When a task adds more steps, this circuit helps you notice

Life is full of processes to learn and then relearn when they become more elaborate. One day you log in to an app with just a password, then the next day you also need a code texted to you. One day you can just pop your favorite microwavable lunch into the oven for six straight minutes, but then the packaging changes and you have to cook it for three minutes, stir, and then heat it for three more. Our brains need a way to keep up. A new study by neuroscientists at The Picower Institute for Learning and Memory at MIT reveals some of the circuitry that helps a mammalian brain learn to add steps.

In Nature Communications the scientists report that when they changed the rules of a task, requiring rats to adjust from performing just one step to performing two, a pair of regions on the brain’s surface, or cortex, collaborated to update that understanding and change the rats’ behavior to fit the new regime. The anterior cingulate cortex (ACC) appeared to recognize when the rats weren’t doing enough and updated cells in the motor cortex (M2) to adjust the task behavior.

“I started this project about 7 or 8 years ago when I wanted to study decision making.” said Daigo Takeuchi, a researcher at the University of Tokyo who led the work as a postdoc at the RIKEN-MIT Laboratory for Neural Circuit Genetics at The Picower Institute directed by senior author and Picower Professor Susumu Tonegawa. “New studies were finding a role for M2. I wanted to study what upstream circuits were influencing this.”

Tripping up the second step

Takeuchi and Tonegawa traced neural circuit connections that led into M2 and found that many originated in the ACC. They began to see the ACC’s role in guiding M2’s sequential decisions when they instilled a genetic manipulation in ACC cells that allowed them to suppress their activity. This “chemogenetic” disabling of the ACC had a very specific effect. When the task rules changed so that instead of having to poke their snout into just one hole to gain a little reward, rats had to poke their nose into a sequence of two holes, the rodents with silenced ACCs took much longer to realize the rule change. Compared to rats with normal ACC activity, they failed for much longer to realize the second poke was necessary. Rats had no trouble, however, going from two steps back to just one, regardless of whether their ACC was silenced.

When the scientists chemogenetically silenced the ACC cells’ terminals in M2, they got the same results as silencing the ACC overall. They also silenced other areas of the cortex, but doing that didn’t affect the ability of the rats to notice and adjust to the rule switch. Together these manipulations confirmed that it was specifically the ACC’s connections with M2 that help the rats notice and adjust to the one-step-to-two-step change.

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