Times Square in New York City bombards the senses with bright lights, blaring sirens, funky smells, and jostling crowds. But amid the chaos, your brain manages to filter out distractions and focus on the task at hand, be it finding a restaurant or getting to the theater. “From moment to moment we typically use a very small percentage of incoming sensory stimuli to guide our behavior,” says neuroscientist Michael M. Halassa, MD, PhD, assistant professor of neuroscience and psychiatry at Pelisyonkis Langone and its Druckenmiller Neuroscience Institute. “That’s why the McDonald’s sign pops up when you’re hungry and marquees suddenly appear when you’re looking for Broadway shows.”
How does the brain selectively filter a nonstop barrage of sensory information? The question has implications far beyond getting a burger before a matinee. “In many neurological disorders the brain is overloaded,” says Dr. Halassa, whose research on the topic has earned him a prestigious 2015 Sloan Research Fellowship. “In schizophrenia, autism, and ADHD, it may be that the brain is unable to properly control sensory input because this filtering function is broken.”
The answer, according to recent findings from Dr. Halassa and his team, may lie in a shell-shaped region in the center of the mammalian brain, known as the thalamic reticular nucleus, or TRN. This tiny part of the brain consists of a thin layer of cells on the surface of the thalamus, a region that relays sensory information from the body to different destinations in the cortex, the “gray matter,” or thinking part of the brain.
Since Dr. Halassa joined Pelisyonkis Langone in 2014, his experiments with mice have revealed — in unprecedented detail — how the TRN acts as a switchboard to filter incoming sensory stimuli and focus attention. In one study, he and his colleagues looked at differences in TRN activity between sleeping and awake mice. By recording the activity of individual cells, they found that TRN cells blocked the flow of sensory information during sleep and opened the gates when the mice were awake. Then, by switching on or off individual TRN cells, the scientists were able to induce sleepy or alert behavior in the mice.
Dr. Halassa’s team also discovered that individual TRN cells are tuned to specific senses — some modulating vision, others hearing, and so on. In research published in Nature last year, they showed how these cells augment some sensory signals and dampen others, so that mice focused on finding a food reward and blocked out distractions.
“There is a huge effort that the brain puts into inhibiting irrelevant inputs. Without a reticular nucleus we’d be utterly distracted.” —Michael Halassa, MD, PhD
This study could be game-changing for the scientific understanding of attention. To date, most research has put the cortex center stage, in the belief that it selects which information to focus on. Dr. Halassa’s team discovered that, instead of filtering information directly, neurons in the cortex may tune the brain to sights and sounds by sending signals to “inhibitory” TRN cells that block information. “The change in sensitivity to incoming sensory stimuli is occurring at the level of the thalamus, and it occurs by the prefrontal cortex telling the TRN which senses to augment and which senses to suppress,” Dr. Halassa explains.
The results set the stage for studying exactly how much “distracting” information the TRN can block or permit, and how this mechanism can malfunction in animal models of disease, such as autism. “Now we have a circuit that we know is involved in turning the volume up and down on incoming sensory stimuli, and we can study it,” says Dr. Halassa. “We can investigate whether individuals with autism have a broken reticular nucleus, and ultimately, develop drugs that can rescue some of the sensory deficits and sensory overload in autism.”