magnetometer channel) or 150 mV (in EOG channel) were rejected for possibly containing extracerebral activity. Prestimulus baseline of 200 ms was used to remove DC offset, and 40 Hz lowpass filter was used during averaging. Before moving the subject to the shielded room, 3D locations of left and right preauricular points and nasion, four head-position indicator (HPI) coils, and a number of extra points from the scalp were digitized to obtain a right-handed head-coordinate frame that was used later in dipole localization and alignment with cortical surface reconstruction for MNE analysis. The HPI coil locations were used in estimating head position at the beginning of each recording block. MEG data analysis Two equivalent current dipoles (ECD) were used to model the current sources of the MEG signals at around N100m peak latency for both hemispheres using a spherical head model and gradiometer data [54]. The data from ECD modeling are referred to as M100 in the manuscript to dissociate them from the sensor- space N100/N100m effect. For each subject, ECDs were estimated using Neuromag (Elekta Neuromag Oy, Helsinki, Finland) Xfit software for the no masker stimulus, as this stimulus provided the best signal-to-noise quality. ECD model explained typically over 80% of the field variability at the N100m peak latency (Fig. 11). These dipoles were used to project the data at different maskers using a fixed dipole approach, assuming that signal sources stay identical with the inclusion of masker. This assumption was verified in a selection of subjects by comparing +500 Hz dipole fits to no masker dipole fits, and results support the findings of previous studies [19] that signal sources are practically unaltered with the addition of a continuous masker sound. As source locations from AttAud and AttVis conditions were practically identical and did not differ (see Fig. 3), the data from both conditions were projected using the AttAud fixed dipole. Sustained responses were modeled, similar to M100, by two per-subject equivalent current dipoles at most prominent part of the sustained field (at around 330–380 ms, before the offset response at around 400 ms). The source locations were close to M100 dipole source in supratemporal plane, but especially the AttAud sustained response sources were localized on average 7 mm more medial than the corresponding M100 sources (see Results and Fig. 3; total distance between sources 14 mm). Due to this, sustained responses were quantified by projecting the data through the two fixed sustained response dipoles and taking the source strength from the corresponding source waveforms, although the M100 dipole was able to capture most of the activity at this latency, as shown in Figure 2. ECD data were analyzed using analysis of variance (ANOVA) with factors MASKERTYPE (+500 Hz, +300 Hz, +200 Hz, +150 Hz, +100 Hz, +50 Hz, 0 Hz), ATTENTION (AttAud, AttVis) and HEMISPHERE (left, right). For MNE auditory cortex grid analysis an additional GRIDPOINT (1, …, 15) factor was used. The reported p-values are Huynh-Feldt corrected with e measure documented where appropriate, but original, uncorrected degrees of freedom are reported. The no masker data were tested separately because (1) auditory stimulus was so clearly different from the masker conditions, and (2) no masker was always presented at the beginning of the experiment, thus probably causing an additional bias in the data. M100 peak measure was quantified as the mean value of peak latency +10 ms time range. Peak detection was done using a semiautomatic algorithm. Notably, in this case, the M100 response latency varied from 100 to over 200 ms with white noise masker Two-Stage Processing of Sounds during Attention Figure 11. Dipole modeling results for a representative subject. Butterfly plot showing MEG fields from all sensors with (A) real data from one subject with AttAud condition and no masker, (B) two-dipole model fitted at around N100m peak latency, (C) residual field (data vs. model). (D) The N100m response is well explained by the dipole model, goodness-of fit curve peaked at 80–90%. doi:10.1371/journal.pone.0046872.g011 (see Fig. 4B), thus a term ‘‘onset response’’ to sound would better describe the measure that was used rather than categorization of deflections by their typical latency. For simplicity, we will use the M100 term throughout the manuscript. Sustained response was defined as mean amplitude of 100-ms range at 300–400 after the sound onset (see Fig. 2B). MNE data analysis was conducted using MNE suite (v2.7.0, http://www.nmr.mgh.harvard.edu/martinos/userInfo/data/ sofMNE.php) for a subgroup of N = 10 subjects for which MRI images were available, and thus in whom surface reconstructions were possible. The inflated surface recon- structions [21,22] were done using FreeSurfer (v4.5.0, http:// surfer.nmr.mgh.harvard.edu/) software. Behavioral data analysis For behavioral data, initially a two-way ANOVA with factors MASKERTYPE and ATTENTION was used. Additional post- hoc contrasts were calculated with AttAud and AttVis conditions separated. All statistical analyses were done in SPSS (version 15.0 for Windows, SPSS Inc., Chicago, IL, USA). PLOS ONE | www.plosone.org 12 October 2012 | Volume 7 | Issue 10 | e46872