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EEG

How to format Hardware and Software filter fields in a .json?

In the modality specific sidecar file _eeg.json, we can specify the software and hardware filters that were applied during the collection or processing of the data. Generally, there are two accepted formats for that:

  1. a string containing "n/a", to show that no filter was used or the information on the filter is not available.

  2. a json object containing one object per filter. This filter-specific object contains key-value pairs to describe filter parameters. As per BIDS, all frequencies SHOULD be in Hz. For example a single hardware filter could be specified as:

"HardwareFilters": {"HighpassFilter": {"CutoffFrequency": 0.1}}

For a formatted example on how to deal with this in the BIDS context, please see this template.

How to specify EEGReference and EEGGround for Biosemi referencing scheme?

Reference and ground electrodes for EEG data can usually be specified using the EEGReference and EEGGround fields in the modality specific sidecar file _eeg.json. Both fields accept a string value such as "Placed on Cz".

However, some manufacturers use special referencing schemes such as the combination of a "Common Mode Sense" (CMS) and a "Driven Right Leg" (DRL) electrode. This is the case for Biosemi, as further documented on their website.

For a formatted example on how to deal with this in the BIDS context, please see this template.