The Brain from Inside Out

by György Buzsáki

There are numerous brain rhythms, from approximately 0.02 to 600 cycles per second (Hz), covering more than four orders of temporal magnitude (Figure 6.1).12 Many of these discrete brain rhythms have been known for decades, but it was only recently recognized that these oscillation bands form a geometric progression on a linear frequency scale or a linear progression on a natural logarithmic scale, leading to a natural separation of at least ten frequency bands.13 The neighboring bands have a roughly constant ratio of e = 2.718—the base for the natural logarithm.14 Because of this non-integer relationship among the various brain rhythms, the different frequencies can never perfectly entrain each other. Instead, the interference they produce gives rise to metastability, a perpetual fluctuation between unstable and transiently stable states, like waves in the ocean. The constantly interfering network rhythms can never settle to a stable attractor, using the parlance of nonlinear dynamics. This explains the ever-changing landscape of the EEG. Traditionally, the frequency bands are denoted by Greek letters.15