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Synchronization

Some systems consist of small units showing rhythmic activity. If the units can influence each other, they start becoming active at the same time. They become synchronized.

Synchrony means sameness in time, so synchronization occurs when two or more processes adopt the same timing. If each unit shows oscillation, the oscillations start to coincide or lock in together.

Steven H. Strogatz wrote a book titled Sync (2003) about synchronization. His most elaborate example involved fireflies, which can synchronize their firing in great numbers. Along some riverbanks in Thailand this produces a stunning "great belt of light" strobing on and off.

As Strogatz noted, firefly synchronization was so puzzling that Science published 20 articles about it between 1915 and 1935. Nobody could explain it. Strogatz tells the story of how biologist John Buck figured out the solution:

By the late 1960s, the pieces of the puzzle began to fall into place. One clue was so obvious that nearly everyone missed it. Synchronous fireflies not only flash in unison: they flash in rhythm, at a constant tempo. Even when isolated from one anoth­er, they still keep to a steady beat...

The second clue came from the work of biologist John Buck... In the mid-1960s, he and his wife, Elisabeth, traveled to Thailand for the first time, in hopes of seeing the spectacular displays for themselves.

In an informal but revealing experi­ment, they captured scores of fireflies along the tidal rivers near Bangkok and released them in their darkened hotel room. ...As the Bucks watched in silent wonder­ment, pairs and then trios began to pulse in unison...until as many as a dozen fireflies were blinking on and off in perfect concert.

Buck experimented with a flashlight and found he could alter the flashing frequen­cy of the fireflies in the room if he timed his flashes correctly. The main require­ment for synchrony is that units with a similar rhythm can influence each other. Sure enough, flashes from Buck's flashlight influenced fireflies.

Some species could be sped up this way, but not slowed down. Other species could be either slowed down or sped up with bursts of light from the flashlight. Strogatz continues:

Thus we are led to entertain an explanation that seemed unthinkable just a few decades ago: the fireflies organize themselves. ...Sync occurs through mutual cuing, in the same way that an orchestra can keep perfect time without a conductor.

What's counterintuitive is that the insects don't need to be intelligent. They have all the ingredients they need: Each firefly contains an oscillator, a little metronome, whose timing adjusts automatically in response to the flashes of others. That's it.

...In 1989 my colleague Rennie Mirollo and I proved that...not only can it work; it will always work, under certain conditions. (Strogatz, 2003)

The principle of synchronization: Synchronization occurs when system compo­nents showing rhythmic activity influence each other, resulting in simultaneous activity of many different units.

Pacemaking

An important example of synchronization involves the heart. Each cell in the heart has its own endogenous rhythm. (Endo­genous means "coming from inside.") A single cell from the heart can be put into a dish alone and it will beat steadily as long as it is alive.

If several cells from the heart are put into a dish together, they soon become synchronized and beat together (an example of self-organization). This synchronization occurs because electrical activity in one influences the others. When one cell beats, it sets off others that were almost ready to beat on their own.

The heart has a nerve that normally acts as a pacemaker. Its natural (endoge­nous) rhythm is a little bit faster than the endogenous rhythm of other heart cells. Therefore, the pacemaker nerve fires first, and that sets off the other heart muscle cells to beat together.

An artificial pacemaker implanted in a heart works by firing electrical impulses into the heart tissue slightly faster than the natural rhythm of the heart cells. Pacemaking can also be called driving a rhythm.

The principle of pacemaking: If individual units in an oscillating system can influence each other, the fastest unit will drive the rhythm of the oscillation and establish a dominant frequency.

Pacemaking requires proper timing. A pacemaker must oscillate just a little bit faster than the natural rhythm of the system. That will drive the rhythm for a group of oscillating units.

In 1980, the research team of Czeisler, Weitzman, Moore-Ede, Zimmerman, and Knauer found that the natural daily rhythm for humans is about 25 hours. They discovered this by having volunteers live in a cave, cut off from normal time cues.

The volunteers could go to sleep when­ever they wanted to, but they never knew what time it was. Most of them adopted a sleep/wakefulness rhythm of 25 hours.

That endogenous sleep/waking rhythm is slightly slower than the 24-hour light/dark rhythm on earth. That, in turn, allows the circadian (24 hour) rhythm to drive our daily wakeful­ness cycle.

Darkness triggers melatonin release that encourages sleep. Because the light/dark cycle is slightly faster than the body's natural 25 hour rhythm, our sleep/waking cycle is driven by the earth's 24 hour cycle: another example of pacemaking.

Elvis's original drummer, D. J. Fontana, was well known for hitting his beats a few milliseconds early. This produced an urgent, driving feeling to the music, audible in such early Elvis hits as Jailhouse Rock. The drumming slightly leads the beats of the music, giving a feeling of pulling the rhythm along.

Delaying or lagging the beat somewhat can produce the opposite effect. This drags the rhythm back, producing a lazy or hesitant feeling to the music.

Notice that in cases of pacemaking, the rhythm does not become faster and faster. Rather, it is triggered to occur at a fixed rhythm.

A professional band will not change pace in response to a drummer playing slightly ahead of the beat (although sometimes an amateur band will, going faster and faster during a song, to comical effect). Ideally, the pace remains the same, but the leading drum beat creates a feeling of urgency, like being pulled ahead.

As it happens, the opposite was shown by a video of Z.Z. Topp playing the song Jailhouse Rock. The drummer was dragging instead of rushing, which made the song feel sluggish.

A company in San Antonio, Texas, made a device called the Russian Dragon. Its purpose was to let musi­cians know if a track being recorded was rushing or dragging, compared to the existing tracks. This gap was shown at the level of milliseconds.

The device went out of production because digital music workstations came along. They made it easy to see if one track was leading or lagging the others. No longer needed, the Russian Dragon sadly slipped into its cave.

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References:

Czeisler, C. A., Weitzman, E. D., Moore-Ede, M. C., Zimmerman, J. C., & Knauer, R. S. (1980) Human sleep: Its duration and organization depend on its circadian phase. Science, 210, 1264-1267.

Strogatz, S. H. (2003) Sync: The Emerging Science of Spontaneous Order. New York: Hachette Books


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