The ERO trials were digitized at a rate of 256 Hz. Trials containing excessive artifact were eliminated prior to averaging (<5% of the trials). An artifact rejection program was utilized to eliminate individual trials in which the EEG exceeded ± 400 μV. ERO analyses were accomplished from the same datasets that were used to generate the ERP data reported in previous publications (Ehlers & Somes, 2002). Since previously published ERP data suggested limited differences in morphology between frontal and parietal sites, only frontal sites were used to analyze ERO data. Data from single trials generated by the stimuli were entered into the time frequency analyses algorithm. The S-transform (ST), a generalization of the Gabor transform (Gabor, 1946), was used (Stockwell et al., 1996).
