Stimulus-locked ERP waveforms were averaged from correct, artifact-free trials using the entire 2-s epoch. The mean number of trials used to compute new and old ERP averages after pooling across lag (M ± SD, min-max range, controls vs. patients) were 56.1 ± 7.3 (37 – 67) vs. 49.0 ± 11.5 (29 – 67) for auditory, and 54.3 ± 8.4 (33 – 65) vs. 50.5 ± 13.4 (19 – 68) for visual stimuli. Whereas about the same number of trials entered into new ERP averages for controls and patients (56.9 ± 7.0 vs. 56.9 ± 7.5), owing to their better performance, controls had more old trials than patients (53.5 ± 8.4 vs. 42.8 ± 12.3; group × condition interaction, F[1,38] = 12.9, p = .0009), but the number of old trials were nonetheless sufficient 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 (Tenke and Kayser, 2001), low-pass filtered at 12.5 Hz (−24 dB/octave), and baseline-corrected using the 100 ms preceding stimulus onset.
