He and a team of researchers thought they might be able to do this by stimulating memory circuits in the brain. They tried the approach with a group of patients who had severe epilepsy.
These people already had electrodes temporarily implanted in their brains as part of their treatment. And that gave the scientists a way to deliver tiny pulses of electricity to areas deep in the brain.
Unfortunately, Kahana says, the stimulation made things worse.
"Not only were we not able to enhance memory," he says, "but we actually reliably and consistently impaired memory in those patients."
So the team began studying more epilepsy patients to figure out why. This time, though, they used the electrodes to monitor brain activity as the volunteers did memory tasks — and the scientists were able to identify electrical patterns that predicted whether a person was going to remember something or not.
Then, the team tried another stimulation experiment, and this one yielded a breakthrough.
"When memory was predicted to be poor," he explains, "brain stimulation enhanced memory, and when it was predicted to be good, brain stimulation impaired memory."
In other words, on a bad memory day, stimulation helped. On a good day, it hurt.
When stimulation was delivered to the right place at the right time, the researchers found, it could improve memory performance among the patients by as much as 50 percent.
The research was funded by the Defense Advanced Research Projects Agency or DARPA. Several years ago, DARPA launched an ambitious effort to help the troops who were returning from Iraq and Afghanistan with memory problems caused by traumatic brain injuries.
Justin Sanchez, who is in charge of the program, says Kahana's work shows DARPA's investment is paying off.
"When I first heard about these results, my mind was absolutely blown," Sanchez says. "It's like the impossible happened."
A medical device based on this research could eventually do for an injured brain what a prosthetic limb does for an injured body, he says. There's a long research road ahead — brain stimulation still has way to go, before it can be proved useful as a memory aid. But it's a start.
"Over 270,000 military personnel are living with traumatic brain injuries and the options for them are very few," Sanchez says. Now we can offer them some hope for the future."
And if the approach pans out, it might help people with a wide range of memory problems, Kahana suggests.
"Traumatic brain injury is certainly high on our list," he says. "But we're also thinking very much about Alzheimer's disease and the other dementias that affect memory."