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Locked-in patients speak through planning signals
Speech Planning Signals
Eddie Chang, chair of neurosurgery at UCSF and a bioengineer, does this work. Speech is the pharynx and larynx controlling exhales, and in a locked-in patient the signals cannot get down to them. So he does not measure from the speech production areas. He has spent decades carefully characterizing the signals coming out of the speech planning areas, and he transforms those signals into electrical signals so the person can talk through a computer.
Beyond Speech
The same approach works for other goals. Researchers working with brain-damaged patients find that going to the areas where the signals are generated to inform the action output ends of the brain gets them far. If the goal is to drink a glass of water, the brain generally figures out the rest.
What Remains Unknown
The neurons involved are extensively interconnected with structures for thinking and planning, and nobody fully understands what a thought is. Stimulating neurons in motor cortex to generate movement without accounting for that could send the system haywire.
Why do locked-in patients still produce speech signals?
Locked-in syndrome often follows a stroke in the brain stem, which cuts the pathways that carry commands from the cortex to the muscles. The cortex itself keeps working, so the patient is fully aware and the brain still plans each word. Many patients can still move their eyes up and down, and some can blink. Those signals in the cortex are what implanted electrode arrays pick up.
How fast can brain implants turn signals into speech?
In 2021, Edward Chang's team at UCSF reported in the New England Journal of Medicine that a man paralyzed by a brain stem stroke could spell out words from a 50-word vocabulary at about 15 words per minute. In a 2023 paper in Nature, a UCSF team decoded speech at about 78 words per minute for a woman with paralysis and animated an avatar to speak. Natural conversation runs at about 150 words per minute. The recordings in both studies came from electrodes placed on the brain's surface.
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