Epoching and averaging
This tutorial fills the gap between the previous tutorial (review and clean continuous recordings), and the introduction tutorials (source analysis of evoked responses). It explains how to epoch the recordings, do some more pre-processing on the single trials, and then calculate their average.
Contents
Import in database
The raw file viewer provides a rapid access to the recordings, but most of operations cannot be performed directly on the continuous files: most of the pre-processing functions, averaging, time-frequency analysis and statistical tests can only be applied on blocks of data that are saved in the database (ie. "imported"). After reviewing the recordings and editing the event markers, you have to "import" the recordings in the database to go with further analysis. The two following operations produce the same result:
In the Event tab: select the menu Import > Import > Import in database (only when the file viewer is open),
In the database tree: Right-click on the nodes "Link to raw file" > Import in database.
If you right-click on the subject node > Import EEG/MEG, and select the tutorial .ds dataset, you would be able to import blocks of the continuous file in the database, but you would not have access to the modified events list or the SSP operators. Therefore, you would not import data cleaned for the ocular and cardiac artifacts. The modified events list and the signal space projectors are saved only in the "Link to raw file" in the Brainstorm database, not in the initial continuous file.
Now, right-click on the file with power line correction: Raw | sin(60Hz 120Hz 180Hz) > Import in dabase
Time window: Time range of interest. Now we want to keep the whole time definition, we are interested by all the stimulations.
Split: Useful to import continuous recordings without events. We do not need this here.
Events selection: Check the "Use events" option, and select both STI001 and STI002. The value between parenthesis represents the number of occurrences of this event.
Epoch time: Time instants that will be extracted before an after each event, to create the epochs that will be saved in the database. Set it to [-100, +300] ms
Use Signal Space Projections: Save and use the SSP vectors created by MaxFilter or during other pre-processing steps. Keep this option selected.
Remove DC Offset: Check this option, and select: Time range: [-100, 0] ms. For each epoch, this will: compute the average of each channel over the baseline (pre-stimulus interval: -100ms to 0ms), and subtract it from the channel at all the times in [-100,+300]ms.
Resample recordings: Keep this unchecked
Create new conditions for epochs: If selected, a new condition is created for each event type (here, it will create a condition "Event_5"). If not selected, all the epochs from all the selected events are saved in the same condition, the one that was selected in the database explorer (if no condition selected, create a condition called "New condition").
- Click on Import and wait.
