
of the wave increases from 1000 to 1500 Hzexactly the range in
which the interaural phase difference becomes ambiguous.
One might imagine that the network of delay lines and
coincidence detectors described in the box vanishes at frequencies
greater than about 1500 Hz. Such a model would be consistent with
the results of pure-tone experiments, but it would be wrong. In
fact, the binaural system can successfully register an ITD that
occurs at a high frequency such as 4000 Hz, if the signal is
modulated. The modulation, in turn, must have a rate that is less
than about 1000 Hz. Therefore, the failure of the binaural timing
system to process sine tones above 1500 Hz cannot be thought of as
a failure of the binaural neurons tuned to high frequency. Instead,
the failure is best described in the temporal domain, as an
inability to track rapid variations.
To summarize the matter of binaural differences, the physiology
of the binaural system is sensitive to amplitude cues from ILDs at
any frequency, but for incident plane waves, ILD cues exist
physically only for frequencies above about 500 Hz. They become
large and reliable for frequencies above 3000 Hz, making ILD cues
most effective at high frequencies. In contrast, the binaural
physiology is capable of using phase information from ITD cues only
at low frequencies, below about 1500 Hz. For a sine tone of
intermediate frequency, such as 2000 Hz, neither cue works well. As
a result, human localization ability tends to be poor for signals
in this frequency region.
The inadequacy of binaural difference cuesThe binaural time and
level differences are powerful cues for the localization of a
source, but they have important limitations. Again, in the
spherical-head approximation, the inadequacy of interaural
differences is evident because, for a source of sound moving in the
midsagittal plane (the perpendicular bisector of a line drawn
through both ears), the signals to left and right earsand therefore
binaural differencesare the same. As a result, the listener with
the hypothetical spherical head cannot distinguish between sources
in back, in front, or overhead. Because of a fine sensitivity to
binaural differences, this listener can detect displacements of
only a degree side-to-side, but cannot tell back from front! This
kind of localization difficulty does not correspond to our usual
experience. There is another problem with this binaural difference
model: If a tone or broadband noise is heard through headphones
with an ITD, an ILD, or both, the listener has the impression of
lateralitycoming from the left or rightas expected, but, as
previously mentioned, the sound image appears to be within the
head, and it may also be diffuse and fuzzy instead of compact. This
sensation, too, is unlike our experience of the real world, in
which sounds are perceived to be externalized. The resolution of
frontback confusion and the externalization of sound images turn on
another sound localization cue, the anatomical transfer
function.
Figure 2. Interaural level differences, calculated for a source
in the azimuthal plane defined by the two ears and the nose. The
source radiates frequency f and is located at an azimuth of 10
(green curve), 45 (red), or 90 (blue) with respect to the listeners
forward direction. The calculations assume that the ears are at
opposite poles of a rigid sphere.
The anatomical transfer functionSound waves that come from
different directions in space are differently scattered by the
listeners outer ears, head, shoulders, and upper torso. The
scattering leads to an acoustical filtering of the signals
appearing at left and right ears. The filtering can be described by
a complex response functionthe anatomical transfer function (ATF),
also known as the head-related transfer function (HRTF). Because of
the ATF, waves that come from behind tend to be boosted in the 1000
Hz frequency region, whereas waves that come from the forward
direction are boosted near 3000 Hz. The most dramatic effects occur
above 4000 Hz: In this region, the wavelength is less than 10 cm
and details of the head, especially the outer ears, or pinnae,
become significant scatterers. Above 6000 Hz, the ATF for different
individuals becomes strikingly individualistic, but there are a few
features that are found rather generally. In most cases, there is a
valley-and-peak structure that tends to move to higher frequencies
as the elevation of the source increases from below to above the
head. For example, figure 4 shows the spectrum for sources in
front, in back, and directly overhead, measured inside the ear of a
Knowles Electronics Manikin for Acoustic Research (KEMAR). The peak
near 7000 Hz is thought to be a particularly prominent cue for a
source overhead. The direction-dependent filtering by the anatomy,
used by listeners to resolve frontback confusion and to determine
elevation, is also a necessary component of externalization.
Experiments further show that getting the ATF correct with virtual
reality techniques is sufficient to externalize the image. But
there is an obvious problem in the application of the ATF. A
priori, there is no way that a listener can know if a spectrally
prominent feature comes from direction-dependent filtering or
whether it is part of the original source spectrum. For instance, a
signal with a strong peak near 7000 Hz may not necessarily come
from aboveit might just come from a source that happens to have a
lot of power near 7000 Hz.
