ENH: clearer error for mixed 10–20 EEG electrode names by AasmaGupta · Pull Request #13622 · mne-tools/mne-python

AasmaGupta · 2026-01-29T21:48:08Z

Reference issue (if any)

What does this implement/fix?

This PR improves the error raised when plotting topomaps with duplicate EEG
electrode positions caused by mixed 10–20 naming conventions
(T3/T4/T5/T6 vs T7/T8/P7/P8).

Instead of a generic "overlapping positions" error, a clearer and more
actionable message is raised that explains the naming conflict and suggests
how to resolve it by dropping one set of channels.

Additional information

A regression test has been added to ensure the clearer error message is raised
when mixed 10–20 electrode naming conventions are present.

for more information, see https://pre-commit.ci

AasmaGupta · 2026-01-29T21:51:23Z

This PR adds a clearer error message for mixed 10–20 EEG electrode naming
conventions, along with a regression test. Happy to adjust wording or test
placement if needed.

larsoner · 2026-01-30T18:52:09Z

doc/changes/dev/13447.enhance.rst

@@ -0,0 +1,3 @@
+Improve the error message raised when mixed 10–20 EEG electrode naming


This file is mis-named and lacks a :newcontrib: entry, can you look at other examples in that directory? And also you'll need an entry in doc/changes/names.inc

Yes, I looked into it and have made the new entry under :newcontib: and updated the file.

larsoner · 2026-01-30T18:52:26Z

mne/channels/tests/test_layout.py

+
+    with pytest.raises(
+        ValueError,
+        match="10.?20",


Why a .? here?

I’ve updated the error message to be explicit and deterministic and replaced the .? with “mixed 10–20 naming conventions”. I hope that helps!

for more information, see https://pre-commit.ci

larsoner · 2026-01-30T19:54:39Z

doc/changes/dev/13447.newcontrib.rst

@@ -0,0 +1 @@
+Improve the error message raised by :func:`mne.viz.plot_topomap` (via :func:`mne.channels.layout._auto_topomap_coords`) when mixed 10–20 EEG electrode naming conventions are detected, by :newcontrib:`Aasma Gupta`.


We should only reference public functions in changelogs

Suggested change

Improve the error message raised by :func:`mne.viz.plot_topomap` (via :func:`mne.channels.layout._auto_topomap_coords`) when mixed 10–20 EEG electrode naming conventions are detected, by :newcontrib:`Aasma Gupta`.

Improve the error message raised by :func:`mne.viz.plot_topomap` when mixed 10–20 EEG electrode naming conventions are detected, by :newcontrib:`Aasma Gupta`.

@AasmaGupta can you please integrate this suggestion?

larsoner

@britta-wstnr I think you opened the original issue... can you look and see if it does what you had in mind?

mne/channels/layout.py

scott-huberty · 2026-02-10T19:13:04Z

mne/channels/tests/test_layout.py

    with pytest.raises(
        ValueError,
-        match="mixed 10-20 naming conventions",
+        match=r"mixed 10.?20 naming conventions",


What is the purpose of the regex here? The error message is hard coded, so can we simply just match the beginning of the message, e.g. match="Duplicate EEG electrode positions detected due to mixed 10-20"

Yeah, that makes sense. I’ve updated the test to simply match the error message and pushed the change.

@scott-huberty
Just a quick follow-up, I’ve updated the error message and adjusted the test accordingly. Please let me know if this looks good or if any further changes are to be made.

scott-huberty · 2026-02-15T19:32:09Z

mne/channels/layout.py

+        duplicates_1020 = {
+            "T3": "T7",
+            "T4": "T8",
+            "T5": "P7",
+            "T6": "P8",
+        }
+
+        names = set(problematic_electrodes)
+        duplicate_pairs = [
+            (old, new)
+            for old, new in duplicates_1020.items()
+            if old in names and new in names
+        ]
+
+        if duplicate_pairs:
+            raise ValueError(
+                "Duplicate EEG electrode positions detected due to mixed 10-20 "
+                "naming conventions.\n"
+                "You appear to have both legacy (T3/T4/T5/T6) and modern "
+                "(T7/T8/P7/P8) electrode names present. The modern convention "
+                "(T7/T8/P7/P8) is recommended.\n\n"
+                "Please drop the legacy channels before plotting, for example:\n"
+                "    inst.drop_channels(['T3', 'T4', 'T5', 'T6'])"
+            )


Suggested change

duplicates_1020 = {

"T3": "T7",

"T4": "T8",

"T5": "P7",

"T6": "P8",

}

names = set(problematic_electrodes)

duplicate_pairs = [

(old, new)

for old, new in duplicates_1020.items()

if old in names and new in names

]

if duplicate_pairs:

raise ValueError(

"Duplicate EEG electrode positions detected due to mixed 10-20 "

"naming conventions.\n"

"You appear to have both legacy (T3/T4/T5/T6) and modern "

"(T7/T8/P7/P8) electrode names present. The modern convention "

"(T7/T8/P7/P8) is recommended.\n\n"

"Please drop the legacy channels before plotting, for example:\n"

