Princeton University researchers have identified how the brain incorporates information about new situations into our existing goals.
Using brain scans of human volunteers, researchers at the Princeton Neuroscience Institute (PNI) found that updating goals takes place in the prefrontal cortex, and appears to involve signals associated with the brain chemical dopamine. When the researchers used a magnetic pulse to interrupt activity in that region of the brain, the volunteers were unable to switch to a new task when playing a game requiring them to push a button after seeing letters pop up on a screen.
“We have found a fundamental mechanism that contributes to the brain’s ability to concentrate on one task and then flexibly switch to another task,” said Jonathan Cohen, Ph.D., co-director of PNI. “Impairments in this system are central to many critical disorders of cognitive function, such as those observed in schizophrenia and obsessive-compulsive disorder.”
Cohen explained that existing research has shown that when new information is used to update a task, behavior or goal, this information is held in a type of short-term memory storage known as working memory. Researchers, however, did not know what mechanisms were involved in updating this information.
To find out, Cohen’s research team used functional magnetic resonance imaging (fMRI) to scan the brains of volunteers playing a game in which they pressed a specific button depending on a particular visual cue.
If the volunteer saw the letter A prior to seeing the letter X, he or she had to press button 1. But if the volunteer saw the letter B prior to seeing the X, the participant had to press button 2. The A and B served as the new information that the person used to update their goal of deciding which button to press. Another version of the task required the same participants to press button 1 upon seeing an X regardless of whether an A or B was shown.
With the fMRI, the researchers detected activity in the right prefrontal cortex during tasks that required the participants to remember whether they saw an A or a B before pressing the correct button, but not during tasks where the participant only had to press the button when prompted by an X.
These results confirmed findings from a previous study led by Cohen that used another scanning method to gauge the timing of brain activity. Using electroencephalography (EEG), the researchers found that the prefrontal cortex showed a spike in brain electrical activity 150 milliseconds after the participant viewed the letter A or B.
For the new study, the researchers demonstrated that the prefrontal cortex is indeed the area of the brain involved with updating working memory by sending a short magnetic pulse to the region. This pulse disrupted cortex activity at the precise time — as revealed by the EEG — the researchers suspected that the prefrontal cortex was updating working memory.
When the researchers introduced the pulse to the right side of prefrontal cortex about 150 milliseconds after the volunteers saw the A or B, the participants were unable to press the correct buttons, Cohen said.
“We predicted that if the pulse was delivered to the part of the right prefrontal cortex observed using fMRI, and at the time when the brain is updating its information as revealed by EEG, then the subject would not retain the information about A and B, interfering with his or her performance on the button-pushing task,” Cohen said.
Finally, the researchers explored their theory that dopamine — a naturally occurring chemical involved in motivation and reward — tags new information entering the prefrontal cortex as important for updating working memory and goals.
Cohen and his team imaged a brain region called the midbrain, which contains clusters of nerve cells called dopaminergic nuclei that are the source of most of the dopamine signals in the brain.
Using high-resolution fMRI, the researchers probed the activity of these dopamine-releasing cells in the brains of volunteers engaged in the game. The researchers found that the activity in these areas correlated with the activity in the right prefrontal cortex and with the ability of the volunteers to press the correct buttons.
“The remarkable part was that the dopamine signals correlated both with the behavior of our volunteers and their brain activity in the prefrontal cortex,” Cohen said. “This constellation of findings provides strong evidence that the dopaminergic nuclei are enabling the prefrontal cortex to hold on to information that is relevant for updating behavior, but not information that isn’t.”
The study was published by the Proceedings of the National Academy of Sciences.
Source: Princeton University