“Patients with brain damage would be the first [candidates]says Song. Such injuries tend to affect specific regions of the brain. Hippocampal injuries would be easier to target than degenerative diseases like Alzheimer’s, which tend to damage many areas of the brain.
“It seems possible to me that one day we could replace a seahorse 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’s a little hard to imagine how a handful of electrodes could replace the millions of neurons in the hippocampus,” he says.
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The electrodes used in the study are about a millimeter wide, and all of the volunteers implanted them deep enough in the brain to reach the hippocampus, about 10 centimeters deep. They’re pretty rudimentary by modern research standards and can only register about 40 to 100 neurons, Song says. Any memory prosthesis designed to treat memory impairment will require brain electrodes with hundreds of points of contact, allowing them to record and stimulate hundreds or thousands of neurons, he says.
Hampson, Song and their colleagues have yet to determine how the memory prosthesis might work in practice. It might not make sense for the device to work all the time, for example – there are a lot of life experiences, like taking out the trash, that people with memory impairments don’t have. no need to remember. “Why waste the [brain] space?” said Jacobs.
Song thinks the prosthesis could be used with some sort of device that can tell whether the device should work or not, perhaps by sensing when the brain needs to be in a ready-to-learn state.
And Song is also unsure if a memory prosthesis should work overnight. It is believed that when we sleep, the hippocampus replays some of the memories encoded during the day, in order to consolidate them in other regions of the brain. Song and his colleagues aren’t sure if a memory prosthesis that replicates this repetition would improve memory, or if it’s a good idea to stimulate the hippocampus while a person is sleeping.
Either way, the prosthesis is still a long way from clinical use, Shapiro says. “I think in principle it could work,” he says. “[But] we have a long way to go before we understand memory well enough to use this kind of approach to replace hippocampal function.