1/f fluctuations in biological systems

The present author makes a review of different types of 1/f fluctuations which have been observed in various levels of biological body. Judging from these observations it seems that the biological rhythm is basically subject to 1/f fluctuations from cellular to behavioral levels. There are three possible generation mechanisms of 1/f-like rhythm fluctuations. Firstly, 1/f ionic conductance fluctuations of a cellular membrane would modulate ion current flowing into a biological cell which would modulate discharge intervals of the neuron. Secondly, a Hopfield-type of artificial neural network consisting of neurons, each of which has binary states 0 and 1 and has connections with all other neurons, show 1/f level fluctuations when random noise is given to each neuron with a proper inhibitory threshold. Thirdly, we mention the clustering Poisson process in which sequences of randomly occurring point events (clusters) overlap each other. As regards biological phenomena, we often ask "How?" and "Why?". The question of "Why?" is still unsolved. 1/f fluctuations were first found in electronic devices, and now they are ubiquitous in nature.