To reduce anomalies in the S transform output at the beginning and the end of the output time series, we used a Hanning window over the initial and final 100 msec of the input time series. The output of the transform for each stimuli and electrode site was calculated by averaging the individual trials containing the time-frequency energy distributions. To quantify S transform magnitudes, a region of interest (ROI) was identified by specifying the band of frequencies and the time interval contained in the rectangular ROI. Total volume was determined as the measure of the energy values in the ROI. These analyses are similar to what has been previously described (Jones et al., 2004). Total volume was defined as the volume of the ROI in energy summed over time and frequency. The ROI frequencies were: delta (1-4 Hz), theta (4-8 Hz), and alpha/beta (8-35 Hz). The ROI time intervals correspond to the P1/N1 ERP component (0 - 50 ms), N2/P3a components (50 - 350 ms) and the P3b component (350-800 ms). ROI frequencies and time intervals are consistent with our previous ERP studies (Ehlers & Somes, 2002; Slawecki et al., 2003).
