Words you'll never speak still cause activity in the brain's speech centers.

Most experimental brain-computer interfaces (BCIs) that have been used for synthesizing human speech have been implanted in the areas of the brain that translate the intention to speak into the muscle actions that produce it. A patient has to physically attempt to speak to make these implants work, which is tiresome for severely paralyzed people.

To go around it, researchers at the Stanford University built a BCI that could decode inner speech—the kind we engage in silent reading and use for all our internal monologues. The problem is that those inner monologues often involve stuff we don’t want others to hear. To keep their BCI from spilling the patients’ most private thoughts, the researchers designed a first-of-its-kind “mental privacy” safeguard.

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