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Figure 11. Brain waves. [Adapted from: Wikipedia]

Brain waves are the results of synchronous oscillations by a large population of neurons. They are divided into several bands: Alpha (α): 8-12 Hz, Beta (β): 16-25 Hz, Gamma (γ): > 30 Hz, Theta (θ): 4-7 Hz, and Delta (δ): 1-4 Hz.

Notice that the median value (10 Hz) of the α band is about twice that of the θ band, and the median value (20 Hz) of the β band is about twice that of the α band. Such "doubling" structure plays a critical role in conscious perception (Chapter 17).