Support of invasive EMG recordings

[klotz-t]

Context:

To standardize Electromyography (EMG) data, EMG-BIDS was introduced with version 1.11.0 (which is amazing!). While this now allows for standardized reporting of data obtained from surface electrodes, the current EMG-BIDS specification still does not capture invasive EMG. However, invasive EMG is also highly relevant in clinical and applied neuropsychology.

Building on the current BIDS specification, we have tested annotating invasive EMG recordings through some additional BIDS examples (see the corresponding pull request). From that exercise, we believe it should be possible to enable the description of invasive EMG data in EMG-BIDS with only a few small modifications to the existing EMG-BIDS specification.

Proposed modifications to support invasive EMG:

The following extensions (in _emg.json) should suffice to make EMG-BIDS capable of handling invasive EMG:

• EMGType (REQUIRED): Specify the type of EMG recording. Can be either surface, invasive, or combined.
• CannulaDiameter (RECOMMENDED if invasive EMG): Diameter of the inserted needle (it is often reported in Gauge units; decide if one wants to stay with that convention or if metric units should be used (more BIDS-like))
• CannulaLength (RECOMMENDED if invasive EMG): Length of the inserted needle in millimeters.
• CannulaMaterial (RECOMMENDED if invasive EMG): Material of the needle.
• InsulatorMaterial (RECOMMENDED if fine wire or concentric needle electrode): Material used to insulate a pair of fine wire electrodes or the signal electrode from the shaft (reference) in a concentric needle.
• SterilizationMethod (RECOMMENDED if invasive EMG): Method used to sterilize the EMG-electrodes, e.g., E-beam or ethylene oxide.
• ElectrodeSurfaceArea (RECOMMENDED if invasive EMG, otherwise also not a bad idea to be reported): Surface area of the electrode contact in mm^2 (in the case of fine wire electrodes, only fully specified through ElectrodeDiameter (does also fit other electrode types) and ElectrodeTipLength (potentially only relevant for that type of electrode))

Minor issues when using EMG-BIDS:

Additionally, here is a list of minor issues we encountered while generating EMG-BIDS datasets.

• As it is, there are fields such as ElectrodeManufacturer or ElectrodeManufacturersModelName that are identified to be included in *electrodes.tsv (if needed); however, they are not specified there, so the BIDS validator only lets you pass if you put these fields in an electrode.json file.
• At the _emg.json level, InterelectrodeDistance is currently described to belong to *electrodes.tsv (if heterogeneous); however, formally it is defined to belong to *channels.tsv. We think it makes sense to have it in the electrodes.tsv file; at least, it should be described consistently.
• It can be kind of tricky that SoftwareFilters is a required field; if channel-specific, there is nothing reasonable to put there (n/a also feels not correct). Maybe there should be an option channel-specific (next to n/a or object). In one example, we have implemented a workaround: in the corresponding object, I set the low- and high-pass cut-offs to channel-specific (to let you pass the validator).

Comparing EMG-BIDS and CEDE

The Consensus for Experimental Design in Electromyography (CEDE) is an existing guide on performing and reporting EMG experiments. While the reporting aspect is mostly addressed from a human-readable perspective, we also conducted an exercise to test whether all required CEDE reporting items (see Table 1 in Besomi et al., 2024) have a place in EMG-BIDS. We identified a few CEDE reporting items that did not have a 1:1 correspondence in EMG-BIDS and which might be easy to add through a few more fields in _emg.json/_electrodes.tsv:

• ElectrodeShape (OPTIONAL): Shape of the electrode contact, e.g., circular or bar. That might also relate to the discussion on how to report the diameter and tip length of a fine wire electrode.
• ElectrideSkinInterface (OPTIONAL): Describe the characteristics of the electrode skin contact, e.g., double adhesive foam filled with conductive paste, pre-gelled adhesive electrode, …
• ElectrodeFixation (OPTIONAL): Describe any procedure that was used to fixate the electrodes (e.g., tape or Velcro strap).
• InterferenceSuppression (OPTIONAL): Techniques applied to reduce power-line interference, e.g., the use of a driven right leg (DRL) circuit.
• Specify for InterelectrodeDistance whether it is center-to-center or edge-to-edge.
• Potentially make Preamplification a free-text field, or consider where to also report preamp placement.