Bipolar eye activity (left and right outer canthi; above and below right eye) was estimated from the raw data by spherical spline interpolation (Perrin et al., 1989) to monitor lateral eye movements and blinks. However, volume-conducted blink artifacts were removed from the raw EEG by spatial PCA generated from identified blinks and artifact-free EEG periods (NeuroScan, 2003). Recording epochs of 2,000 ms (250 ms prestimulus baseline) were extracted off-line, tagged for A/D saturation, and lowpass filtered at 50 Hz (−24 dB/octave). A reference-free approach identified residual artifacts on a channel-by-channel and trial-by-trial basis (Kayser & 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.
