Improve tests of maxwell_filter_prepare_emptyroom (#13208)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: mscheltienne

doc/changes/devel/13208.bugfix.rst.rst

@@ -0,0 +1 @@
+Handle channels with potential electrode location in :func:`~mne.preprocessing.maxwell_filter_prepare_emptyroom`, by `Mathieu Scheltienne`_.

mne/preprocessing/maxwell.py

@@ -131,12 +131,17 @@ def maxwell_filter_prepare_emptyroom(
     * Set the following properties of the empty-room recording to match the
       experimental recording:
 
-      * Montage
+      * Montage (required for the fiducials defining the head coordinate frame)
       * ``raw.first_time`` and ``raw.first_samp``
 
     * Adjust annotations according to the ``annotations`` parameter.
     * Adjust the measurement date according to the ``meas_date`` parameter.
 
+    .. note::
+
+        Note that in case of dual MEG/EEG acquisition, EEG channels should not be
+        included in the empty room recording. If provided, they will be ignored.
+
     .. versionadded:: 1.1
     """  # noqa: E501
     _validate_type(item=raw_er, types=BaseRaw, item_name="raw_er")

mne/preprocessing/tests/test_maxwell.py

@@ -1896,7 +1896,7 @@ def test_prepare_emptyroom_bads(bads):
     assert raw_er_prepared.info["bads"] == ["MEG0113", "MEG2313"]
     assert raw_er_prepared.info["dev_head_t"] == raw.info["dev_head_t"]
 
-    montage_expected = raw.copy().pick(picks="meg").get_montage()
+    montage_expected = raw.pick(picks="meg").get_montage()
     assert raw_er_prepared.get_montage() == montage_expected
 
     # Ensure the originals were not modified
@@ -1906,6 +1906,27 @@ def test_prepare_emptyroom_bads(bads):
     assert raw_er.get_montage() is None
 
 
+@testing.requires_testing_data
+def test_prepare_empty_room_with_eeg() -> None:
+    """Test preparation of MEG empty-room which was acquired with EEG enabled."""
+    raw = read_raw_fif(raw_fname, allow_maxshield="yes", verbose=False)
+    raw_er = read_raw_fif(erm_fname, allow_maxshield="yes", verbose=False)
+    assert "eeg" in raw
+    assert "eeg" in raw_er
+    raw_er_prepared = maxwell_filter_prepare_emptyroom(raw_er=raw_er, raw=raw)
+    assert raw_er_prepared.info["dev_head_t"] == raw.info["dev_head_t"]
+    montage_expected = raw.get_montage()
+    assert raw_er_prepared.get_montage() == montage_expected
+
+    raw_er = raw_er.pick("meg")
+    assert "eeg" in raw
+    assert "eeg" not in raw_er
+    raw_er_prepared = maxwell_filter_prepare_emptyroom(raw_er=raw_er, raw=raw)
+    assert raw_er_prepared.info["dev_head_t"] == raw.info["dev_head_t"]
+    montage_expected = raw.pick("meg").get_montage()
+    assert raw_er_prepared.get_montage() == montage_expected
+
+
 @testing.requires_testing_data
 @pytest.mark.slowtest  # lots of params
 @pytest.mark.parametrize("set_annot_when", ("before", "after"))