Stimulus-locked ERP waveforms were averaged from artifact-free trials using the entire 2-s epoch (correct responses only for button press). The mean number of trials used to compute target ERP averages across response mode (M ± SD, minimum in any condition, controls vs. patients) were 29.2 ± 2.2 (14) vs. 29.3 ± 1.8 (21) for the tonal task, and 29.7 ± 1.8 (22) vs. 29.0 ± 2.0 (19) for the phonetic task. There were no significant differences between patients and controls, response mode, or task, as virtually identical mean number of trials were observed for each response mode and task in each group. Furthermore, a satisfactory signal-to-noise ratio for each condition was confirmed by visual inspections of the individual ERP waveforms of each participant. ERP waveforms were screened for electrolyte bridges,71 low-pass filtered at 12.5 Hz (−24 dB/octave), and baseline-corrected using the 100 ms preceding stimulus onset.
