Flexibility demands influence motor cortex’s involvement in execution of motor sequences, rat study finds

The motor cortex is a part of the mammalian brain and is known to support the planning and control of voluntary body movements. Some past neuroscience studies, however, found that the motor cortex may not be necessary for executing some learned skills and motor sequences, raising the question of when and how it is recruited.

Researchers at Harvard University recently carried out a study on rats to address this question. Their paper, published in Nature Neuroscience, shows that the involvement of the motor cortex in motor sequence execution depends on the extent to which a given task requires flexibility.

“We previously found that motor cortex is not necessary to generate highly automatized learned skills or motor sequences,” Bence P. Ölveczky, senior author of the paper, told Medical Xpress.

“This was a departure from common wisdom, which assumed that learned motor skills are stored in the motor cortex. If this is the case, however, when is the motor cortex necessary for generating motor sequences?”

Building on their previous research, the researchers decided to explore whether and how the motor cortex is involved in the generation of sequences in response to sensory cues. They hypothesized that this brain region conveys information processed in cortical circuits (e.g., the meaning of a given stimulus) to subcortical motor centers in the brain.

To test this hypothesis, Ölveczky and his colleagues performed a series of experiments involving adult rats. These rats were trained to “play” a small piano with only three keys, producing sequences of three notes.

“There were two contexts: in one context, the rats only had to ever perform one specific sequence,” explained Ölveczky. “This is equivalent to you learning and overtraining on a single piano piece for weeks and months. You become really good, or ‘automatic,’ as we call it.”

In the second experimental condition, rats were still trained on this sequence, but they learned to generate other three-element sequences that changed with every trial. These sequences depended on instructive cues provided to the rats, which they had previously learned to interpret.

“The analogy here is to a piano player who reads sheet music and produces action sequences based on cues (i.e., musical notes on a sheet) that they have learned to interpret,” said Ölveczky.

“We then lesioned the motor cortex and found that animals that learned single overtrained sequences were unaffected, whereas the rats that had to interpret cues to get the sequence right were very much affected.”

These experiments yielded various interesting insights. Firstly, the researchers found that the rats without a motor cortex could still play the sequence they were overtrained on.

This confirmed the team’s previous findings, which suggested that the execution of motor sequences that have been practiced extensively, and thus became automatic, is not supported by the motor cortex. In contrast, the ability to generate new sequences in response to specific cues, which requires greater flexibility, appeared to be supported by the motor cortex.

Interestingly, the researchers found that the overtrained sequence, which was independent of the motor cortex when trained in isolation, became motor cortex dependent when trained alongside more flexible sequences.

“To keep with the pianist analogy, this is the case when, in the morning, you (over)train the one piece you will play for the concert, but in the evening you also play different tunes from sheet music,” said Ölveczky. “This is interesting to us because it suggests that the same behavior can be implemented in different brain circuits depending on how it is learned.”

Essentially, the researchers found that if a sequence is learned alongside other ones and supported by the same movements, it becomes reliant on the motor cortex. Nonetheless, if the specialized movements used to learn a specific sequence are not re-used in other contexts, the brain appears to consolidate this behavior in subcortical circuits.

“We believe this is possible because the progression of rote and highly stereotyped behaviors can be unambiguously defined in terms of past behavior,” said Ölveczky.

“All the brain needs to do is connect past action to future action. But if the mapping between past action and future action is ambiguous and depends on environmental cues, then the cortex gets involved. This has implications for how we practice and learn our own skills.”

The recent study by Ölveczky and his colleagues offers valuable insights that could inform future learning methods. Specifically, it suggests that overtraining on a single motor sequence could prevent learners from re-using the movements they learned in other contexts and to produce different behaviors.

“Avoiding this inflexibility might be why good piano teachers will never have you practice the same piece repeatedly, but instead interleave other exercises, like scales and etudes,” added Ölveczky. “In our next studies, we want to further explore the logic of how neural circuits generate learned skills under different conditions.”

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