Fix interpolate_to spline target positions

doc/changes/dev/14266.bugfix.rst

@@ -0,0 +1 @@
+Fix bug where :meth:`mne.io.Raw.interpolate_to` and related methods with ``method="spline"`` did not center the target sensor positions on the fitted sphere origin, by `Eric Larson`_.

mne/channels/interpolation.py

@@ -452,7 +452,9 @@ def _interpolate_to_eeg(inst, sensors, origin, method, reg):
     if method == "spline":
         origin_val = _check_origin(origin, inst.info)
         pos_from = inst.info._get_channel_positions(picks_good_eeg) - origin_val
-        pos_to = np.stack(list(ch_pos.values()), axis=0)
+        # Use info_to (rather than ch_pos directly) so that the target positions
+        # are in the head frame, and center both sets on the fitted origin
+        pos_to = info_to._get_channel_positions() - origin_val
 
         def _check_pos_sphere(pos):
             d = np.linalg.norm(pos, axis=-1)

mne/channels/tests/test_interpolation.py

@@ -585,6 +585,18 @@ def test_interpolate_to_eeg(montage_name, method, data_type):
     assert inst_interp.info["bads"] == bads
 
 
+def test_interpolate_to_eeg_same_positions():
+    """Test that spline interpolate_to onto src pos is a no-op (gh-14153)."""
+    raw = read_raw_fif(raw_fname).pick("eeg").crop(0, 1).load_data()
+    montage = make_dig_montage(
+        ch_pos=dict(zip(raw.ch_names, raw.info._get_channel_positions())),
+        coord_frame="head",
+    )
+    raw_interp = raw.copy().interpolate_to(montage, method="spline")
+    assert raw_interp.ch_names == raw.ch_names
+    assert_allclose(raw_interp.get_data(), raw.get_data(), rtol=1e-5, atol=1e-12)
+
+
 @pytest.mark.slowtest  # ~5s locally
 @pytest.mark.filterwarnings("ignore:Projection vector.* reduced .*:RuntimeWarning")
 def test_interpolate_to_meg(monkeypatch):