As patients learn to use newly outfitted prostheses, their ability to control the movement of these devices usually improves over time. Researchers at UC Berkeley have discovered that the brain actually develops memory circuits, coding for the new prostheses, that are activated later when using the device. In effect, the brain accepts the artificial limbs as its own.
In this study, [published] July 21 in the open-access journal PLoS Biology, macaque monkeys using brain signals learned how to move a computer cursor to various targets. What the researchers learned was that the brain could develop a mental map of a solution to achieve the task with high proficiency, and that it adhered to that neural pattern without deviation, much like a driver sticks to a given route commuting to work.
The study, conducted by scientists at the University of California, Berkeley, addresses a fundamental question about whether the brain can establish a stable, neural map of a motor task to make control of an artificial limb more intuitive.
To demonstrate this, Carmena and Karunesh Ganguly, a post-doctoral fellow in Carmena’s laboratory, used a mathematical model, or “decoder,” that remained static during the length of the study, and they paired it with a stable group of neurons in the brain. The decoder, analogous to a simplified spinal cord, translated the signals from the brain’s motor cortex into movement of the cursor.
It took about four to five days of practice for the monkeys to master precise control of the cursor. Once they did, they completed the task easily and quickly for the next two weeks.
As the tasks were being completed, the researches were able to monitor the changes in activity of individual neurons involved in controlling the cursor. They could tell which cells were firing when the cursor moved in specific directions. The researchers noticed that when the animals became proficient at the task, the neural patterns involved in the “solution” stabilized.
“The solution adopted is what the brain returned to repeatedly,” said Carmena.
That stability is one of three major features scientists associate with motor memory, and it is all too familiar to music teachers and athletic coaches who try to help their students “unlearn” improper form or techniques, as once a motor memory has been consolidated, it can be difficult to change.
Other characteristics of motor memory include the ability for it to be rapidly recalled upon demand and its resistance to interference when new skills are learned. All three elements were demonstrated in the UC Berkeley study.
In the weeks after they achieved proficiency, the primates exhibited rapid recall by immediately completing their learned task on the first try. “They did it from the get-go; there was no need to retrain them,” said Carmena.
Press release: Brain can develop motor memory for prosthetics, study finds…
Image: In experiments that involved moving a cursor from a central starting point to a nearby target, researchers found that the brain is capable of creating a stable mental representation of a disembodied device.
Article in PLoS Biology: Emergence of a Stable Cortical Map for Neuroprosthetic Control
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