Imagine trying to focus on a friend's voice at a noisy party or blocking the phone conversation of the person sitting next to you on the bus while trying to read. Both of these tasks require that your brain somehow suppress the distracting signal so you can focus on your chosen input.
MIT neuroscientists have now identified a brain circuit that helps us do just that. The circuit they identified, which is controlled by the prefrontal cortex, filters out unwanted background noise or other distracting sensory stimuli. When this circuit is engaged, the prefrontal cortex selectively suppresses sensory input as it enters the thalamus, the place where most sensory information enters the brain.
"This is a basic operation that cleans up all signals entering a targeted way," said Michael Halassa, an assistant professor of brain and cognitive science, a member of the MIT McGovern Institute for Brain Research, and the senior author. of the study.
The researchers are now investigating whether deterioration of this circuit can be involved in the hypersensitivity to noise and other stimuli often seen in people with autism.
Miho Nakajima, a MIT postdoc, is the lead author of the paper, published on June 1
Our brains are constantly bombarded with sensory information, and we can automatically reconcile much of it without even realizing it. Other disturbances that are more urgent, such as your side buddy's phone call, require a deliberate effort to suppress.
In a paper from 2015, Halassa and his colleagues examined how careful attention can be shifted between different types of sensory input, by training mice to shift focus between a visual and auditory cue. They found that during this task, mice suppress the competing sensory input so they can focus on the queue that will serve them a reward.
This process appeared to originate from the prefrontal cortex (PFC), which is critical to complex cognitive behaviors such as planning and decision making. The researchers also found that part of the thalamus that treats eyesight was inhibited as the animals focused on sound sounds. However, there are no direct physical connections from the pre-frontal cortex to the sensory thalamus, so it was unclear how PFC exercised that control, Halassa says.
In the new study, the researchers again trained mice to shift attention between visual and auditory stimuli, so mapping the brain connections involved. They first examined PFC's output, which was essential to this task, systematically inhibiting PFC projection terminals in each target. This allowed them to find that the PFC compound to a brain group known as the striatum is needed to suppress visual entry when the animals are aware of the auditory cue.
Further mapping revealed that the striatum then sends input to a region called Globus pallidus, which is part of the basal ganglia. The basal ganglia then suppresses activity in the part of the thalamus that treats visual information.
Using a similar experimental setup, the researchers also identified a parallel circuit that suppresses auditory input when the animals are watching the angle of view. In that case, the circuit travels through parts of the striatum and the thalamus associated with the treatment of sound rather than vision.
The results provide some of the first evidence that the basal ganglia known to be essential for motion planning also play a role in controlling attention, Halassa says.
"What we have understood here is that the connection between PFC and sensory treatment at this level is mediated through the basal ganglia, and in that sense, the basal ganglia affects control of sensory treatment," he says. "We now have a very clear idea of how the basal ganglia can be involved in purely attentive processes that have nothing to do with engine production."
The researchers also found that the same circuits are used not only to switch between different types of sensory input, such as visual and auditory stimuli, but also to suppress distracting input within the same sense , such as blocking background noise while focusing on a person's voice.
The team also showed that when the animals become aware that the task gets noisy, their performance actually improves as they use this circuit to focus their attention.
Halassa's laboratory is now doing similar experiments in mice genetically engineered to develop symptoms similar to those of people with autism. A common feature of autism spectrum disorder is hypersensitivity to noise, which may be due to deterioration of this brain cycle, Halassa says. He now studies whether increasing the activity of this circuit can reduce the sensitivity to noise.
"Controlling noise is something that patients with autism have problems with all the time," he says. "Now there are several nodes in the path that we can start looking at to try to understand this."
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Neuroscientists track a brain circuit that filters unwanted sensory input (2019, June 12)
June 12, 2019
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