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Amplification and Damping

In systems with periodicity, timing is everything. If cycles are timed to coincide, they become strengthened or amplified. Synchronization grows within a system through a process called recruitment.

Recruitment is just a label for the ongoing process of synchronization. As more units are added to a pulsing rhythm, the pulse grows larger and even more units join them (a positive feedback process).

Buck, the researcher mentioned on the previous page who used a flashlight to stimulate fireflies, noticed recruitment. He wrote that "pairs and then trios began to pulse in unison...until as many as a dozen pairs of fireflies were blinking on and off in perfect concert."

When small rhythms are synchronized this way, they can become very powerful. Recall the description of riverbanks in Thailand, thick with fireflies, flashing like a powerful strobe light.

This positive feedback effect, in which many small rhythms are combined into one larger rhythm, can be destructive in some situations. Poorly constructed bleachers in a stadium, or balconies in an apartment complex, may collapse if multiple people start dancing or stamping their feet in unison.

A bleacher or balcony will normally rebound after a weight is dropped on it. It oscillates if people are jumping up and down on it. However, if many people start stamping their feet in unison, the vibra­tions are synchronized, amplifying the motion until (sometimes) the system breaks.

An example of recruitment in the nervous system is the epileptic seizure. It starts as a small disturbance in the nervous system, and then the disturbance builds upon itself in a positive feedback reaction.

Additional nerve cells are recruited as the feedback reaction becomes larger. If enough neurons are recruited, the surges of activity are more than the system can handle and it shuts down, resulting in a grand mal seizure with unconsciousness.

Some people with epilepsy can feel this activity starting and relax themselves to prevent it from spreading. People with so-called petit mal (small sickness) epilepsy might have no more symptoms than a brief fluttering of the eyelids or moment of inattention (a petit mal seizure).

Grey Walter (1950) found that non-epileptic people could be induced to have epileptic activity in the brain (the "spike and dome" pattern on the EEG characteristic of seizures, not uncon­sciousness) merely by flashing a strobe light at the same frequency as the person's dominant brain wave. Walter used a feedback arrangement so the natural rhythm in a person's brain triggered the strobe.

Normally, each flash of a strobe light drives a big wave of activity through the brain, easily seen on EEG machines. This is called photic driving. (Driving, you might recall, is another name for pacemaking.) When the flash of the strobe is timed to reinforce a normal brain rhythm, the normal wave is amplified, sometimes resulting in seizure-like activity.

Repetitive flashes can trigger grand mal seizures resulting in unconsciousness in about 3% of children who are prone to seizures. The most famous example of this occurred when the "Denno Senshi Porygon" episode of Pokémon was broadcast in Japan. It had strong flickering scenes and produced seizures in a few children.

Sometimes drivers have accidents because sunlight flickers through a series of posts or trees on the side of the road, creating a strobe-like effect, causing a driver to lose consciousness. Fortunately, like the Pokémon effect, this is rare.

An opposite effect occurs if periodic disturbances are timed to cancel each other out. This is called damping (or, erroneously, dampening) a rhythm.

If you are in a canoe with several other people and they start singing a jolly song while swaying from right to left in unison, the boat will soon be in danger of capsizing. You can help keep it upright and prevent dampening by damping the rhythm, swaying the opposite way each time the group leans left or right.

Sound waves consist of oscillations: pressure waves in the air, commonly shown as a wavy line with peaks and valleys. If two speakers are wired the wrong way (out of phase) the sound waves cancel each other out.

The peak of one sound wave corre­sponds to the valley of the other sound wave. This creates a "dead spot" in the middle of the room.

That is why a pair of speaker wires is always marked in some way, with a ridge or white line on one wire but not the other. They must be installed the same way on both speakers (e.g. the marked side going to the red terminal, the unmarked side to the black).

The "dead spot" effect from waves canceling each other out is exploited intentionally in noise-canceling headphones. They use a small microphone to pick up noise from the environment. The exact opposite waveform (anti-noise) is fed into the audio signal to cancel out the noise.

As shown below, the anti-noise (bottom) is the same shape as the noise (top) but turned upside-down (phase reversed). Peaks of noise correspond to valleys of anti-noise.

Two sound waves, inversions of each other, cancel each other out
Two sound waves cancel out if one is the inversion of the other.

The idea is to cancel out noise from the environment while leaving normal audio unaffected. The music or voice a person wants to hear is passed through to the ears, as it normally would be. Noise-canceling headphones work well to enable listening to voice or music in a noisy environment like an airplane.

A Case Study

We have reviewed several principles independently: hierarchical organization, feedback, oscillation, and more. In natural systems these may be intertwined in the same system.

Here is an example from an article in the journal Science by Sanchez, Welch, Nicastro, and Dogic (2011). In this single example we will see hierarchical organization, emergence, oscillation, recruitment, and synchrony.

The four researchers analyzed the mechanisms of flagella (whipping tails that allow single cells to move) and cilia (sheets of hair-like cells capable of wave-like motion, found in numerous body structures such as the human inner ear). The authors explained how these systems are built up from tiny tubes (microtubules) and molecular motors (dyneins), both well known to biologists.

First, the tubes self-assemble into active bundles. That is an example of self-organization. Hundreds of molecular motors "spontaneously synchronize their activity to generate large-scale oscillations." That is recruitment and synchronization.

The little tubes are individually driven by molecular motors (dyneins) which all have a similar rhythm. They are gathered in bundles, so they spontaneously synchronize, moving together as one, producing the lashing back and forth motion that allows flagella (protozoa, bacteria, spermatozoa) to swim.

The authors, Sanchez, Welch, Nicastro, and Dogic, made the following comments:

New behavior frequently emerges as complex structures are assembled from simpler components. In biology, an example of such hierarchical structure is the axoneme [the central strand of a cilium or flagellum].

...The activity of thousands of dyneins [molecular motors] within each axoneme is regulated to form oscillatory beating patterns, an emergent behavior that is qualitatively different from the linearly moving isolated dynein motors...

...At the next level of hierarchy, axonemes are the essential building blocks of dense ciliary fields, where thousands of individual cilia beat in tightly coordinated fashion. (Sanchez, Welch, Nicastro, and Dogic, 2011, p.456)

The researchers first analyzed the building blocks or components of the system (microtubules and dyneins). Then they identified how the components were assembled into a hierarchical structure, producing emergent properties: oscilla­tion, recruitment, and synchrony.

Their work is important because flagella (the whipping tails) and cilia (sheets of hair like cells) are two types of structures found in numerous biological systems. They are also highly conserved in evolution, maintained in the same form for millions of years. The same structures are found in bacteria and humans.

Once these problems were solved in evolution (over a billion years ago in the case of flagella) the genetic solutions were passed on unchanged (conserved) precisely because the solution was unique and effective. Organisms became dependent on these mechanisms and died if they could not manufacture flagella or cilia.

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References:

Sanchez, T., Welch, D., Nicastro, D. &and Dogic, Z (2011) Cilia-like beating of active microtubule bundles. Science, 333, 456-459.

Walter, W. G. (1950) The functions of the electrical rhythms of the brain. Journal of Mental Sciences, 46, 1


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