A continuous interval comprising 256 seconds of eyes-closed resting EEG data was analyzed. The raw data were subjected to wavelet filtering and reconstruction to eliminate very high and low frequencies (Bruce & Gao, 1994; Strang & Nguyen, 1996). The s12 wavelet was used to perform a six-level analysis, and the output signal was reconstructed with levels d6–d3, roughly equivalent to applying a bandpass filter with a range of 2–64 Hz to the data. Subsequently, eye movements were removed by using the method developed by Gasser et al. (Gasser and Laemmli, 1987; Gasser et al., 1986). The filtered artifact-free data were transformed into bipolar derivations. Bipolar derivations were used in preference to monopolar derivations to improve the spatial resolution of the electrical sources (Ingber and Nunez, 1995; Nunez et al., 1997). Bipolar derivations were analyzed in 254 overlapping 2-second epochs by use of a Fourier transform. After windowing effects were minimized by application of a Hamming function (Hamming, 1983), the resulting spectral densities, sampled at 0.5 Hz intervals, were aggregated into bands, divided by the bandwidth, and then averaged across epochs.
