Researchers have found that a split-second decision to change an action once it has already started involves lightning-speed choreography in not just one brain region — as previously thought — but several.

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If you don't make your altered decision within 100 milliseconds, it may be too late to put it into action.

The discovery may improve our understanding of why older people are more prone to falls, as well as why addictive behaviors such as binge eating and drinking are hard to stop, says a team of neuroscientists from Johns Hopkins University in Baltimore, MD, who report their findings in the journal Neuron.

Every year in the United States, emergency departments treat around 2.8 million older people for fall injuries.

Falls can result in broken bones, and they can also cause hip fractures and head injuries. The total cost of medical care for fall injuries in the U.S. is $31 billion per year.

Consider what happens when we start putting a foot forward and then realize we need to change the action to avoid stepping on a patch of ice. If the decision to change the movement is made even just a few milliseconds too late, we cannot stop our foot from landing on the ice.

"We have to process all of these pieces of information quickly," explains senior study author Susan Courtney, a professor of psychological and brain sciences. "The question is," she ponders, "[w]hen we do succeed, how do we do that? What needs to happen in order for us to stop in time?"

'Stop signal task'

For their investigation, the researchers monitored the brains of human volunteers and a monkey as the subjects performed a computer-based "stop signal task." Psychologists and neuroscientists use this task to study response inhibition under laboratory conditions.

The stop signal task tests a person's ability to stop an action that has already started and measures the reaction time for doing it. In this case, the action was to focus on a certain shape on a screen during which time "go" or "stop" instructions would appear suddenly in the form of other color-coded shapes.

The researchers monitored what happened during the task — as represented by changes in eye movement — in two different ways. In the humans, they used functional MRI scans to see which areas of the brain lit up during different stages of the task.

In the monkey, they used an implanted electrode to monitor single-cell changes in the brain that occurred at the same eye movement points in the computer task for the human subjects.

Having a macro and a micro view of what was happening gave the team a better fix on how the different parts of the brain communicated with each other during response inhibition.

Several brain regions are involved

It has long been thought that only one brain system is involved when people change their minds about a planned behavior at the last minute.

But after mapping brain activity in humans and monkeys, Prof. Courtney and her team found that a split-second decision to change, stop, or reverse an action that is already underway involves very fast coordination between an area in the premotor cortex and two in the prefrontal cortex of the brain.

The team suggests that problems arise when something goes wrong with the communication between these areas.

"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."

Prof. Susan Courtney

As well as discovering that more than one part of the brain is involved, the team also found that timing was critical to the success of a split-second change of mind to stop or change an action.

Timing is critical

For example, imagine that you are approaching traffic lights as they turn yellow but you decide that you are going to put your foot down and speed through. But, just as you make the decision to go, you spot a police car and decide to stop instead.

The researchers found that if the decision to change is made within 100 milliseconds, the chances are that the change of mind will succeed in altering the original course of action.

But, if it takes at least 200 milliseconds, the chances are that the original — and not the altered — plan will succeed because the signal to move the foot is already traveling to the muscles.

Prof. Courtney explains that knowing what happens in the brain when we try to stop a planned behavior could help us to better understand thought processes and decisions in addiction. For example, "The sooner I can turn off the plan to drink," Prof. Courtney explains, "the less likely I'll carry out the plan. It's very relevant."