This is why you can't always stop something you've already started doing

WHEN WE TRY to stop a body movement at the last second, perhaps to keep ourselves from stepping on what we just realised was ice, we can’t always do it – and scientists have found out why.

Stopping a planned behaviour requires extremely fast choreography between several distinct areas of the brain, researchers at John Hopkins University have found.

If we change our mind about taking that step even a few milliseconds after the original “go” message that has been sent to our muscles, we simply cannot stop our feet.

“We have to process all of these pieces of information quickly,” author Susan Courtney said.

“The question is: When we do succeed, how do we do that? What needs to happen in order for us to stop in time?”

Scientists had believed only one brain region was active when people changed plans. However, the findings of Courtney’s team suggest it takes a lightning-fast interaction between two areas of the prefrontal cortex and another in the pre-motor cortex stop, reverse or otherwise change a plan already in progress.

There’s even another brain area that continues to process what we should have done if we are unable to stop. Courtney jokingly called it the “oops” area.

In addition to all three areas of the brain communicating successfully, the key to being able to stop, the researchers found, is timing.

The researchers used a practical example to explain their discovery.

Suppose you’re driving and approaching an intersection when the light turns yellow. You decide to accelerate and speed through. But just after you send that decision to the part of the brain that will move your foot to hit the gas, you notice a police car and change your mind.

“Which plan is going to win?” author Kitty Xu said.

“The sooner you see the police car after deciding to go through the light, the better your chance of being able to move your foot to the break instead.”

By soon, Xu means milliseconds.

If you attempt to change your mind after 100 milliseconds or less, you most likely can. If it takes you 200 milliseconds or more – that’s less than a quarter of a second – you’re still going through with the original plan.

That’s because the original signal is already on its way to the muscles by then – past the point of no return, the researchers explained.

“If you’re already executing the plan when you see the police car you’re going to go through the light,” Xu said.

The research

The team devised a near-identical computer task for human and non-human subjects (monkeys).

While having their brain actively monitored, both the people and one monkey saw one of two shapes on the screen – one shape meant that blue means stop and yellow means go, the other shape meant the opposite.

A black circle would then appear and participants would try to move their eyes to look at it quickly. But then a blue or yellow dot might appear, and subjects would have to stop or continue their planned eye movement.

The researchers were able to observe what happened across the full brain with the human fMRI results, while electrodes implanted in the monkey’s brain measured single cells. From here, the researchers were able to get a more holistic view of how the prefrontal cortex and pre-motor cortex communicate with each other.

“We know people with damage to these parts of the brain have trouble changing plans or inhibiting actions. We know as we age, our brain slows down and it takes us longer to find words or to try to make these split-second plan changes. It could be part of the reason why old people fall,” Courtney said.

Knowing more about how the brain can stop an intended activity could also be revealing for those dealing with addictions, she added.

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