Figure 3. Interaural time differences, given by the difference
in arrival times of waveform features at the two ears, are useful
localization cues only for long wavelengths. In (a), the signal
comes from the right, and waveform features such as the peak
numbered 1 arrive at the right ear before arriving at the left.
Because the wavelength is greater than twice the head diameter, no
confusion is caused by other peaks of the waveform, such as peaks 0
or 2. In (b), the signal again comes from the right, but the
wavelength is shorter than twice the head diameter. As a result,
every feature of cycle 2 arriving at the right ear is immediately
preceded by a corresponding feature from cycle 1 at the left ear.
The listener naturally concludes that the source is on the left,
contrary to fact.Confusion of this kind between the source spectrum
and the ATF immediately appears with narrow-band sources such as
pure tones or noise bands having a bandwidth of a few semitones.
When a listener is asked to say whether a narrow-band sound comes
from directly in front, in back, or overhead, the answer will
depend entirely on the frequency of the soundthe true location of
the sound source is irrelevant.5 Thus, for narrow-band sounds, the
confusion between source spectrum and location is complete. The
listener can solve this localization problem only by turning the
head so that the source is no longer in the midsagittal plane. In
an interesting variation on this theme, Frederic Wightman and Doris
Kistler at the University of WisconsinMadison have shown that it is
not enough if the source itself movesthe listener will still be
confused about front and back. The confusion can be resolved,
though, if the listener is in control of the source
motion.6Fortunately, most sounds of the everyday world are
broadband and relatively benign in their spectral variation, so
that listeners can both localize the source and identify it on the
basis of the spectrum. It is still not entirely clear how this
localization process works. Early models of the process that
focused on particular spectral features (such as the peak at 7000
Hz for a source overhead) have given way, under the pressure of
recent research, to models that employ the entire spectrum.
The Binaural Cross-Correlation Model
In 1948, Lloyd Jeffress proposed that the auditory system
processes interaural time differences by using a network of neural
delay lines terminating in ee neurons.10 An ee neuron is like an
AND gate, responding only if excitation is present on both of two
inputs (hence the name ee). According to the Jeffress model, one
input comes from the left ear and the other from the right. Inputs
are delayed by neural delay lines so that different ee cells
experience a coincidence for different arrival times at the two
ears. An illustration of how the network is imagined to work is
shown in the figure. An array of ee cells is distributed along two
axes: frequency and neural internal delay. The frequency axis is
needed because binaural processing takes place in tuned channels.
These channels represent frequency analysisthe first stage of
auditory processing. Any plausible auditory model must contain such
channels.Inputs from left ear (blue) and right ear (red) proceed
down neural delay lines in each channel and coincide at the ee
cells for which the neural delay exactly compensates for the fact
that the signal started at one ear sooner than the other. For
instance, if the source is off to the listeners left, then signals
start along the delay lines sooner from the left side. They
coincide with the corresponding signals from the right ear at
neurons to the right of = 0, that is, at a positive value of . The
coincidence of neural signals causes the ee neurons to send spikes
to higher processing centers in the brain.The expected value for
the number of coincidences Nc at the ee cell specified by delay t
is given in terms of the rates PL(t) and PR(t) of neural spikes
from left and right ears by the convolution-like integral
where TW is the width of the neurons coincidence window and TS
is the duration of the stimulus.11 Thus, Nc is the cross
correlation between signals in the left and right ears. Neural
delay and coincidence circuits of just this kind have been found in
the superior olive in the midbrain of cats.12
The experimental artMost of what we know about sound
localization has been learned from experiments using headphones.
With headphones, the experimenter can precisely control the
stimulus heard by the listener. Even experiments done on cats,
birds, and rodents have these creatures wearing miniature
earphones. In the beginning, much was learned about fundamental
binaural capabilities from headphone experiments with simple
differences in level and arrival time for tones of various
frequencies and noises of various compositions.7 However, work on
the larger question of sound localization had to await several
technological developments to achieve an accurate rendering of the
ATF in each ear. First were the acoustical measurements themselves,
done with tiny probe microphones inserted in the listeners ear
canals to within a few millimeters of the eardrums. Transfer
functions measured with these microphones allowed experimenters to
create accurate simulations of the real world using headphones,
once the transfer functions of the microphones and headphones
themselves had been compensated by inverse filtering.Adequate
filtering requires fast, dedicated digital signal processors linked
to the computer that runs experiments. The motion of the listeners
head can be taken into account by means of an electromagnetic head
tracker. The head tracker consists of a stationary transmitter,
whose three coils produce low-frequency magnetic fields, and a
receiver, also with three coils, that is mounted on the listeners
head. The tracker gives a reading of all six degrees of freedom in
the head motion, 60 times per second. Based on the motion of the
head, the controlling computer directs the fast digital processor
to refilter the signals to the ears so that the auditory scene is
stable and realistic. This virtual reality technology is capable of
synthesizing a convincing acoustical environment. Starting with a
simple monaural recording of a conversation, the experimenter can
place the individual talkers in space. If the listeners head turns
to face a talker, the auditory image remains constant, as it does
in real life. What is most important for the psychoacoustician,
this technology has opened a large new territory for controlled
experiments.Making it wrongWith headphones, the experimenter can
create conditions not found in nature to try to understand the role
of different localization mechanisms. For instance, by introducing
an ILD that points to the left opposed by an ITD that points to the
right, one can study the relative strengths of these two cues. Not
surprisingly, it is found that ILDs dominate at high frequency and
ITDs dominate at low frequency. But perception is not limited to
just pointlike localization; it also includes size and shape.
