Neuroscientists establish a brain circuit that filters out unwanted sensory inputs

Imagine that you are trying to focus on a friend's voice at a loud party or that you are blocking the phone conversation of the person sitting next to you on the bus while you try to read. Both of these tasks require your brain to somehow suppress the distracting signal so that you can focus on the input you have chosen.

MIT neuroscientists have now identified a brain circuit that helps us achieve this. The identified circuit, controlled by the prefrontal cortex, filters out unwanted background noises or other annoying sensory stimuli. When this circuit is activated, the prefrontal cortex selectively suppresses sensory inputs as it flows into the thalamus, the site where most sensory information enters the brain.

"This is a fundamental operation that eliminates all the signals that arrive, in a targeted way," says Michael Halassa, assistant professor of brain and cognitive science, a member of MIT's McGovern Brain Research Institute. author of the study.

Researchers are currently studying whether impairments in this circuit might be involved in the hypersensitivity to noise and other stimuli often seen in people with autism.

Miho Nakajima, MIT's postdoc, is the lead author of the newspaper, which appears in the June 12 issue of neuron. Researcher L. Ian Schmitt is also the author of the article.

Changing attention

Our brain is constantly bombarded with sensory information and we are able to listen to it largely automatically, without even realizing it. Other more intrusive distractions, such as the telephone conversation of your mate companion, require a conscious effort of suppression.

In a 2015 article, Halassa and colleagues explored how attention can be deliberately shifted between different types of sensory information, causing mice to switch their attention between a visual and auditory signal. They found that during this task, the mice suppress the concurrent sensory input, allowing them to focus on the signal that will earn them a reward.

This process appears to have originated in the prefrontal cortex (PFC), which is essential for complex cognitive behaviors such as planning and decision making. The researchers also found that part of the thalamus that treats vision was inhibited when animals focused on sound signals. However, there is no direct physical connection between the prefrontal cortex and the sensory thalamus, so it was difficult to know exactly how the PFC exercised this control, Halassa says.

In the new study, researchers again trained mice to change their attention between visual and auditory stimuli, and then mapped the brain connections involved. First, they examined the PFC outputs that are essential to this task, by systematically inhibiting PFC projection terminals in each target. This allowed them to discover that the connection of the PFC to a region of the brain known as the striatum is necessary to suppress the visual inputs when the animals are attentive to the auditory signal.

Another mapping revealed that the striatum then sends information to a region called globus pallidus, which is part of the basal ganglia. The basal ganglia then suppress the activity in the part of the thalamus that processes the visual information.

Using a similar experimental setup, the researchers also identified a parallel circuit that removes auditory data when animals pay attention to the visual signal. In this case, the circuit traverses parts of the striatum and thalamus associated with sound processing rather than vision.

The findings offer one of the first evidence that basal ganglia, which are known to be essential to the planning movement, also play a role in controlling attention, Halassa says.

"What we understood here is that the connection between the PFC and the sensory processing at this level passes through the basal ganglia and, in this sense, that the basal ganglia influence the control of sensory processing" he explains. "We now have a very clear idea of ​​how basal ganglia may be involved in purely attentional processes that have nothing to do with motor preparation."

Sensitivity to noise

The researchers also discovered that the same circuits are used not only to switch between different types of sensory inputs such as visual and auditory stimuli, but also to suppress annoying inputs in the same direction, for example to block noises background while focusing on the voices.

The team also showed that when the animals are warned that the task is going to be noisy, their performance really improves because they use this circuit to focus their attention.

The Halassa lab is currently conducting similar experiments on mice genetically engineered to develop symptoms similar to those of people with autism. A common feature of autism spectrum disorders is the hypersensitivity to noise, which could be caused by an alteration of this brain circuit, says Halassa. He is currently studying whether strengthening the activity of this circuit could reduce the sensitivity to noise.

"Noise control is something that autistic patients have constant problems with," he says. "There are now several nodes in the lane on which we can look to try to understand that."

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