MEG was recorded with a 306-channel Elekta Neuromag™ system (Elekta Oy, Helsinki, Finland) located at the MEG Core of Aalto Neuroimaging, Aalto University. The signals were filtered to 0.1–330 Hz and digitized at the rate of 1 kHz. Four head-position indicator (HPI) coils were attached to the subject's scalp for head position estimation and alignment to a common coordinate frame. The visual cues and feedback (see below) were delivered on a screen located approximately 50 cm in front of the subject's eyes by a projector outside the shielded room. During the recording, the raw MEG data were continuously written in 300-ms segments to a network-transparent ring buffer [33 (link),37 (link)] hosted by the MEG acquisition workstation (6-core Intel Xeon CPU at 2.4 GHz, 64-bit CentOS Linux, version 5.3). This buffer was read over a local network connection by another computer (64-bit Ubuntu Linux, version 12.04-LTS), which processed the data in real time using functions implemented in the MNE-Python software [38 (link)] and presented the visual stimuli using PsychoPy version 1.83 [39 (link)].
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