91.

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		2.4.6— Example of Bifurcations: Motor and Sensory Phase Transitions

		Some mechanical and electrical systems consist of many individual interacting parts. However, cooperation between the parts can lead to coherent behavior of the entire system, which Haken called ''synergetics." This coherent behavior can change by a large amount when the value of a control parameter changes by a small amount. These changes have similar characteristics to phase transitions, such as the change of a substance from a liquid to a gas. For example, there is an increase in fluctuations as the control parameter approaches its value at the phase transition.

		These phase transitions can be described by a potential energy function. The shape of this potential energy function depends on the control parameter. As the control parameter changes, the shape of the potential energy function changes, and thus the behavior of the system changes.

		Kelso et al. showed that if you tap the index finger on your left hand in time with the tick of a metronome, you can also move the index finger on your right hand in the opposite direction. As the frequency of the metronome increases, the movement of your right finger suddenly shifts to motion in the same direction as that of your left finger. This change in behavior has the characteristics of a phase transition. They also found strong evidence of such bifurcations in the motor control of arms, hands, and legs.

		Living systems have many components. It has been hard to understand how the activity of all these different nerves and muscles organize to produce coherent motion in our body. The finding of these phase transitions in the human body suggests that biological components interact in a way that self-organizes them into coherent function. Determining how a measured property depends on a control parameter that can be varied in an experiment gives us a way to test for this type of synergistic self-organization.

 

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