Tutorial 7: Event markers

Authors: Francois Tadel, Elizabeth Bock, John C Mosher

Lists of events

You probably noticed colored dots on top of the recordings in the MEG figure. They represent the event markers saved in this dataset. In this documentation, they can be called indifferently events, markers or triggers. Some are stimulus triggers that were generated by the stimulation computer (Psychtoolbox-3), others are the subject responses recorded from a button box. This tutorial shows how to manipulate these markers.

Adding events

The markers can represent either stimulation triggers or subject responses that were recorded during the acquisition. It can also be useful to add new markers during the analysis of the recordings, to identify events of interest that are not detected at the time of the recordings, such as artifacts (eye movements, heartbeats, subject movements) or specific patterns of brain activity (epileptic spikes).

Extended events

You can also use this interface to create events that have a temporal extension, i.e., that last for more than one time sample. This can be used to define bad segments in the recordings.

Bad segments

It is very common to have portions of the recordings heavily contaminated by events coming from the subject (eye blinks, movements, heartbeats, teeth clenching) or from the environment (stimulation equipment, elevators, cars, trains, building vibrations...). Some of them are well defined and can be removed efficiently, some are too complex to be modeled efficiently. For this last category, it is usually safer to mark the noisy segments as bad, and ignore them for the rest of the analysis.

To mark a segment of recordings as bad, the procedure is the same as for defining an extended event: select a time window, and then tag it as bad with one of the following methods.

It creates a new event group BAD, and adds an extended event to it. Later, when epoching this file (extracting time blocks and saving them in the database), the trials that contain a bad segment will be imported but tagged as bad, and ignored in the rest of the analysis.

You can create multiple groups of bad segments, for instance to identify different types of artifacts. Any event group that contains the tag "BAD" will be considered as indicating bad segments.



Hide event groups

When you have too many events in the viewer, seeing the ones you are interested in can be difficult. This will be the case for insteance after we detect the heartbeats in the signal, we will have one event every second, which is not always interesting to see. Each event category can be selectively hidden.


Channel events

Some events can be attached to only one or a few channels. This is useful for instance for reviewing clinical EEG recordings, where neurologists are tagging epileptic activity only on a subset of the channels.



Additional comments can be added to the event, in case additional details should be displayed in the file viewer. This is mostly useful for reviewing clinical recordings as well.


Display modes

Three display modes are available for the event markers: dots, lines or hidden. Select the corresponding menu in the display options, or press CTRL+L multiple times.

events_dots.gif events_lines.gif


Custom shortcuts

When reviewing long recordings and adding manually lots of events (eg. when marking manually epileptic spikes), using the menus presented above is not convenient because they require many mouse clicks.

Using the menu Events > Edit keyboard shortcuts, you can associate custom events to the key 1 to 9 of the keyboard. Define the name of the event type to create for each key, and then simply press the corresponding key to add/delete a marker at the current time position. Three options are available for each event type:


Saving modifications

Now you can delete all the event groups that you've just created and leave only the initial ones (button, standard, deviant): select the event groups and press Delete, or use the menu Events > Delete group.

When you close the continuous file viewer, or the last figure that shows a part of the raw file, the dataset is unloaded, the file is released and the memory is freed.

If you edited the events for this file, you are asked whether to save the modifications or not. If you answer "Yes", the modifications are saved only in the database link (Link to raw file), not in the original file itself. Therefore, you would see the changes the next time you double-click on the "link to raw file" again, but not if you open the original .ds file in another protocol or with an external program.


Note that events you edit are not automatically saved until that moment. As you would do with any other type of computer work, save your work regularly, to limit the damages of a program or computer crash. In the Record tab, use the menu File > Save modifications.


Other menus





On the hard drive

The nodes "Link to raw file" you see in the database explorer are represented by .mat files on the hard drive. They contain all the header information extracted from the raw files, but do not contain a full copy of the recordings.

All the additional information created from the Brainstorm interface (event markers, bad channels, SSP projectors) are not saved back to the original raw files, they are only saved in the "Link to raw file". The names of these files start with the tag data_0raw_, they share the same structure as all the imported epochs (introduced later in the tutorials).

To explore the contents of these link files, right-click on them use the popup menus File > View file contents or File > Export to Matlab.


sFile structure: This structure is passed directly to all the input/output functions on continuous files.

Useful functions

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Tutorials/EventMarkers (last edited 2020-09-08 15:56:54 by FrancoisTadel)