People usually only have to learn how to ride a bicycle once; and once a person learns how, they can get on and start riding even if they haven't in many years. It's easier to remember motor skills than facts or details about names or faces because motor memories are encoded in the brain in a different way. Now researchers have observed those memories forming in a mouse model. The findings, which have been reported in Neuron, can help explain why these memories are so persistent. They could also offer new insights into neurodegenerative movement disorders such as Parkinson's disease.
"We think motor memory is unique," said corresponding study author Jun Ding, an associate professor at Stanford University. Ding noted that some Alzheimer's disease patients who had been musicians play music well, even though they cannot recall their own families. "Clearly, there's a huge difference in the way that motor memories are formed."
It's thought that patterns in neural activity are involved in memory formation. These patterns may involve neural networks of hundreds or even thousands of cells in various parts of the brain. In this work, the researchers were interested in learning more about how that neural activity arises when a new skill is being learned.
In this study, the investigators trained mice to grab food pellets by sticking their paws through a little slot. Scientists also used genetic tools to engineer mice so the neural activity in the motor cortex could be observed. In those mice, neurons that were active during learning were fluorescent. If the mice would use those same neurons in recall later on, they would be identifiable.
The trained mice were tested on their skill several weeks later. The mice were able to rememberer the food-grabbing skill. The researchers also found that the same neurons that were active during learning were also active while the mice recalled the skill; the study authors noted that they saw the memory form.
With two-photon microscopy, the researchers saw neurons reprogram themselves while mice learned. In the motor cortex, neurons gained new inputs at the synapses, and also generated new connections in a part of the brain called the dorsolateral striatum. This connection enables the neurons in the motor cortex that learned the skill to exert control over movement.
Motor memories are also thought to be redundant. The researchers suggested that when learned skills are repeated, new connections are built by the same cells, refining the neural network, and the skill. Eventually, it can be so refined as to seem automatic.
Ding noted that there may be many influences on the persistence of memory, however. Repetition is one reason, but rewards, such as those that trigger the release of dopamine, may also play a role.
Follow up work is now planned to investigate whether a blockage in motor memory is related to Parkinson's disease. Patients might be able to improve movement through repetition and practice of motor skills, for example. However, if the neural pathways are destroyed by Parkinson's, another approach would be needed.
"Obviously, we're still a long way from a cure, but understanding how motor skills form is critical if we want to understand why they're disrupted by disease," said Ding.
Sources: Stanford University, Neuron
