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 in the visual condition (i.e., 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). Residual artifacts due to amplifier drift, muscle or movement-related activity, or residual eye activity were identified on a channel-by-channel and trial-by-trial basis by employing 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 eight or less channels were affected; otherwise, a trial was rejected. Artifact detection and electrode replacement was verified by visual inspection.
