8 May 2009

Internal and External Noise, in Ward, Dynamical Cognitive Science

by Corry Shores
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Lawrence M. Ward

Dynamical Cognitive Science

Ch. 14, Noise

Internal and External Noise

One very useful distinction is that between external and internal noise (van Kampen, N.G. 1987, "Some theoretical aspects of noise." In C.M. Van Vliet, ed., Ninth International Conference on Noise in Physical Systems, 3-10. Singapore: World Scientific).

External noise is caused by a force outside of a system acting on that system. A good example is the turbulence of the earth's atmosphere acting on the light rays from distant stars, causing them to twinkle. This kind of noise has two properties: (1) it does not affect the actual behavior of the system of interest (the amount of light generated by a distant star is not affected by the atmosphere's turbulence); and (2) it is connected to that system by some coupling mechanism and, in principle, can be switched off (or by-passed, as when the Hubble telescope in earth orbit views the star from outside the earth's atmosphere).

In contrast to external noise, internal noise is an integral part of the system of interest, and participates completely in its temporal unfolding. "It originates from the fact that matter consists of particles, or, as in photons, from the fact that energy is quantized." (van Kampen 1987, 4) A good physical example is the current noise described in chapter 19: the electrical current in a wire with a constant voltage source (say a battery) fluctuates randomly. According to van Kampen (1987), such internal noise often arises from the cooperative effect of many system elements (e.g., electrons in current noise) rather than from fluctuations in the behavior of individual elements. (116-117)

Ward, Lawrence M. Dynamical Cognitive Science. London: MIT Press, 2002.

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