Removing artifacts from MEG recordings using Signal Space Projections can sometimes be very efficient.  Here are a few tricks to help you remove artifacts from your data that are ongoing or repeating.

We typically use one of three strategies to define an SSP projector:

1. PCA analysis over one continuous epoch (i.e. the entire recording), for which the artifact is ongoing

2. PCA analysis for concatenated 'events', for which the artifact is discrete and repeating in the recording

3. Visual identification of one topography that represents the artifact and is converted to an SSP projector.

Example 1: Metal in the mouth (dental retainer) that moves when the subject breathes.

Identify the artifact and channel(s) that are most affected.  Scanning though the data and performing a PSD can help with this: {{/brainstorm/attachement%3Adental_breathing_artifacts.png?action=content|attachement:dental_breathing_artifacts.png|width="100%"}}

Channel MRT57, on the MEG traces shows the effect of the metal.  In addition, this channel has high power in the lower freq (PSD) and the topography shows a dipole below the posterior part of the sensor array.

Since the artifact is ongoing and not necessarily a well-defined 'event', we will use the first strategy.

First make a 'start' event at the beginning of the recording (or when the artifact starts)
Using the Artifacts menu, select SSP: Generic

Time window:  this should be the time window of the artifact, in this example I use the first 100 seconds of the recording

Event name: empty

Event window: ignore this

Frequency band: from the PSD I can see that the artifact is mostly defined in [0.5,10]Hz.  If you know the freq band of the artifact, you should adjust this accordingly, if not, you can use a more broadband to get an overall view.

Sensor types: MEG

Compute using existing: uncheck this box

Save averaged artifact: uncheck this box

Method to calculate: choose PCA