麻豆传媒

DON鈥橳 think about anything in particular. Just tap away with your finger and
count the number of taps over a couple of minutes. The rate you fall into will
probably be about one beat every 600 milliseconds鈥攁 little slower than one
tap every half second.

Why? Some researchers believe that the tapping rate echoes a primal pulse
inside each of us. The beat, scientists say, is central to our existence. We use
it not only to make music, but to coordinate our brains and bodies, keep track
of time, and filter the stream of events that assail us in this unpredictable
world.

Carolyn Drake of CNRS, Ren茅 Descartes University in Paris is one
researcher who beleives we have an internal beat. She asks people to tap their
fingers just like in the exercise above and finds that adults鈥� tapping rates are
remarkably consistent鈥攏ot only during each session, but also a week or a
month later. What鈥檚 more, Drake says, 鈥渨hen you look at the rate people walk,
their heartbeat and infant suckling rates, they are all in the same range.鈥�
Without an external cue, each activity tends to follow the same drummer.

The first person to notice this preferred timing was cognitive psychologist
Paul Fraisse in the 1940s and 1950s. Then in the 1970s, neuropsychologist Mari
Reiss Jones of Ohio State University in Columbus suggested that the physical
tempo corresponds to an internal pulse that regulates our attentions. Other
neuroscientists have been slow to accept the idea, but now they are starting to
come round.

One reason is the phenomenon of 鈥渁bsolute tempo鈥�. Some conductors can beat 60
to the minute, or any other tempo you ask for, with astonishing accuracy. Daniel
Levitin of McGill University in Montreal thinks we all have this ability. He
asks people to sing a well-known song that they have only heard in one version,
and finds that they get the tempo just right. Hard to explain if we don鈥檛 have
some precise internal metronome to guide us.

But apart from making music, what is the timekeeper for? There must be
cultural advantages in being able to join in a knees-up or a hoedown, but it
goes a lot deeper than that.

Reiss Jones and Drake believe that we perceive the world in pulses, rather
than as a stream of consciousness, and that our internal metronome sets the
pace. Senses such as sight and hearing rely on the idea of contrast, because
it鈥檚 more efficient to confine your attention to places where things
change鈥攖he edge of an object, for example. The same is true of time, says
Drake. She believes we sample the world roughly twice a second, checking on each
sense to see if sounds or sights have changed. She goes on to suggest that we
perceive the world best during these pulsed check-ups. It鈥檚 how we filter out
information that would otherwise overwhelm us.

The internal pulse also marks out time, suggests Drake. 鈥淗ow otherwise can
you appreciate the fact that one event comes after another, or the fact that one
note is twice the length of another?鈥� Without instinctive rhythm, we would be
lost in a confusing world where everything happens at once.

We can, of course, deviate from a 600-millisecond pulse when we need to. Lots
of important things happen on longer and shorter timescales. So our internal
beat must somehow adapt, shifting when external rhythms change, or when we focus
on a different tempo. 鈥淭here may be a very complex dance between an organism and
the organism鈥檚 environment,鈥� says Reiss Jones.

That鈥檚 why people are good at picking out rhythms. According to Ed Large of
Florida Atlantic University in Boca Raton, we often mistakenly think rhythm is
solely a feature of the music. 鈥淵ou listen to a piece of music and you hear a
rhythm,鈥� says Large, who studies neural networks and sidelines as a jazz
musician. What really happens is that as you listen, the pattern of notes and
rests triggers an oscillation in the brain. After enough oscillations, the brain
鈥済ets it鈥�, and the dominant pulse in the mind shifts to match one of the periods
in the music. This 鈥渆ntrainment鈥� might even trigger the body to move, tap or
gyrate in time.

According to Bjorn Merker of the Royal University College of Music in
Stockholm, more complicated rhythms are merely variations on simple cadences
that humans can entrain to. You have to start with the beat鈥攖he
subdivision of time that makes the next downstroke of the conductor鈥檚 baton
predictable. 鈥淏eneath different kinds of rhythm is a very simple thing, a steady
dub, dub, dub,鈥� he says. 鈥淚f it is not there, the rest falls apart quickly.鈥�

Most people prefer fairly simple rhythms, because they are easy to entrain
to. Drake鈥檚 experiments suggest that when there is a main beat of 400
milliseconds, for example, the brain automatically sets up other oscillations at
multiples and fractions of this鈥�100, 200, 800 and 1600 milliseconds. So
when each second or fourth beat of a bar is emphasised, for example, two pulses
can easily be excited in the brain. More complicated rhythms such as 5/3 need
other multiples, so they may fall outside the average listener鈥檚 ability to
discriminate, and the less able performer鈥檚 ability to play.

Another reason to reset your metronome is to join in with a group. From an
evolutionary perspective, coordinating rhythms with other animals can have real
benefits. When a group of chimps hoot in unison, for example, the sound is far
more likely to reach a potential mate, say, than shouting alone, says Merker,
who has studied the way male chimps call and dance in synchrony during their
carnival displays. Even insects can entrain to the sounds and sights of their
neighbours. Cicadas, crickets and fireflies buzz or beam together as part of
clever mating games.

Mass communication

Although humans might not shout and whoop in chorus to attract a
mate鈥攅xcept perhaps in nightclubs鈥攚e do communicate en masse.
Physicist Albert-L谩szl贸 Barab谩si from the University of
Notre Dame in Indiana and his colleagues studied crowds applauding at the end of
opera and theatre performances (Nature, vol 403, p 849).

