repeated deviant), repetition probability (high vs. low) and temporal regularity (anisochronous vs. isochronous). To check for differences in the distribution of MMN as a function of repetition and/or repetition probability, four regions of interest (ROIs) were defined: left (F5,
FC5, C5); center-left (F1, FC1, C1); center-right (F2, FC2, C2); and right (F6, FC6, C6). Scalp potential (SP) measures and scalp current density (SCD) values were computed for each ROI as the mean across electrode locations. Voltage measures were transformed into current density estimates by computing the second spatial derivative of the interpolated voltage distribution (Perrin et al., 1989, 1990), with maximum degree of Legendre polynomials EPZ015666 set to 50, order of splines (m) equal to 4, and a smoothing parameter of 10−5. This way we obtained reference-free distribution maps of local current sources/sinks (radial current flow through the skull measured in mA/m3; BIBF 1120 mw Srinivasan, 2005). Four-way anovas with factors repetition, repetition probability, laterality (central vs. lateral) and side (left vs. right) were separately run for each temporal regularity condition on SP measures and SCD estimates. IBM SPSS Statistics for Windows, Version 20.0 (IBM; Armonk, NY, USA) was used for statistical analyses. Brain electrical tomographic procedures
were applied to detect the presence of differences in MMN generator location, using the distributed inverse solution VARETA approach (Variable Resolution Electrical Tomography; Bosch-Bayard et al., 2001). VARETA reconstructs brain sources by estimating the spatially smoothest intracranial primary current density (PCD) distribution that is compatible with the observed scalp voltages, and restricts the allowable solutions to the gray matter on the basis of probabilistic Montréal Neurological Ureohydrolase Institute (MNI) 3D brain tissue maps (Evans et al., 1993; Trujillo-Barreto et al., 2004). Statistical parametric maps (SPMs) of the PCD estimates were then constructed based
on a voxel by voxel Hotelling T2 test against zero (threshold: P < 10−4) to determine the sources of the MMN component separately for each condition and for the relevant contrast between solutions. The PCD is a vector quantity, that is, at each voxel the three projections of the PCD vector onto the three orthogonal directions in the 3D Cartesian space are estimated. This asks for a multivariate T2 statistic at each voxel to test for changes in magnitude as well as orientation of the PCD vector. Significance threshold correction to account for spatial dependencies between voxels was calculated by means of random field theory (Worsley et al., 1996). Results are shown as 3D images. To verify if indeed a slower stimulus rate (SOA = 600 ms, 1.