Millions of people live with sensory and communication deficits after stroke, trauma or neurodegenerative disease, and BCIs are a genuine avenue for restoring some of what was lost. The field trying to deliver that is split in two.

Invasive neuroprosthetics and non-invasive electrophysiological decoders have grown into largely separate literatures, with different terminology and no consistent basis for comparison. That fragmentation has a practical cost: a clinician or funder cannot easily tell whether two approaches are solving the same problem differently or solving different problems.

A two-by-two framework is a modest-sounding contribution that does real work here. Once results from both camps sit in the same structure, the genuine trade-offs become visible, and so do the gaps nobody is working on because they fall between the two communities.