In order to substantiate the differential effects between low and high beta band frequencies, we considered them separately in further analyses. The histogram of all beta band peak and sub-peak frequencies in the individual power spectra showed a clear bimodal pattern (Fig. 2). The optimal fit of a mixture of two normal distributions was obtained for centre frequencies at 16.65 and 24.79 Hz, with standard deviations of 1.99 and 2.81 Hz, respectively. The cut-off frequency between low- and high-beta ranges was found to be 20.36 Hz. By comparison, the difference in Akaike information criterion value with a fit of a single normal distribution was 169, significantly favouring the bimodal over the unimodal distribution.
