An MIT study shows that the brain is predisposed for reactive and reflexive movements

When you cycle to the store you may have two very different reasons for swerving: a simple reflex when something whizzes in your path, or executive control when you see road signs indicating the correct path.

A new study by MIT neuroscientists shows how the brain is wired for both, monitoring the specific circuits involved and their effect on visually provoked actions.

The research, published in Nature Communications, demonstrates in mice that neurons in the anterior cingulate cortex (ACC) area of ​​the prefrontal cortex, a region at the front of the brain associated with understanding rules and implementing plans, project connections into an evolutionarily older region called superior colliculus (SC).

The SC executes basic commands responsive, thoughtful The key conclusion of the study is that the purpose of the CCA’s links with the supervisory board is to bypass the supervisory board when executive control is needed.

ACC provides the inhibitory control of this ancient structure. This inhibitory control is a dynamic entity that depends on the activity and its rules. This is how a reflex is modulated by cortical control “.

Mriganka Sur, senior author of the study, professor of neuroscience at Newton, department of brain and cognitive sciences at MIT, Picower Institute for Learning and Memory

Lead author Rafiq Huda, assistant professor of cell biology and neuroscience at Rutgers University and a former postdoc in the Sur lab, added that by looking at specific circuits between the ACC and both the SC and the visual cortex (VC), the researchers could resolve uncertainty about how the cortex regulates the most basic brain regions during decision making.

“There has been an ongoing debate about what exactly the role of the cortex is in sensorimotor decisions,” Huda said. “We were able to provide some answers by looking at the level of the different projection pathways of the ACC, which would not have been possible by looking at all ACCs at the same time. Our work provides evidence of the possibility that inhibitory control of subcortical structures such as ‘SC is a unifying principle for how the ACC, and the prefrontal cortex in general, modulates decision-making behavior. “

Sense and rotation

To make their findings, the team first traced the circuits in and out of the ACC from both the VC and the SC, confirming that the ACC was in a prime position to integrate and process information about what mice have seen and what to do about it. During the study, they chose to focus on these structures on the left side of the brain.

After tracing these ACC-SC and ACC-VC circuits on the left side, the team then trained the mice to play a video game that required both sensation (seeing a cue on one side or the other of the screen) and action (spin a trackball to move the cue).

A group of mice had to move the cue inward, towards the center of the screen. The other group had to move the cue outward towards the edge of the screen. In this way, the signals could be visually on both sides and different groups of mice had to move them according to different rules.

As the mice worked, the scientists observed the activity of neurons in various regions to learn how they responded during each activity. Then the researchers manipulated the activity of neurons using optogenetics, a technique in which cells are genetically engineered to become controllable by flashes of light.

These manipulations allowed scientists to see how inhibition of neural activity within and between regions would change behavior.

Under natural conditions, the SC would reflexively direct the movement of the mouse’s head, such as rotating towards a stimulus to center it in view. But the scientists had to keep their heads still to make their observations, so they devised a way for the mice to guide the screen stimulus with their paws on a trackball. In the paper, they show that these two actions are equivalent for mice to move a signal in their field of view.

Optogenetic inactivation of the circuits between ACC and VC on the left side of the brain demonstrated that the ACC-VC connection was essential for mice to process signals on the right side of their visual field. This was equally true for both groups, regardless of how they should have moved a signal when they saw it.

The manipulations involving the SC turned out to be particularly intriguing.

In the group of mice that saw a stimulus on the right and were supposed to shift the signal inward to the center of the screen, when the scientists turned off the neurons inside the left CS, they found that the mice struggled compared to the mice. not manipulated. In other words, under normal conditions, the left CS helped move a stimulus from the right side to the center of the visual field.

When the scientists turned off the input from the ACC to the SC, the mice performed the task correctly more often than the non-manipulated mice. When the same mice saw a stimulus on the left and had to move it inward, they did the wrong task more often.

The job of the ACC inputs, it seems, was to ignore the tilt of the SC. When the override was disabled, the SC’s preference for moving a right-hand cue to the center was deselected. But the mouse’s ability to move a left stimulus to the center was undermined.

“These findings suggest that the SC and ACC-SC pathway facilitates opposite actions,” the authors wrote. “Importantly, these findings also suggest that the ACC-SC pathway does this by modulating the innate response bias of the SC.”

The scientists also tested the effect of ACC-SC inactivation in the second group of mice, whose job was to move the signal outward. There they saw that inactivation increased incorrect responses in tests with right signals.

This result makes sense in the context of the rules that prevail over Reflex. If the ingrained reflex in the left hemispheric SC is to bring a right hand signal into the middle of the field of view (by turning the head to the right), then only a working ACC-SC override could force it to successfully shift the signal past the right , and therefore farther to the periphery of the visual field, when the rule of the task requires it.

Sur said the findings accentuate the importance of the prefrontal cortex (in this case, the ACC in particular) in equipping mammals with the intelligence to follow rules rather than reflexes when needed. It also suggests that developmental deficits or lesions in the ACC could contribute to psychiatric disorders.

“Understanding the role of the prefrontal cortex, or even a segment, is critical to understanding how executive control may, or may not, develop in dysfunctional conditions,” Sur said.