A Century of the Alpha Rhythm and Its Relatives: A Unified Theory at Last

Berger first recorded human EEG on in 1924, noting the ~10 Hz alpha rhythm is the most prominent activity. Alpha is concentrated over visual cortex, sometimes displays a split peak, and is suppressed by visual inputs; the beta rhythm occurs at its harmonic. Later, the ~10 Hz mu and tau rhythms were found, concentrated over motor and auditory cortex respectively and suppressed by associated sensory input. Early theories argued that separate groups of neurons fire at ~10 Hz at the relevant locations, but these lacked explanatory power. More recently, the alpha rhythm was argued to be a natural mode of activity in the cortex or corticothalamic (CT) system and analyzed using neural field theory (NFT). Here, we show that just 4 corticothalamic activity eigenmodes can explain the key features of alpha, mu, and tau rhythms, including their frequency structure and topography. CT loops account for the basic 10 Hz frequency and fundamental-harmonic correlations, with splitting arising from breaking of degeneracy due to cortical folding. We have observed split-beta and predict that split-mu, split-tau, and harmonic mu and tau rhythms will occur, plus split harmonic mu and tau. Spatial localization of power is due to constructive interference of modes in the relevant sensory region, supported by enhanced CT gains and suppressed when gains are reduced by attention.