“Brain injury patients would be the first [candidates],” says Song. Such injuries tend to affect specific regions of the brain. Injuries to the hippocampus would be easier to target than degenerative diseases like Alzheimer’s, which tend to involve damage across many regions of the brain.
“It seems possible to me that one day we could replace a hippocampus with something else,” says Jacobs. But he points out that it will be difficult to fully replicate a healthy hippocampus—the structure contains tens of millions of neurons. “It is a little hard to imagine how a handful of electrodes could be replacing the millions of neurons in the hippocampus,” he says.
The electrodes used in the study are around a millimeter wide, and all the volunteers had them implanted deep enough into the brain to reach the hippocampus—around 10 centimeters deep. They are fairly crude by modern research standards and are only able to record from around 40 to 100 neurons, says Song. Any memory prosthesis designed to treat memory disorders will require brain electrodes with hundreds of contact points, allowing them to record from, and stimulate, hundreds or thousands of neurons, he says.
Hampson, Song, and their colleagues haven’t yet worked out how the memory prosthesis might work in practice. It might not make sense to have the device running all the time, for example—there are plenty of life experiences, such as taking out the garbage, that people with memory disorders don’t need to remember. “Why waste the [brain] space?” Jacobs says.
Song thinks the prosthesis might be used alongside some sort of device that can tell whether the device needs to be running or not—perhaps by detecting when the brain needs to be in a ready-to-learn state.
And Song doesn’t yet know if a memory prosthesis should run overnight, either. It is thought that when we sleep, the hippocampus replays some of the memories encoded during the day, in order to consolidate them in other brain regions. Song and his colleagues don’t know if a memory prosthesis that replicates this replay would improve memory, or whether it’s a good idea to stimulate the hippocampus at all while a person is sleeping.
Either way, the prosthesis is still some way from clinical use, says Shapiro. “I think in principle it could work,” he says. “[But] we have a long way to go before we understand enough about memory to be able to use this sort of approach to replace hippocampal function.”