Hello,

MEG experimental data often comes with an associated "empty-room" recording, which is supposed to capture the signal recorded with no participant present.

BIDS allows to indicate which empty-room recording belongs to an experimental recording via the AssociatedEmptyRoom key in *_meg.json. Specifically, the specs state the following:

Relative path in BIDS folder structure to empty-room file associated with the subject’s MEG recording. The path needs to use forward slashes instead of backward slashes (for example, "sub-emptyroom/ses-/meg/sub-emptyroom_ses-_task-noise_run-_meg.ds").

Two questions came to my mind:

• The path is supposed to be "relative", but it's unclear to me what the reference point is supposed to be. Intuitively, I assumed it's either the location of *_meg.json, or of the experimental recording. However, the path in the example seems to be relative to the BIDS root. What is the correct way to specify AssociatedEmptyRoom?
• The path in the example has empty ses and run entities. I didn't know this was even valid. Is that a mistake?

For reference, this is the schema currently used for validation of AssociatedEmptyRoom. It's … under-complex and therefore didn't provide me with any additional clues:
https://github.com/bids-standard/bids-validator/blob/2f0ca2458cff47e1fd1f898c3a81aef370558085/bids-validator/validators/json/schemas/meg.json#L48-L59

Thanks for you help,

Richard