Inner speech in motor cortex and implications for speech neuroprostheses

Speech brain-computer interfaces (BCIs) show promise in restoring
communication to people with paralysis, but have also prompted discussions
regarding their potential to decode private inner speech. Separately,
inner speech may be a way to bypass the current approach of requiring
speech BCI users to physically attempt speech, which is fatiguing and can
slow communication. Using multi-unit recordings from four participants, we
found that inner speech is robustly represented in motor cortex, and that
imagined sentences can be decoded in real-time. The representation of
inner speech was highly correlated with attempted speech, though we also
identified a neural “motor-intent” dimension that differentiates the two.
We investigated the possibility of decoding private inner speech and found
that some aspects of free-form inner speech could be decoded during
sequence recall and counting tasks. Finally, we demonstrate high-fidelity
strategies that prevent speech BCIs from unintentionally decoding private
inner speech.