In a grainy black-and-white video shot at the Mayo Clinic in Minnesota, a patient sits in a hospital bed, his head wrapped in a bandage. He's trying to recall 12 words for a memory test but can only conjure three: whale, pit, zoo. After a pause, he gives up, sinking his head into his hands.

In a second video, he recites all 12 words without hesitation. "No kidding, you got all of them!" a researcher says. This time the patient had help, a prosthetic memory aid inserted into his brain.

Over the past five years, the U.S. Defense Advanced Research Projects Agency (Darpa) has invested $77 million to develop devices intended to restore the memory-generation capacity of people with traumatic brain injuries. Last year two groups conducting tests on humans published compelling results.