In the present study, Northwestern Medicine scientists have discovered a neural mechanism for making rapid, fine adjustments in precise motor activity

The study, led by Lee Miller, professor of physiology at Northwestern University Feinberg School of Medicine, found that the brain's premotor cortex may use a "neural scratch pad" between it and the primary motor cortex to calculate fine adjustments in reaching plans.

"This kind of short-term motor learning appears to require structural changes within the brain, making it faster and more flexible," said Miller, also a professor of biomedical engineering at Northwestern's McCormick School of Engineering. The study will be published Oct. 18 in the journal Neuron.

Neuroscientists have been probing how the brain adapts motor behavior on short time scales for many years. Previous research has shown that it takes about 15 minutes for the brain to significantly adjust motor behavior, so scientists would expect to find a matching neural signature-decaying or changing activity over this time during which behavior is adjusted.

Activity in the Motor cortex

In the current study, the investigators widened their scope, measuring activity in both the motor cortex and the premotor cortex during reaching movements in primate models. They found that when the motion was pushed off course and had to be adjusted back, functional relations between neurons in the two areas remained unchanged. This seemed to rule out physical changes in the brain as the mechanism responsible for the adjustment.

However, parsing the signals in the premotor cortex to the motor cortex revealed two distinct sets. One set contained signals that appeared to control activity in the motor cortex, called the potent projection. The second set of keys was taken out on the way to the engine cortex, called null projection. This allows the brain to simulate the effects of different motor plans without ever performing the action, Miller said.

The second set of signals essentially canceled themselves out on the way to the motor cortex, called null projection. This allows the brain to simulate the effects of different motor plans without ever performing the action, Miller said.

"The brain can try different things in a simulation without screwing up what it's already doing," Miller said. "Then when it's got it figured out, it can send these modified instructions to the motor cortex through the potent projection," simulation without screwing up what it's already doing," Miller said. "Then when it's got it figured out, it can send these modified instructions to the motor cortex through the potent projection."

The time course of these changing computations in the null projection matched up with the missing 15-minute interval, according to Matt Perich, first author of the study and former graduate student in the Miller laboratory.

"This suggests that something is happening within the premotor cortex: the null projection may undertake its own type of computation or error processing, and it eventually tells the engine what to do," Perich said.

The fact that these short-term adjustments appear to require the physical change in the brain may simplify the development of brain-computer interfaces, literal connections to the brain that do things like control prosthetic limbs physical change in the brain may simplify the development of brain-computer interfaces, literal connections to the brain that do things like control prosthetic limbs.

"The structure is surprisingly stable," Perich said. "Now that we have a more fundamental understanding of what this process looks like, we hope to find ways of incorporating this into the interfaces." More investigation is required to fully understand how these null projections are used to adjust motor function, according to Miller.

"Our decoders have a good enough to look at that null activity and make sure they do not make the arm move or do something it's not supposed to be, since the brain would not have," Miller said.do, since the 

In another related study, published in  Nature Communications, Miller and colleagues found that when faced with uncertainty about how to move, the premotor cortex only prepares one completed "reach plan" at a time. This type of decision-making and planning OCCURS before the activity Reaches the engine cortex, suggesting a role for null projections beyond error correction.

"These are very powerful experimental and analytical approaches that we are beginning to take to this process of decision making, planning, and movement execution," Miller said.