Frequently Asked Questions

As this starter-kit grows it gets harder to know where to find information, so this page is a collection of frequently asked questions.

Please add to this list! It will always be :construction: in construction :construction: and we really encourage everyone to update it to be more useful. If there’s a question you’ve asked or answered more than twice then it’s an FAQ and it should be in the repository.

Ideally the questions will link to an answer elsewhere in the repository to maximise the different ways of finding out more about BIDS.


For questions related to the BIDS apps, please visit the BIDS apps website.

General questions

What is a json file?

You can find more information about json (and tsv) files in the Metadata-file-formats page.

What does [this word] mean?

We’re building a glossary to de-jargonise some of the terms you need to know to work with data in BIDS format. Check it out here.

How to specify the micro sign in Matlab?

The symbol used to indicate µ has unicode U+00B5, which is in Matlab char(181).

Is your data type not covered in the current BIDS specification?

BIDS extensions proposals (BEPs) aim to extend the BIDS specification to new data types. A list of extensions proposals can be found on the main BIDS webpage under Get Involved. Guidelines for contributing to these extensions or starting your own can be found in the BIDS Extension Proposals Guide.

If only part of your data is covered under BIDS, an option to allow additional files currently not covered in BIDS to pass the validator is the .bidsignore file, which works just like .gitignore. It allows you to list all the files (or directories, with wildcards) that are not BIDS compliant and should be ignored by the validator. Of course you should still try to adhere to upcoming BEPs and the general BIDS philosophy for file names and metadata where possible, but this gives a little extra flexibility.

How do I convert my data to BIDS?

We strongly recommend you pick a BIDS converter to help you convert your data.

A list of converters can be found on the BIDS website

How to import Excel files?

See these bids tools to import and export a participants.tsv file: bids-matlab-tools

I had to split the testing of one of my participants across 2 days, should I use 2 different session folders to organize the data of that participant?

No. The session level in the BIDS folder hierarchy can be used to group data that go “logically” together: this means that you can put in the same session folder data that were acquired on different days, but that are “linked” to one another in a way that make sense to how you want to organize your data.

If you want to keep track of what data was acquired when you can use the scans.tsv files.

For some examples, see this issue in the bids-starter kit.

If you deal with EEG data, you may want to read this comment in another issue as well before considering combining within the same session folder recordings acquired on different occasions.

MRI specific questions

What defacing tools can I use?

If you want to share your BIDS data set, chances are that you will have to anonymize it and therefore prevent identification of the participants from their anatomical scans. You will need to de-identification (or deface) the anatomical images. There are several options to do that

  • If you already have a valid BIDS data set you can simply use the BIDSonym BIDS app on it. To do that it relies on several tools that you can also use before you have finalized your BIDS data set

Otherwise you can also use:

  • Fieldtrip under matlab can do it.

  • SPM8 and SPM12: when in the batch editor fo to –> SPM menu –> Util –> De-face

EEG specific questions

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.

How to specify units in microVolt?

BIDS requires physical units to be specified according to the SI unit symbol and possibly prefix symbol (for example: mV, μV for milliVolt and microVolt). In Matlab use native2unicode(181,'latin1') to get the correct symbol for micro.


Are BIDS and NWB compatible?

Yes, BIDS and NWB are compatible.

How to combine BIDS and NWB?

An NWB data file is an allowed format in the iEEG-BIDS data structure. This means that one subject (AAA) with a session (BBB) can have a BIDS folder with raw iEEG data in NWB format:


The same subject can have another session (CCC) with raw fMRI data in BIDS:



How to store subject phenotypes?

In the phenotype folder, according to the details provided for phenotypes: bids-root/phenotype

Is there a standard for epilepsy phenotypes?

Yes, open this epilepsyClassification2017 and follow the examples in the phenotype templates.