[FIX] Clarify that electrodes.tsv is optional for MEG, for use with simultaneous (i)EEG

src/modality-specific-files/magnetoencephalography.md

@@ -98,8 +98,10 @@ it SHOULD be stored separately under a new `/eeg` data type
 If however EEG is recorded simultaneously **with the same MEG system**,
 it MAY be stored under the `/meg` data type.
 In that case, it SHOULD have the same sampling frequency as MEG (see `SamplingFrequency` field below).
-Furthermore, the EEG sensor coordinates SHOULD be specified using MEG-specific coordinate
-systems (see [coordinates section](#coordinate-system-json-_coordsystemjson) below and
+Furthermore, EEG sensor coordinates SHOULD be recorded in an
+[`electrodes.tsv`](electroencephalography.md#electrodes-description-_electrodestsv)
+file using MEG-specific coordinate systems
+(see [Coordinate System JSON](#coordinate-system-json-_coordsystemjson) below and
 the [Coordinate Systems Appendix](../appendices/coordinate-systems.md)).
 
 ### Sidecar JSON (`*_meg.json`)

src/schema/rules/checks/meg.yaml

@@ -0,0 +1,14 @@
+# Rules for MEG data that cannot be defined in tables
+---
+MEGElectrodesRecommended:
+  issue:
+    code: MEG_EEG_ELECTRODES_RECOMMENDED
+    message: |
+      A nonzero EEGChannelCount for MEG datasets indicates the presence of EEG
+      channels. A corresponding `electrodes.tsv` file should be be added.
+    level: warning
+  selectors:
+    - suffix == "meg"
+    - sidecar.EEGChannelCount > 0
+  checks:
+    - type(associations.electrodes) != 'null'

src/schema/rules/files/raw/channels.yaml

@@ -72,6 +72,15 @@ electrodes:
     acquisition: optional
     space: optional
 
+# MEG has an additional entity available
+electrodes__meg:
+  $ref: rules.files.raw.channels.electrodes
+  datatypes:
+    - meg
+  entities:
+    $ref: rules.files.raw.channels.electrodes.entities
+    processing: optional
+
 optodes:
   suffixes:
     - optodes