All OERP waveforms at each electrode were transformed into reference-free CSD estimates (μV/cm2 units; 10 cm head radius; 50 iterations; m = 4; smoothing constant λ = 10−5) using a spherical spline surface Laplacian (Perrin et al., 1989). To determine their common sources of variance, CSD waveforms were submitted to temporal PCA derived from the covariance matrix, followed by unrestricted Varimax rotation of the covariance loadings (Kayser & Tenke, 2003, 2006c). The input data matrix consisted of 401 variables (time interval −250 to 1750 ms) and 4,154 observations stemming from 67 participants, two intensities, and 31 electrode sites, including the nose.
