By Benedict Carey
Scientists have designed a brain implant that restored lost memory function and strengthened recall of new information in laboratory rats -- a crucial first step in the development of so-called neuroprosthetic devices to repair deficits from dementia, stroke and other brain injuries in humans.
Though still a long way from being tested in humans, the implant demonstrates for the first time that a cognitive function can be improved with a device that mimics the firing patterns of neurons. In recent years neuroscientists have developed implants that allow paralyzed people to move prosthetic limbs or a computer cursor, using their thoughts to activate the machines. In the new work, being published Friday, researchers at Wake Forest University and the University of Southern California used some of the same techniques to read neural activity. But they translated those signals internally, to improve brain function rather than to activate outside appendages.
"It's technically very impressive to pull something like this off, given our current level of technology," said Daryl Kipke, a professor of bioengineering at the University of Michigan who was not involved in the experiment. "We are just scratching the surface when it comes to interacting with the brain, but this experiment shows what's possible and the great potential of interacting with the brain in this way."
In a series of experiments, scientists at Wake Forest led by Sam A. Deadwyler trained rats to remember which of two identical levers to press to receive water; the animals first saw one of the two levers appear and then (after being distracted) had to remember to press the other lever to be rewarded. Repeated training on this task teaches rats the general rule, but in each trial the animal has to remember which lever appeared first, to inform the later choice.
The rats were implanted with a tiny array of electrodes, which threaded from the top of the head down into two neighboring pieces of the hippocampus, a structure that is crucial for forming these new memories, in rats as in humans. The two slivers of tissue, called CA1 and CA3, communicate with each other as the brain learns and stores new information. The device transmits these exchanges to a computer.
To test the effect of the implant, the researchers used a drug to shut down the activity of CA1. Without CA1 online, the rats could not remember which lever to push to get water. They remembered the rule -- push the opposite lever of the one that first appeared -- but not which they had seen first.
The researchers, having recorded the appropriate signal from CA1, simply replayed it, like a melody on a player piano -- and the animals remembered. The implant acted as if it were CA1, at least for this one task.
"Turn the switch on, the animal has the memory; turn it off and they don't: that's exactly how it worked," said Theodore W. Berger, a professor of engineering at U.S.C. and the lead author of the study, being published in The Journal of Neuroengineering and Rehabilitation. His co-authors were Robert E. Hampson and Anushka Goonawardena, along with Dr. Deadwyler, of Wake Forest, and Dong Song and Vasilis Z. Marmarelis of U.S.C.
In rats that did not receive the drug, new memories faded by about 40 percent after a long distraction period. But if the researchers amplified the corresponding CA1 signals using the implant, the memories eroded only about 10 percent in that time.
The authors said that with wireless technology and computer chips, the system could be easily fitted for human use. But there are a number of technical and theoretical obstacles. For one, the implant must first record a memory trace before playing it back or amplifying it; in patients with significant memory problems, those signals may be too weak. In addition, human memory is a rich, diverse neural process that involves many other brain areas, not just CA3 and CA1; implants in this area will be limited.
Still, some restored memories -- Where is the bathroom? Where are the pots and pans stored? -- could make a big difference in the lives of someone with dementia. "If you're caring for someone in the house, for example," Dr. Berger said, "it might be enough to keep the person out of the nursing home."