" inst.drop_channels(['T3', 'T4', 'T5', 'T6'])"

)

# Check for duplicate 10-20 channels that are aliases for the same position

LEGACY_TO_MODERN_1020 = {"T3": "T7", "T4": "T8", "T5": "P7", "T6": "P8"}

names = set(problematic_electrodes)

conflicts = {

old: new

for old, new in LEGACY_TO_MODERN_1020.items()

if old in names and new in names

}

if conflicts:

overlap_info = ", ".join(f"{old}/{new}" for old, new in conflicts.items())

drop_list = ', '.join(repr(old) for old in conflicts)

raise ValueError(

"The following electrodes are aliases for the same physical location "

f"(10-20 vs 10-10): {overlap_info}\n. To fix this call "

f"`.drop_channels([{drop_list}])` on your Raw/Epochs/Evoked object."

)

A couple notes on that suggestion:

The error message is now a bit shorter and provides an actionable fix. The repr(old) just makes sure that e.g. 'T3' is printed instead of T3 so that the suggestion is valid Python code.

I added a code comment at the beginning of this code block as a hint to future devs.

condensed duplicates_1020 to a single line. IMO the new name is more descriptive.

Make sure to remember to update the match=... In your test! Otherwise it will fail after incorporating this change.

scott-huberty

Sorry for the delay @AasmaGupta almost there! Once you integrate the remaining suggestions I think this will be good to merge.

scott-huberty · 2026-02-15T19:36:52Z

mne/channels/layout.py

+        duplicates_1020 = {
+            "T3": "T7",
+            "T4": "T8",
+            "T5": "P7",
+            "T6": "P8",
+        }
+
+        names = set(problematic_electrodes)
+        duplicate_pairs = [
+            (old, new)
+            for old, new in duplicates_1020.items()
+            if old in names and new in names
+        ]
+
+        if duplicate_pairs:
+            raise ValueError(
+                "Duplicate EEG electrode positions detected due to mixed 10-20 "
+                "naming conventions.\n"
+                "You appear to have both legacy (T3/T4/T5/T6) and modern "
+                "(T7/T8/P7/P8) electrode names present. The modern convention "
+                "(T7/T8/P7/P8) is recommended.\n\n"
+                "Please drop the legacy channels before plotting, for example:\n"
+                "    inst.drop_channels(['T3', 'T4', 'T5', 'T6'])"
+            )


A couple notes on that suggestion:

The error message is now a bit shorter and provides an actionable fix. The repr(old) just makes sure that e.g. 'T3' is printed instead of T3 so that the suggestion is valid Python code.

I added a code comment at the beginning of this code block as a hint to future devs.

condensed duplicates_1020 to a single line. IMO the new name is more descriptive.

Make sure to remember to update the match=... In your test! Otherwise it will fail after incorporating this change.

scott-huberty · 2026-02-15T19:40:36Z

Also @AasmaGupta please create a CircleCI account by signing in via GitHub . After you do this, the build_docs job will run.

ENH: clearer error for mixed 10–20 EEG electrode names

01c1a09

AasmaGupta requested review from agramfort, dengemann, jasmainak and mscheltienne as code owners January 29, 2026 21:48

AasmaGupta mentioned this pull request Jan 29, 2026

Handling duplicate electrode names in 10–20 system #13447

Open

[pre-commit.ci] auto fixes from pre-commit.com hooks

e2786b7

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ENH: add towncrier entry for mixed 10–20 EEG naming error

44c708d

AasmaGupta requested review from drammock and larsoner as code owners January 29, 2026 21:55

larsoner reviewed Jan 30, 2026

View reviewed changes

AasmaGupta and others added 4 commits January 31, 2026 00:42

DOC: fix towncrier entry and test regex

3c3dd78

[pre-commit.ci] auto fixes from pre-commit.com hooks

7a422b8

for more information, see https://pre-commit.ci

DOC: fix towncrier entry type for mixed 10–20 EEG naming

ccb9527

TEST: simplify for mixed 10-20 naming

6466c33

larsoner reviewed Jan 30, 2026

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scott-huberty reviewed Jan 30, 2026

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mne/channels/layout.py Show resolved Hide resolved

ENH: recommend modern 10–10 electrode names and update test regex

5709131

scott-huberty reviewed Feb 10, 2026

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TEST: simplify regex match for duplicate 10-20 electrode error

99f130d

scott-huberty reviewed Feb 15, 2026

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scott-huberty requested changes Feb 15, 2026

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		@@ -0,0 +1,3 @@
		Improve the error message raised when mixed 10–20 EEG electrode naming

		@@ -0,0 +1 @@
		Improve the error message raised by :func:`mne.viz.plot_topomap` (via :func:`mne.channels.layout._auto_topomap_coords`) when mixed 10–20 EEG electrode naming conventions are detected, by :newcontrib:`Aasma Gupta`. No newline at end of file

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AasmaGupta commented Jan 29, 2026

Reference issue (if any)

What does this implement/fix?

Additional information

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AasmaGupta commented Jan 29, 2026

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scott-huberty commented Feb 15, 2026

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