Brain Circuit

Brain Circuit; imagine trying to focus on a friend’s voice at a noisy party, or blocking out; but 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 you can focus on your chosen input.

MIT neuroscientists have now identified a brain circuit that helps us to do just that. The circuit they identify, which is controlled by the prefrontal cortex; but filters out unwanted background noise or other distracting sensory stimuli. When this circuit is engage; the prefrontal cortex selectively suppresses sensory input as it flows into the thalamus; but the site where most sensory information enters the brain.

Hypersensitivity to noise

“This is a fundamental operation that cleans up all the signals that come in, in a goal-directed way,” says Michael Halassa; an assistant professor of brain and cognitive sciences; a member of MIT’s McGovern Institute for Brain Research, and the senior author of the study. The researchers are now exploring whether impairments of this circuit may be involved in the hypersensitivity to noise and other stimuli that is often seen in people with autism.

Miho Nakajima, an MIT postdoc, is the lead author of the paper; which appears in the June 12 issue of Neuron. Research scientist L. Ian Schmitt is also an author of the paper.

Shifting attention

Our brains are constantly bombard with sensory information; and we are able to tune out much of it automatically, without even realizing it. Other distractions that are more intrusive, such as your seatmate’s phone conversation, require a conscious effort to suppress.

In a 2015 paper, Halassa and his colleagues explore how attention can be consciously shifted; between different types of sensory input; by training mice to switch their focus between a visual and auditory cue. They found that during this task, mice suppress the competing sensory input; allowing them to focus on the cue that will earn them a reward.

Focusing on sound cues

This process appeared to originate in the prefrontal cortex (PFC); which is critical for complex cognitive behavior such as planning and decision-making. The researchers also found that a part of the thalamus that processes vision; was inhibited when the animals were focusing on sound cues. However, there are no direct physical connections from the prefrontal cortex to the sensory thalamus, so it was unclear exactly how the PFC was exerting this control, Halassa says.

In the new study, the researchers again trained mice to switch their attention between visual and auditory stimuli; then mapped the brain connections that were involved. They first examined the outputs of the PFC that were essential for this task; by systematically inhibiting PFC projection terminals in every target. This allowed them to discover that the PFC connection to a brain region known as the striatum; but is necessary to suppress visual input when the animals are paying attention to the auditory cue.