Some companies are working to move beyond that. Facebook, for example, is exploring methods for optically reading brain activity from outside the skull. Such a system would shine light into the brain to directly read chemical changes.

“What if you could type directly from your brain?” Regina Dugan of Facebook said this spring as she unveiled the company’s efforts to build this kind of optical interface. “It sounds impossible, but it’s closer than you may realize.” In a few years, she said, Facebook hopes to have a system that allows people to type with their thoughts five times faster than they now type using a smartphone keyboard.

That is well beyond the realm of current research, and a number of neuroscientists question whether it will ever be possible, arguing that such speed will only come with devices planted inside the skull.

Several start-ups are now working to do just that. But some, including a Silicon Valley start-up called Paradromics, hope to do this as a way of treating people with medical conditions like blindness, deafness and paralysis.