[ENH] added PPG as an accepted channel type for EEG, MEG and iEEG

[robertoostenveld]

See #561 and bids-standard/bids-examples#197 (comment)

This does not close ... #561 since I think that the ACCEL requires some more thought and discussion with other stakeholders.

[sappelhoff]

Thanks @robertoostenveld would we have to add this entry to the tables in the MEG and iEEG sections as well?

I think the current understanding among the ephys modalities is, that these types are shared anyhow.

@rwblair could you please update this in the validator and revert the changes in the bids-examples PR where you changed PPG to PD?

[robertoostenveld]

would we have to add this entry to the tables in the MEG and iEEG sections as well?

the way I read the specification is that each of the MEG, EEG and iEEG chapters lists the channel types that are common for that specific modality, but also allows for the specification of channel types from the others. I.e. the lists are shared between MEG, EEG and iEEG, and a complete list would be the union of the three.

I think better (more maintainable, and more clear for people who read it) would be to have the channel types in an appendix. That would also be good for upcoming modalities (NIRS, motion). But that is considerably more work, and not in scope of this small PR.

[sappelhoff]

agreed, thanks.