All data recording and preprocessing closely followed the procedures detailed elsewhere (e.g., Kayser et al., 2007). Briefly, nose-referenced EEG (30 channels) and bipolar EOG activity were continuously recorded at 200 samples/s with a gain of 10k (5k horizontal, 2k vertical EOG) within 0.1–30 Hz (−6 dB/octave). Volume-conducted blink artifacts were removed from the raw EEG by spatial PCA. Recording epochs of 2000 ms (250 ms prestimulus baseline) were extracted off-line, tagged for A/D saturation, and low-pass filtered at 20 Hz (−24 dB/octave). A reference-free approach identified residual artifacts on a channel-by-channel and trial-by-trial basis (Kayser & Tenke, 2006d). A trial was rejected if it contained artifact in more than eight channels; otherwise, artifactual data were replaced by spherical spline interpolation (Perrin, Pernier, Bertrand, & Echallier, 1989) from artifact-free channels. These procedures for artifact detection and reduction were originally developed to optimize the signal-to-noise ratio in problematic ERP recordings, such as those stemming from artifact-prone psychiatric samples, but these routines also help in reducing the problem of latency jitter in olfactory ERPs (Lorig, 2000).
