presentation, does not have an effect on auditory-evoked MEG responses as compared to a passive baseline [30]. Further, as the visual task difficulty level was kept constant during different masker types, the possible cross-modal visual task-induced effect should be constant with different noise maskers (although there is some variation in d’ but not in RT, see Fig. 8). Thus, taken together, we can say relatively confidently that the observed effects arose from partly frequency-specific [71] selective attentional enhancements of relevant or suppression of irrelevant sounds during auditory attention, and were not caused by a general suppression induced by the visual task. Materials and Methods Subjects Altogether fourteen healthy subjects participated in the study (age 21–46, mean age + S.D. 29.3+6.2, 10 males). One of the subjects was native English speaker, one a native Russian speaker, and the rest were native Finnish speakers. All subjects had normal hearing and normal or corrected-to-normal sight, and all but two were right-handed. All subjects were university students or staff, and were not paid for participation. A subset of 10 subjects (7 males) was used in minimum norm estimate (MNE) analysis as their cortical surface reconstructions from whole-head anatomical MRI images were readily available. Ethics Statement All subjects signed a written informed consent before the study. The entire study had a prior approval by the Ethics Committee of the Hospital District of Helsinki and Uusimaa, Finland, and the experiment was run in accordance with the Helsinki Declaration. Experimental setup Stimulus and experimental setup is depicted in Figure 1. The stimulus setup is nearly identical to our earlier EEG study [18], the main differences being the inclusion of more notch widths, only one auditory task (AttAud), and visual control task (AttVis) instead of passive baseline. In the current experiment, auditory and visual stimuli were identical during both AttAud and AttVis conditions, only the masker sound was changed in each block. The subjects were instructed to follow the instructions on the screen and to respond as accurately as possible. They were cued to attend either modality at the beginning of the 8-minute block, and at each condition change every two minutes (see Fig. 1C) by text instructions shown on the screen. The evoked responses were discarded during these presentations of task instructions. The order of the AttAud and AttVis conditions were pseudo- randomized. The target response was detected using an optical response device, where the subjects lift their right index finger to answer. The frequency of the deviants for both auditory and visual stimuli were adjusted during the design of the experiment based on several psychoacoustic sessions. The aim was to keep the attentional demand high and to avoid a ceiling effect in the course of the experiment by using stimuli only slightly above the threshold. Auditory stimuli were constructed at 16-bit, 48-kHz with Matlab (R14, MathWorks Inc., Natick, MA, USA). The standard stimulus for which evoked responses were recorded was 300-ms, 1000-Hz tone with 5-ms onset and offset ramps. The 1020 Hz target tone (identical in other aspects to the standard) was presented at random intervals 10% of the time. The continuous masker sounds (16-bit, 48-kHz) were created in Matlab by filtering in frequency domain 10-minute Gaussian white noise with symmetrical stopbands or notches around 1000 Hz. For example, Two-Stage Processing of Sounds during Attention for +150 Hz masker, stopband of 850—q1150 Hz was used. Thus, due to logarithmic nature of frequency perception [72], lower range of the notched-noise masker contributes more to the frequency masking. The tones were presented with a mean onset- to-onset interval of two seconds (range 1800–2200 ms). The slow rate was used in order to obtain good enough MEG signal-to-noise with the narrowest notches and the white noise masker. Visual stimuli consisted of Gabor patches or flankers with an identical orientation and predefined spatial frequency. Similar to the auditory stimulus, 10% of the flankers were deviants with a slightly higher spatial frequency. The still frames were concate- nated to a 5 FPS video file using Xvid encoder (http://www.xvid. org) with parameters to maximize image quality. Each flanker was presented for 400 ms (2 frames) and the onset-to-onset interstim- ulus interval for each flanker was constantly 1.8 seconds (9 frames). Ten video clips were created with a different presentation order, and they were presented in random order in each block so that subjects could not learn the presentation pattern. The sounds were presented through a high-quality 60660 cm panel speaker (Panphonics SSH-SQW sound shower, Panphonics, Espoo, Finland), which is able to reproduce frequency response of 400 Hz–16 kHz (26 dB/oct.). The speaker was mounted on the wall of the shielded room in front of the MEG device, directed to the subject’s head, so a natural binaural perception was possible. The visual stimuli were presented through a back-projector screen located in front of the subject at a distance of 1.5 m. As the auditory and visual stimuli were presented asynchronously in their own streams with a different rate, they could not be fused to an audiovisual object. Stimulus presentation was controlled by a computer running Presentation software (v12.0, Neurobehavioral Systems Inc., Albany, CA, USA). Before the MEG acquisition began, the subject was introduced to the task and stimuli outside of the MEG shielded room. The subject then entered the room and was seated comfortably under the MEG device. After this, individual 50% hearing threshold was estimated using an up-down procedure [73], where the level of the white noise masker was changed so that the embedded tone-in- noise was barely audible. Following this threshold estimate, the no- masker condition (with only tones playing) was presented to the subject, lasting *8 minutes. Finally, 8-minute blocks spanning all masker sounds (+500 Hz, +300 Hz, +200 Hz, +150 Hz, +100 Hz, +50 Hz, 0 Hz) were presented in randomized order, counterbalanced across subjects. MEG acquisition Magnetoencephalography (MEG) was measured in a magnet- ically shielded room located in the O.V. Lounasmaa Laboratory of Aalto University using a whole-head neuromagnetometer (Vector- view, Elekta Neuromag Oy, Helsinki, Finland) with 306 channels. The device has 102 sensor elements, each with two orthogonal gradiometers and a magnetometer. The continuous MEG data was recorded at 2000 Hz, with a passband of 0.1–650 Hz, except for the additional microphone channel, where lowpass filtering was disabled in order to detect onset of tones and to verify that the auditory stimulus presentation was accurate with a minimal jitter. To detect eye blinks and movements, one electro-oculogram (EOG) channel was recorded with the electrodes placed below and on the outer canthus of the left eye. Auditory-evoked responses to both standard and deviant tones were averaged. MEG was online- averaged to assess the initial quality of the data and to verify that at least 100 artifact-free epochs were obtained, but the actual analyses were made with offline-averaged data. In the data analysis, offline-averaged 2200–800 ms time-locked epochs ex- ceeding 3000 fT/cm (in any gradiometer channel), 400 fT (in any PLOS ONE | www.plosone.org 11 October 2012 | Volume 7 | Issue 10 | e46872