Audiences lapsed into synchronised clapping at roughly half the average
initial rate of individual members of the crowd. This seemed odd to
Barab谩si鈥檚 team, since the average noise intensity decreased with such a
slowing鈥攏ot a good way to convey enthusiasm. But the volume of each clap
rose greatly, the scientists noted. Just like chorusing chimps, audience members
seem to realise unconsciously that they have a larger impact as a group working
in synchrony. We probably use the same mechanism to coordinate group activities
such as rowing, dancing and marching.

The rhythm instinct may also enable us to learn language. Infants seem to
concentrate on the rhythm of language, first simple patterns, then more complex
ones, long before putting the significant sounds together into meaningful words
and sentences.

Frank Ramus at France鈥檚 national agency for scientific research in Paris took
phrases spoken in different languages and replaced each syllable with the sound
鈥渟a鈥�. He then asked native speakers to articulate these strings of sa-sa sounds
with the rhythm of the original phrase, and played the gibberish back to
infants. He found that infants as young as two months can tell the difference
between the rhythms of their native language and those of another. 鈥淪ensitivity
to rhythm may be built into babies鈥� brains to initiate language acquisition,鈥�
Ramus says.

We might have to go back even further in development to find the origins of
adults鈥� intrinsic, 600-millisecond beat. Reiss Jones and her supporters once
thought that a single cluster of neurons in the brain, dubbed an internal
oscillator, supplied this frequency. This seems to be true of insects and other
lower organisms, but it can鈥檛 be true of people. A single oscillator wouldn鈥檛 be
able to change its frequency by more than about 10 per cent, says Drake, so this
can鈥檛 explain entrainment to the wide range of beats in music, for example.

Reiss Jones now thinks that people are born with many oscillators. She and
others hypothesise that, perhaps because of early experiences, infants begin to
focus heavily on certain well-used tempos, while leaving others dormant.

Drake played a series of evenly spaced beeps at a particular tempo to
suckling infants between 2 and 4 months old. Initially, babies find such sounds
interesting, and turn their heads towards them, but eventually they get bored
and stop turning. If they hear a new tempo, however, they turn again.

Drake found that the infants were more likely to notice and respond when the
speed changed to around 3 beats per second鈥攁 pulse every 300 milliseconds,
to be precise. Babies also seemed to prefer a second pulse at 600 ms, more in
time with adults and, Drake notes, the heartbeat of the mother. Whether infants
process at these rates because they are born with a particular anatomy that
picks it up, or they simply use those tempo detectors that are closest to what
they first hear, Drake can鈥檛 say. 鈥淚t鈥檚 a chicken-and-egg argument at the
尘辞尘别苍迟.鈥�

After this, the preferred tempo appears to slow with age. Drake played two
simple but different tempos and asked volunteers aged between 4 and 20 to tell
her which was the faster of the two. Children chose most accurately when there
were intervals of around 400 milliseconds between the beats, and adults at
around 600-millisecond intervals.

As people age, Drake suggests, they have to coordinate more complex chunks of
information. Many different kinds of information are needed to drive a car or
dance the Lindy Hop, so to give the brain enough time to process each chunk in
between samplings, the time between pulses must be stretched out.

I got rhythm鈥�

Musical experience also seems to slow the preferred pulse down, and add to
its flexibility. In her tapping experiments, Drake found that musically inclined
adults spontaneously focus on two distinct slower tempos, 800 and 1600
milliseconds, and can shift these tempos a long way if they need to. It seems
musicians鈥� acoustic palettes are like those of experienced wine tasters.
鈥淢usicians are much less constrained in the details they can hear,鈥� says Drake.
鈥淣on-musicians, on the other hand, focus on intermediate rhythms rather than
fast or slow ones. They get bored relatively quickly because it all sounds the
蝉补尘别.鈥�

If rhythm is built into our brains, are there any clues about where?
Neurologist Stephen Rao and his colleagues at the Medical College of Wisconsin
in Milwaukee used a brain-scanning technique called functional magnetic
resonance imaging to pinpoint which areas are active when people make
split-second decisions about rhythms they hear. The researchers played two tones
at one tempo and then a second pair of tones at a different tempo. Next, 17
volunteers were asked which pair was quicker. As they answered, two brain
regions became active: the basal ganglia, a collection of small regions deep
inside the brain which help to coordinate our movements; and the parietal lobes,
which are important in spatial awareness and language. Rao and his colleagues
believe these regions hold the neurons that perceive rhythm.

The same regions may monitor the passing of time. Mind-altering drugs seem to
disrupt people鈥檚 ability to keep track of short periods of time, either slowing
or speeding our ability to count off a minute or two. Parkinson鈥檚 disease also
alters the sense of time passing and, interestingly, the illness severely
affects the basal ganglia
(麻豆传媒, 1 November 1997, p 52). So the
stretching of the pulse as we age could even have something to do with the way
time seems to pass more quickly the older we are. Whether time passes faster for
musicians, though, might be hard to test.

Others disagree with the idea of a specific pacemaker limited to one or two
brain regions. Scott Kelso of Florida Atlantic University in Boca Raton, who
makes mathematical models of oscillating brain regions, says the concept is too
simplistic. 鈥淚n different situations, different parts of the brain can express
different rhythmic behaviours,鈥� he says. When Kelso asked people to tap in sync
with a beat provided on a metronome, he found that their left-central and
anterior parietal areas of their brains鈥攁s well as other regions involved
in sensory-motor coordination鈥攚ere active. However, these activity
patterns shifted to include many more frontal areas as people were asked to tap
between the downbeats of the metronome鈥攁 syncopated rhythm. There seem to
be other timekeeping areas needed to perform more difficult tasks like this.

But even if Kelso is right, and the primal beat doesn鈥檛 come from a single
drum, it seems to be inescapable. Whatever disastrous music lessons or dance
classes you might have suffered in the past, take heart. You鈥檝e got rhythm.