MEG data acquisition and pre-processing
MEG data was continuously recorded (600 Hz sample rate, 0.1 Hz to 200 Hz online-filter) using a 306-channel (102 magnetometers, 204 orthogonal gradiometers pairs) system (Elekta-Neuromag® VectorView, Helsinki, Finland, 2005). For the depiction of sensor space MEG data only gradiometers will be shown as their magnetic flux topographies are easier to interpret. However, all cortical source space analyses (see below) were done based on all gradio- and magnetometer sensors. For artifact monitoring the electrooculogram (EOG) was recorded with electrodes attached above, below, left to and right to the outer canthus. The electrocardiogram (ECG) was recorded with electrodes placed at the left mid clavicle and lower right rib bone. Additionally, one electrode was attached to the left earlobe serving as the ground electrode. EOG and ECG were recorded simultaneously using standard Au electrodes (NSC Electromedicina) with the same sample frequency and on-line filters as the MEG data.
MEG data were spatially filtered using the temporal signal space separation (tSSS) algorithm implemented in Neuromag® MEG software (Taulu and Hari 2009). Eye blinks and cardiac artefacts were removed from data by independent component analysis (ICA-JADE) implemented in the Brainstorm software (Tadel et al. 2011) (https://neuroimage.usc.edu/brainstorm/). Then for each pre-cue trial (attend central fixation cross and attend peripheral rings) and participants epochs from -1 s before the pre-cue onset were extracted from the continuous MEG data. For the post-cue trials (attend left and right) epochs of 2.9 s after cue onset were derived from the MEG recordings for each participant. Movement artefacts and horizontal eye movements as monitored by the EOG were determined for each epoch by visual inspection and excluded from analysis. The number of trials analyzed for the pre-cue task and post-cue combinations (fixation left post-cue: M = 106.1 ± 2.0 s. e. m., fixation right post-cue: M = 106.8 ± 1.8 s. e. m., rings left post-cue: M = 104.9 ± 2.23 s. e. m., rings right-post-cue: M = 103.9 ± 2.16) were equal (main effect pre-cue task F(1, 19) = 2.39, p = 0.138, η2 = 0.11; main effect post-cue F(1, 19) = 0.02, p = 0.879, η2 = 0.001; interaction pre-cue task by post-cue: F(1, 19) = 0.97, p = 0.336, η2 = 0.05). For each pre-cue task, post-cue condition and participant the epochs were averaged separately in order to obtain stimulus evoked MEG data epochs.