Error: Compensation grade of ICA (3) and Raw (0) do not match

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I’m working on making a script for “re-BIDSifying” the data imposing the compensation grade to 0 for experimental data. Now I’m running into a strange behavior that I think can be summed up with the following MWE:

from pathlib import Path

import mne

from mne_bids import (

    BIDSPath,

    read_raw_bids,

    write_raw_bids

)




old_bids_root = Path("/home/.../BIDS")

new_bids_root = Path("/home/.../BIDS_for_MNE-BIDS-Pipeline")

target_grade = 0

bp = BIDSPath(

    root=old_bids_root,

    subject="063",

    session="01",

    task="rest",

    run="01",

    datatype="meg"

)

raw = read_raw_bids(bids_path=bp, verbose=False)

print(f"Initial compensation grade: {raw.compensation_grade}")

raw.apply_gradient_compensation(target_grade)

new_bp = bp.copy().update(root=new_bids_root)

write_raw_bids(

        raw,

        bids_path=new_bp,

        overwrite=True,

        verbose=False

    )

new_raw = mne.io.read_raw_ctf(new_bp.fpath, preload=False)

print(f" → written {new_bp.fpath} with compensation grade {new_raw.compensation_grade}")

(NOTE: I created a new bids_path to go around the issue signalled here )

But strangely the compensation grade of the new raw is still 3:

Initial compensation grade: 3
Compensator constructed to change 3 → 0
...
Current compensation grade : 3
→ written /home/.../BIDS_for_MNE-BIDS-Pipeline/sub-063/ses-01/meg/sub-063_ses-01_task-rest_run-01_meg.ds with compensation grade 3

In the sidecar json:

"SoftwareFilters": {

        "SpatialCompensation": {

            "GradientOrder": 0

        }

    }

Why this happens? I saw I could impose in write_raw_bids to have format = ‘FIF’ and that would solve the issue, but I would prefer to keep .ds