Acquisition Setting For MEG Recording
In order to record your data, you have to specify your MEG acquisition settings: the name of your project, which channels and triggers you are going to record, the hardware high-pass and low-pass filter, the frequency sampling etc.
These parameters are inserted through the acquisition software interface. Please read the Acquisition Software Manual for a detailed guide.
Hereafter the main steps for a standard acquisition are briefly described. A project called standard_project and associated acquisition settings called standard settings have been generated for this purpose.
Open the software and define your project
If it's not open yet, double-click the Neuromag icon and start the data acquisition control program (megacq) by double clicking the Acquisition icon.
Click on Change button next to Project and select standard_project. Click the top-left button File -> Load settings and select standard settings. These settings have been saved as the standard default settings for a generic MEG experiment. You are of course free to use different settings for your experiment, but it is recommended that you do it by modifying the standard settings to minimize errors.
IMPORTANT: When you define your project, do define and save your settings. Crucially, DO NOT FORGET to load them every time you open your project (they are not automatically loaded when you open your project), otherwise you will record with the settings of the previous user, which might be completely different from yours.
Subject
You need to select a subject only if you want to record data. You do not need to select the subject if you just want to explore the signal.
Acquisition settings
Click the "Change" button corresponding to "Acquisition". One window opens with all the channels that you can record (MEG, EEG, triggers, auxiliary channels).
- MEG channels
By default, all MEG channels are selected. In case of noisy channels, do NOT de-select them, mark them on the MEG notebook, and set them as bad channels when running Maxfilter.
- EEG channels
Select EEG channels only if you record them (not included in the 'standard settings').
- EOG/ECG channels
Select EEG 61, EEG 62 and EEG 63 to record vertical and horizontal eye movements (EOG V and EOG H), and the cardiac signal (ECG). IMPORTANT: to set them as bipolar channels, right click on EEG 61 and 62 and select EOG, right click on EEG 63 and select ECG. Failing to do that will cause the system to record unipolar (only one electrode) and have bad signals. EOG H and V and ECG will be displayed on the bipolar EEG channels button during acquisition.
- Trigger (STI) channels
See Trigger Encoding for the choice of trigger channels. The standard way is to use channel STI101 only, as channels STI1 to 16 bring redundant information.
It is highly recommended to encode all the relevant events of your experiment (stimulus and response) in the STI101 with distinct values, so that all the relevant information is included in one single file. The alternative is to encode events always with the same value, and then re-label them integrating a separate event file, but this is an insidious source of error if for any reason one trigger is not/badly recorded and one trial is lost.
- Other channels
It is not recommended to change the selection of the other channels of the standard settings, as they are needed by the acquisition system for crucial basic operations. In particular, STI201 is needed to record the HPI positions (See Section 5.2 of the Acquisition Software Manual), MISC channels are needed for the Maxshield active compensation.
- Recording parameters
- Sampling frequency. Default: 1000 Hz.
- Hardware high-pass filtering. Default: 0.1 Hz (you can set it lower, but be aware that DC recordings might be problematic because of slow trends).
- Hardware low-pass filtering. Default: 330 Hz. Its maximum depends on the sampling frequency.
- Maxshield noise compensation. Default: On. In our experience Maxshield is very efficient and should be used for all experiments. Deactivate at your own risk. Please remember that when Maxshield is On, you should always apply Maxfilter to analyse your data.
External stimulus generation
This button allows the generation of stimuli with the Neuromag software. Since in our lab we systematically use either E-prime or Psychtoolbox programmed from the PC Stim, you do not need to touch this button.
On-line averaging
If activated, this useful function displays the ERFs computed on-line (e.g. while you are recording) with respect to events that you will have defined (see Trigger Encoding for event definition). See Section 3.5 of the Acquisition software manual to read how to implement On-line averaging.