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Hitting the perfect tennis serve takes hours and hours of practice, but for scientists studying complex motor behaviors, there has always been a big unanswered question: What does the brain learn from those hours gone by. on the court? Is it just the timing required to hit the perfect serve, or is it the precise path along which to move the hand?
The answer, according to Harvard researchers, is both, but in separate circuits.
Bence Ölveczky, John L. Loeb associate professor of natural sciences, discovered that the brain uses two largely independent neural circuits to learn the temporal and spatial aspects of a motor skill. The study is described in a September 26 article in Neuron.
“What we are studying is the structure of learning motor skills,” said lveczky. “What we were able to show is that the brain breaks something complex into modules – in this case for synchronization and motor implementation – in order to take advantage of the hierarchical structure of the motor system, and it prints learning at different levels independently.
To find out how these independent circuits work, Lveczky and his colleagues turned to a creature well known for its ability to learn: the zebra finch. Small birds are regularly used in learning studies as each male learns to sing a unique song from his father.
In a series of experiments, Ölveczky’s team used traditional conditioning techniques to alter the rhythm of a bird’s song by speeding up or slowing down certain “syllables” of song. They could also change the activated vocal muscles and cause the bird to sing at a higher or lower pitch.
“But when you change the pitch of a syllable, the duration doesn’t change, and when you change the duration, the pitch doesn’t change,” said lveczky. “It appears that the neural circuits of the two characteristics are separate.”
Further evidence that the learning circuits of motor implementation and synchronization are distinct has emerged when researchers injured the basal ganglia of birds – the region of the brain long believed to play an essential role in the disease. learning to sing.
“The idea was that there was a circuit for learning song in general,” said lveczky. “We found that if we bind the basal ganglia and repeat the height change experiment, the bird could no longer use the information it got from our feedback to change its behavior – in other words, he couldn’t learn. “
Experiments aimed at altering the timing of birds, however, were equally effective, suggesting two distinct learning circuits – with only one involving the basal ganglia.
This independence and modularity is essential, said lveczky, because it allows different characteristics of a behavior to be changed independently if circumstances change. Parallel learning of different characteristics can also speed up the learning process and allow for the flexibility we see in birdsong and many human motor skills.
“If you learn something – it can be your tennis serve or any behavior – and you have to slow it down or speed it up to deal with a new eventuality, you don’t have to completely relearn the tennis serve. all. , you can just change the timing, and everything else will stay exactly the same.
“In fact, ‘slow practice’, a technique used by many piano and dance teachers, makes good use of this modularity,” said lveczky. “Students are first taught to slowly perform the movements of a piece. Once they learn it, all they have to do is find the right timing. The technique works because the two processes – driving the motor and timing – do not interfere with each other.
The hope among researchers, said Lveczky, is that a better understanding of how birds learn complex motor tasks such as singing unique songs will help shed new light on the neural foundations of learning in the ‘man.
“For us, it’s an inspiration to look at similar types of questions in mammals,” he said. “The flexibility with which we can alter the spatial and temporal structure of our motor output is similar to that of songbirds, but our understanding of how the mammalian brain engages the underlying learning process is not not as advanced as for songbirds. The intriguing parallels in circuitry and behavior, however, suggest a general principle of how the brain analyzes the process of learning motor skills.