Rivalry experiments such as contradictory ILDs and ITDs lead to a
source image that is diffuse: The image occupies a fuzzy region
within the head that a listener can consistently describe. The
effect can also be measured as an increased variance in
lateralization judgments.
The curves show the spectrum of a small loudspeaker as heard in
the left ear of a manikin when the speaker is in front (red),
overhead (blue), and in back (green). A comparison of the curves
reveals the relative gains of the anatomical transfer function.
left) The KEMAR manikin is, in every gross anatomical detail, a
typical American. It has silicone outer ears and microphones in its
head. The coupler between the ear canal and the microphone is a
cavity tuned to have the input acoustical impedance of the middle
ear. The KEMAR shown here is in an anechoic room accompanied by
Tim, an undergraduate physics major at Michigan State.Incorporating
the ATF into headphone simulations considerably expands the menu of
bizarre effects. An accurate synthesis of a broadband sound leads
to perception that is like the real world: Auditory images are
localized, externalized, and compact. Making errors in the
synthesis, for example progressively zeroing the ITD of spectral
lines while retaining the amplitude part of the ATF, can cause the
image to come closer to the head, push on the face, and form a blob
that creeps into the ear canal and finally enters the head. The
process can be reversed by progressively restoring accurate ITD
values.8 A wide variety of effects can occur, by accident or
design, with inaccurate synthesis. There are a few general rules:
Inaccuracies tend to expand the size of the image, put the images
inside the head, and produce images that are in back rather than in
front. Excellent accuracy is required to avoid frontback confusion.
The technology permits a listener to hear the world with someone
elses ears, and the usual result is an increase in confusion about
front and back. Reduced accuracy often puts all source images in
back, although they are nevertheless externalized. Further
reduction in accuracy puts the images inside the back of the
head.Rooms and reflectionsThe operations of interaural level and
time difference cues and of spectral cues have normally been tested
with headphones or by sound localization experiments in anechoic
rooms, where all the sounds travel in a straight path from the
source to the listener. Most of our everyday listening, however, is
done in the presence of walls, floors, ceilings, and other large
objects that reflect sound waves. These reflections result in
dramatic physical changes to the waveforms. It is hard to imagine
how the reflected sounds, coming from all directions, can
contribute anything but random variation to the cues used in
localization. Therefore, it is expected that the reflections and
reverberation introduced by the room are inevitably for the worse
as far as sound localization is concerned. That is especially true
for the ITD cue.The ITD is particularly vulnerable because it
depends on coherence between the signals in the two earsthat is,
the height of the cross-correlation function, as described in the
box above. Reverberated sound contains no useful coherent
information, and in a large room where reflected sound dominates
the direct sound, the ITD becomes unreliable.By contrast, the ILD
fares better. First, as shown by headphone experiments, the
binaural comparison of intensities does not care whether the
signals are binaurally coherent or not. Such details of neural
timing appear to be stripped away as the ILD is computed. Of
course, the ILD accuracy is adversely affected by standing waves in
a room, but here the second advantage of the ILD appears: Almost
every reflecting surface has the property that its acoustical
absorption increases with increasing frequency; as a result, the
reflected power becomes relatively smaller compared to the direct
power. Because the binaural neurophysiology is capable of using
ILDs across the audible spectrum with equal success, it is normally
to the listeners advantage to use the highest frequency information
that can be heard. Experiments in highly reverberant environments
find listeners doing exactly that, using cues above 8000 Hz. A
statistical decision theory analysis using ILDs and ITDs measured
with a manikin shows that the pattern of localization errors
observed experimentally can be understood by assuming that
listeners rely entirely on ILDs and not at all on ITDs. This
strategy of reweighting localization cues is entirely
unconscious.The precedence effectThere is yet another strategy that
listeners unconsciously employ to cope with the distorted
localization cues that occur in a room: They make their
localization judgments instantly based on the earliest arriving
waves in the onset of a sound. This strategy is known as the
precedence effect, because the earliest arriving sound wavethe
direct sound with accurate localization informationis given
precedence over the subsequent reflections and reverberation that
convey inaccurate information. Anyone who has wandered around a
room trying to locate the source of a pure tone without hearing the
onset can appreciate the value of the effect. Without the action of
the precedence effect on the first arriving wave, localization is
virtually impossible. There is no ITD information of any use, and,
because of standing waves, the loudness of the tone is essentially
unrelated to the nearness of the source.
