A tiny cluster of brain cells in the brainstem may act as the brain’s own focus filter, and when researchers switched those cells off in mice, the animals became unusually distractible.
The study, led by researchers at Johns Hopkins University and published in Nature Communications, identified inhibitory neurons in an evolutionarily old part of the brain that help animals ignore distractions and focus on the most important information.
“A hallmark of ADHD is that even faint distractors draw attention away, and that’s exactly what we see here when these neurons are silenced,” senior author Shreesh Mysore said. “But the very next day, when the neurons are turned back on, the same animal can ignore distractors again, even very strong ones.”
The researchers said the finding points to a brain system shared across vertebrates, including humans.
For years, scientists thought attention was controlled mainly by the prefrontal cortex, a brain region especially developed in humans and other primates. But the researchers said that view does not explain how many other animals focus attention without a highly developed prefrontal cortex.
“If we really go back in evolution, for hundreds of millions of years, birds have had this ability, fish have had this ability. And they do not typically have a highly developed prefrontal cortex, so how does the brain solve this problem?” lead author Ninad Kothari said. “We were able to identify an evolutionarily old region in the brainstem which affords this ability.”
The team built an attention task for mice based on tests used in people. Mice watched visual cues on a screen and got a reward when they responded correctly to information displayed directly in front of them while ignoring distracting cues off to the side.
The mice could do the task until researchers temporarily switched off the brainstem neurons.
“When we inactivate these neurons, the mice become hyper distractable,” Kothari said.
The researchers then tested if the mice were struggling because of problems with vision or movement. They ruled those out and found the animals had specifically lost the ability to compare competing information and focus on the most relevant signal.
“The only thing impaired was their ability to take the competing pieces of information, compare them, and pay attention to the location with the most important information,” Mysore said. “This part of the brain is like an attentional selection engine. It helps solve the question: ‘What is most important information I should pay attention to right now?’”
The researchers said future studies may look at these neurons in people with ADHD and autism and test if the cells work differently in those conditions.
“All the evidence to date suggests that these neurons exist in humans too,” Mysore said. “But are they responsible for selective spatial attention in humans? An exciting hypothesis is that they play a crucial role.”
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