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.