The prototype is among the earliest fruits of a widespread effort to embrace technology that was once science fiction — and in some ways still is. Driven by recent investments from the United States government and by the herd mentality that so often characterizes the tech world, a number of a start-ups and bigger companies like Facebook are working on ways to mentally control machines. They are also looking for smoother ways to use virtual reality technology. “Neurotechnology has become cool,” said Ed Boyden, a professor of biological engineering and brain and cognitive sciences at the M.I.T. Media Lab who advises one of those start-ups.