Recording epochs of 2,000 ms (including a 300 ms pre-stimulus baseline) were extracted off-line from the blink-corrected continuous data, tagged for A/D saturation, and low-pass filtered at 20 Hz (−24 dB/octave). To maximize the number of artifact-free epochs, volume-conducted horizontal eye movements, which were systematically prompted by reading the word stimuli, were reduced by computing the linear regressions between the horizontal EOG and the EEG differences of homologous lateral recording sites (i.e., Fp2 - Fp1, F8 - F7, etc.) for each epoch, and the correlated eye activity was then removed by applying ± beta weight/2 to each lateral EEG signal (cf. Kayser et al., 2006, 2007, 2009). Residual artifacts, including muscle or movement-related activity, or residual eye activity were identified by a semi-automated routine on a channel-by-channel and trial-by-trial basis using the frequency distribution of a reference-free electrical distance measure (Kayser and Tenke, 2006d). Artifactual surface potentials were replaced by spherical spline interpolation (Perrin et al., 1989) using the data from artifact-free channels if possible (i.e., when less than 25% of all EEG channels contained an artifact); otherwise, a trial was rejected.
