We can pick out a conversation in a loud room, amid the rise and fall of other voices or the hum of an air conditioner. We can spot a set of keys in a sea of clutter, or register a raccoon darting into the path of our onrushing car. Somehow, even with massive amounts of information flooding our senses, we’re able to focus on what’s important and act on it.

Attentional processes are the brain’s way of shining a searchlight on relevant stimuli and filtering out the rest. Neuroscientists want to determine the circuits that aim and power that searchlight. For decades, their studies have revolved around the cortex, the folded structure on the outside of the brain commonly associated with intelligence and higher-order cognition. It’s become clear that activity in the cortex boosts sensory processing to enhance features of interest.

But now, some researchers are trying a different approach, studying how the brain suppresses information rather than how it augments it. Perhaps more importantly, they’ve found that this process involves more ancient regions much deeper in the brain — regions not often considered when it comes to attention.

By doing so, scientists have also inadvertently started to take baby steps toward a better understanding of how body and mind — through automatic sensory experiences, physical movements and higher-level consciousness — are deeply and inextricably intertwined.

Hunting for Circuits

For a long time, because attention seemed so intricately tied up with consciousness and other complex functions, scientists assumed that it was first and foremost a cortical phenomenon. A major departure from that line of thinking came in 1984, when Francis Crick, known for his work on the structure of DNA, proposed that the attentional searchlight was controlled by a region deep in the brain called the thalamus, parts of which receive input from sensory domains and feed information to the cortex. He developed a theory in which the sensory thalamus acted not just as a relay station, but also as a gatekeeper — not just a bridge, but a sieve — staunching some of the flow of data to establish a certain level of focus.

But decades passed, and attempts to identify an actual mechanism proved less than fruitful — not least because of how enormously difficult it is to establish methods for studying attention in lab animals.

That didn’t stop Michael Halassa, a neuroscientist at the McGovern Institute for Brain Research at the Massachusetts Institute of Technology. He wanted to determine exactly how sensory inputs got filtered before information reached the cortex, to pin down the precise circuit that Crick’s work implied would be there.

He was drawn to a thin layer of inhibitory neurons called the thalamic reticular nucleus (TRN), which wraps around the rest of the thalamus like a shell. By the time Halassa was a postdoctoral researcher, he had already found a coarse level of gating in that brain area: The TRN seemed to let sensory inputs through when an animal was awake and attentive to something in its environment, but it suppressed them when the animal was asleep.

In 2015, Halassa and his colleagues discovered another, finer level of gating that further implicated the TRN as part of Crick’s long-sought circuit — this time involving how animals select what to focus on when their attention is divided among different senses. In the study, the researchers used mice trained to run as directed by flashing lights and sweeping audio tones. They then simultaneously presented the animals with conflicting commands from the lights and tones, but also cued them about which signal to disregard. The mice’s responses showed how effectively they were focusing their attention. Throughout the task, the researchers used well-established techniques to shut off activity in various brain regions to see what interfered with the animals’ performance.

As expected, the prefrontal cortex, which issues high-level commands to other parts of the brain, was crucial. But the team also observed that if a trial required the mice to attend to vision, turning on neurons in the visual TRN interfered with their performance. And when those neurons were silenced, the mice had more difficulty paying attention to sound. In effect, the network was turning the knobs on inhibitory processes, not excitatory ones, with the TRN inhibiting information that the prefrontal cortex deemed distracting. If the mouse needed to prioritize auditory information, the prefrontal cortex told the visual TRN to increase its activity to suppress the visual thalamus — stripping away irrelevant visual data.

The attentional searchlight metaphor was backward: The brain wasn’t brightening the light on stimuli of interest; it was lowering the lights on everything else.

Despite the success of the study, the researchers recognized a problem. They had confirmed Crick’s hunch: The prefrontal cortex controls a filter on incoming sensory information in the thalamus. But the prefrontal cortex doesn’t have any direct connections to the sensory portions of the TRN. Some part of the circuit was missing.

Until now. Halassa and his colleagues have finally put the rest of the pieces in place, and the results reveal much about how we should be approaching the study of attention.

Obscuring, Dimming, Blinking

With tasks similar to those they’d used in 2015, the team probed the functional effects of various brain regions on one another, as well as the neuronal connections between them. The full circuit, they found, goes from the prefrontal cortex to a much deeper structure called the basal ganglia (often associated with motor control and a host of other functions), then to the TRN and the thalamus, before finally going back up to higher cortical regions. So, for instance, as visual information passes from the eye to the visual thalamus, it can get intercepted almost immediately if it’s not relevant to the given task. The basal ganglia can step in and activate the visual TRN to screen out the extraneous stimuli, in keeping with the prefrontal cortex’s directive.

“It’s an interesting feedback pathway, which I don’t think has been described before,” said Richard Krauzlis, a neuroscientist at the National Eye Institute at the National Institutes of Health in Maryland who did not participate in this study.

Furthermore, the researchers found that the mechanism doesn’t just filter out one sense to raise awareness of another: It filters information within a single sense too. When the mice were cued to pay attention to certain sounds, the TRN helped to suppress irrelevant background noise within the auditory signal. The effects on sensory processing “can be much more precise than just suppressing the whole thalamic region for one sensory modality, which is a rather blunt form of suppression,” said Duje Tadin, a neuroscientist at the University of Rochester.

“We often neglect how we get rid of the things that are less important,” he added. “And oftentimes, I think that’s a more efficient way of dealing with information.” If you’re in a noisy room, you can try raising your voice to be heard — or you can try to eliminate the source of the noise. (Tadin studies this kind of background suppression in other processes that happen more quickly and automatically than selective attention.)

Halassa’s findings indicate that the brain casts extraneous perceptions aside earlier than expected. “What’s interesting,” said Ian Fiebelkorn, a cognitive neuroscientist at Princeton University, is that “filtering is starting at that very first step, before the information even reaches the visual cortex.”