Tutorial 11: Bad channels

Authors: Francois Tadel, Elizabeth Bock, Sylvain Baillet

It is common during the acquisition to have a few sensors that are recording values that will not be usable in the data analysis. In MEG, a sensor can be damaged or unstable. In EEG, the quality of the connection between the electrode and the scalp is sometimes too low to record anything interesting.

It is important to identify the sensors with poor signal quality at an early stage of the pre-processing, because the efficiency of the artifact removal will depend on it. If you try to remove blink and cardiac artifacts with some bad sensors it may not work very well, and worse, it will propagate the bad signals to all the channels.

This tutorial will explain the various ways we have to handle the bad channels. Note that the recordings from this auditory experiment do not contain any bad sensors, therefore the entire tutorial is optional. If you are not interested, you can skip it and will still be able to follow the next tutorials.


Identifying bad channels

Some bad channels are easy to detect, their signals look either completely off or totally flat compared with the other surrounding sensors. Some others are more difficult to identify. The examples below are taken from other datasets.


Selecting sensors


Marking bad channels


From the database explorer

Many options to change the list of bad channels are available from the database explorer.


Epoching and averaging

The list of bad channels is saved separately for each dataset.

At this stage of the analysis, the database contains only links to continuous files. When you import epochs from a continuous file, the list of bad channels will be copied from the raw file to all the imported data files.

Then you will be able to redefine this list for each epoch individually, tagging more channels as bad, or including back the ones that are ok. This way it is possible to exclude from the analysis the channels that are too noisy in a few trials only, for instance because of some movement artifacts.

When averaging, if an epoch contains one bad channel, this bad channel is excluded from the average but all the other channels are kept. If the same channel is good in other trials, it will be considered as good in the average. This means that not all the channels have the same number of trials for calculating the average.

This may cause the different channels of an averaged file to have different signal-to-noise ratios, which may lead to confusing results. However, we decided to implement the average in this way to be able to keep more data in the studies with a low number of trials and a lot of noise.


On the hard drive

The list of bad channels is saved for each data file separately, in the field ChannelFlag.
This vector indicates for each channel #i if it is good (ChannelFlag(i)= 1) or bad (ChannelFlag(i)= -1).

Right-click on a link to a continuous file > File > View file contents:


This information is duplicated in the sFile structure (F field) in order to be passed easily to the low-level reading functions. If you are planning to modify the list of bad channels manually, you need to change two fields: mat.ChannelFlag and mat.F.channelflag

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Tutorials/BadChannels (last edited 2016-03-17 16:26:54 by FrancoisTadel)