New Delhi, June 16 -- BOSTON - The voice that comes out of Casey Harrell's laptop is recognisably his. The team at Massachusetts General Hospital trained the system on a clip of him speaking before amyotrophic lateral sclerosis took the muscle control he needed to form words, and the synthesiser now reproduces something close to the cadence his family remembers. He thinks about what he wants to say, and a few hundred milliseconds later the room hears it.

Over the past two years, that interface has carried close to two million words for him. Birthday calls. Bedtime stories for his daughter. Arguments with insurance administrators. None of those exchanges would have happened without the array of electrodes the BrainGate 2 team placed in hi...