Figure 5. Precedence effect demonstration with two loudspeakers
reproducing the same pulsed wave. The pulse from the left speaker
leads in the left ear by a few hundred microseconds, suggesting
that the source is on the left. The pulse from the right speaker
leads in the right ear by a similar amount, which provides a
contradictory localization cue. Because the listener is closer to
the left speaker, the left pulse arrives sooner and wins the
competitionthe listener perceives just one single pulse coming from
the left.
The operation of the precedence effect is often thought of as a
neural gate that is opened by the onset of a sound, accumulates
localization information for about 1 ms, and then closes to shut
off subsequent localization cues. This operation appears
dramatically in experiments where it is to the listeners advantage
to attend to the subsequent cues but the precedence effect prevents
it. An alternative model regards precedence as a strong reweighting
of localization cues in favor of the earliest sound, because the
subsequent sound is never entirely excluded from the localization
computation.Precedence is easily demonstrated with a standard home
stereo system set for monophonic reproduction, so that the same
signal is sent to both loudspeakers. Standing midway between the
speakers, the listener hears the sound from a forward direction.
Moving half a meter closer to the left speaker causes the sound to
appear to come entirely from that speaker. The analysis of this
result is that each speaker sends a signal to both ears. Each
speaker creates an ILD andof particular importancean ITD, and these
cues compete, as shown in figure 5. Because of the precedence
effect, the first sound (from the left speaker) wins the
competition, and the listener perceives the sound as coming from
the left. But although the sound appears to come from the left
speaker alone, the right speaker continues to contribute loudness
and a sense of spatial extent. This perception can be verified by
suddenly unplugging the right speakerthe difference is immediately
apparent. Thus, the precedence effect is restricted to the
formation of a single fused image with a definite location. The
precedence effect appears not to depend solely on interaural
differences; it operates also on the spectral differences caused by
anatomical filtering for sources in the midsagittal
plane.9Conclusions and conjecturesAfter more than a century of
work, there is still much about sound localization that is not
understood. It remains an active area of research in
psychoacoustics and in the physiology of hearing. In recent years,
there has been growing correspondence between perceptual
observations, physiological data on the binaural processing system,
and neural modeling. There is good reason to expect that next year
we will understand sound localization better than we do this year,
but it would be wrong to think that we have only to fill in the
details. It is likely that next year will lead to a qualitatively
improved understanding with models that employ new ideas about
neural signal processing. In this environment, it is risky to
conjecture about future development, but there are trends that give
clues. Just a decade ago, it was thought that much of sound
localization in general, and precedence in particular, might be a
direct result of interaction at early stages of the binaural
system, as in the superior olive. Recent research suggests that the
process is more widely distributed, with peripheral centers of the
brain such as the superior olive sending informationabout ILD,
about ITD, about spectrum, and about arrival orderto higher centers
where the incoming data are evaluated for self-consistency and
plausibility, and are probably compared with information obtained
visually. Therefore, sound localization is not simple; it is a
large mental computation. But as the problem has become more
complicated, our tools for studying it have become better. Improved
psychophysical techniques for flexible synthesis of realistic
stimuli, physiological experiments probing different neural regions
simultaneously, faster and more precise methods of brain imaging,
and more realistic computational models will one day solve this
problem of how we localize sound. Bill Hartmann is a professor of
physics at Michigan State University in East Lansing, Michigan
([email protected]; http://www.pa.msu.edu/acoustics). He is the
author of the textbook Signals, Sound, and Sensation (AIP Press,
1997).
The author is grateful to his colleagues Brad Rakerd, Tim
McCaskey, Zachary Constan, and Joseph Gaalaas for help with this
article. His work on sound localization is supported by the
National Institute on Deafness and Other Communication Disorders,
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5.J. Blauert, Spatial Hearing, 2nd ed., J. S. Allen, trans., MIT
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Acoust. Soc. Am. 105, 2841 (1999). 7.N. I. Durlach, H. S. Colburn,
in Handbook of Perception, vol. 4, E. Carterette, M. P. Friedman,
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