Before the prefrontal cortex evolved, another system in the brain may already have been directing attention, scientists have now revealed.
In a recent study published in Nature Communications, researchers at Johns Hopkins University have identified a small group of brainstem neurons that act as an internal attention filter.
When these neurons were deactivated in mice, the animals became highly distractible, mirroring one of the hallmark features of ADHD. Restoring activity in these neurons allowed the mice to regain normal focus.
An Old Problem With No Modern Answer
Selective spatial attention is the brain’s ability to focus on relevant information while ignoring distractions. This process allows a person to follow a conversation in a noisy environment or recognize a familiar face in a crowd. Conditions such as ADHD and autism also often involve disruptions in selective attention. For many years, scientists believed the prefrontal cortex was primarily responsible for this function.
However, this view does not fully explain how attention works across different species. Primates have a well-developed prefrontal cortex, while other vertebrates have a less pronounced version, yet birds, fish, and turtles still effectively focus their attention.
“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?” said lead author Ninad Kothari, a postdoctoral fellow in Johns Hopkins’ Department of Psychological and Brain Sciences. “We were able to identify an evolutionarily old region in the brainstem which affords this ability.”
A Filter Hiding in Plain Sight
The researchers traced this behavior to a network of inhibitory neurons located deep within the brainstem, a region found in all vertebrates. Mysore’s laboratory had previously studied this area in birds, frogs, and turtles before examining it in mice.
The team created an attention task, similar to those used in human studies, to test the role of these neurons. Mice viewed visual cues on a screen and earned rewards for responding to a target in front of them while ignoring a distracting signal off to the side. This test measured how well the mice could pick out important information and ignore distractions. The mice successfully performed the task until researchers deactivated the brainstem neurons, at which point they became easily distracted.
Ruling Out the Obvious Explanations
Before concluding that these neurons controlled attention, the researchers tested for other possible explanations, such as vision or motor problems, that might account for the mice’s behavior. However, the researchers ruled out these alternative explanations.
“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,” said senior author Shreesh Mysore, a neuroscientist who studies neural circuits tied to behavior. “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 attention deficit was not permanent. When the neurons were reactivated, the same mice that had previously been unable to ignore faint distractions were once again able to ignore even strong distractors.
“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,” 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.”
What This Could Mean for Human Brains
“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.”
If researchers find that these neurons function differently in people with attention disorders, the circuit could become a target for future therapies.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.