ABSTRACT
Measurements involving biological systems are complicated by the complexity of cells and tissues, particularly if fields are expected to interact weakly and if field-induced changes are found to be small. The chapter examines how Brownian noise, the simple random motion of molecules due to thermal agitation, interferes with the coupling of an electromagnetic field to a biochemical system. Some organisms have evolved ability to sense and effectively "measure" electric and magnetic fields. More solid estimates of detection thresholds can probably only be derived after biophysical mechanisms are revealed. The chapter shows that the thermal noise that a signal has to compete against sets fundamental limits on detectability. It suggests how evolution has come up with structures to optimize the signal-to-noise ratio in sensory perception. The chapter describes how amplification generally adds noise to a signal.
