According to new research by UC San Francisco, mice use a unique long distance connection in their brains to explore new environments. This prompts them to pay more attention to the most important features of the environment. This link, which originated in the prefrontal cortex and stretches to the hippocamp, is evidence that the brain’s higher cognitive region refines operations occurring in distant brain parts.
“This circuit is an entry point to understanding how the brain allows prefrontal cortex exert top-down regulation over other parts of the mind,” Vikaas Sohal MD, PhD, senior author of the study, published April 28, 2022 in Cell. “It’s a long-range, inhibitory pathway that connects two brain regions. It’s something we haven’t seen before.”
The prefrontal cortex (PFC), also known as the “CEO” of the brain, controls executive functions such as attention, planning, and decision making. The hippocampus stores and processes spatial information. This helps us navigate the environment.
Ruchi Malik, PhD, the study’s principal author, stated that the new circuit helps to focus attention on the important things in the environment, and ignore other sensory stimuli.
It’s like the PFC is taking in all this sensory information and saying, “Hey, hippocamp, we’re in this particular context, so pay close attention to this particular information.”
Ruchi Malik, University of California, San Francisco Study Lead Author
She uses the parking lot as an example to show how the PFC can exert that kind of control over the hippocampus. Malik said that in order to remember where you parked, PFC would tell the PFC to instruct the hippocampus selectively to pay attention to landmarks. Then, when you return, they will recall and seek those landmarks.
Fine-tuning attention through inhibition of neurons
This circuit is unique because it focuses attention in a complex way. It increases and focuses activity in certain microcircuits by switching off signals that could otherwise slow down those microcircuits. The PFC sends a clear signal to the hippocampus telling it what to pay attention to. It also has a highly precise way to adjust that message as the environment changes.
The team demonstrated this by putting mice in small arenas for 10 minutes. There were small objects around. The mice would then inspect the objects for a few minutes before moving on. The researchers discovered that the brain activity of the mice was synchronized by looking at the signals.
Researchers were able to see the signal enhancements in the hippocampus when the mouse passed it again.
Malik said, “There was a dialogue taking place; the hippocampus was mapping out the locations of objects in the universe and the PFC were instructing the PFC on the relevance of each location.”
The team also discovered that data could indicate which neurons were firing at any given moment and identify where the mouse is at that moment. This confirmed that brain activity changes as the mouse investigates or approaches an object that the PFC deems important.
This suggests that, as the hippocampus maps the environment, it is also fine-tuning to produce certain patterns in neural activity when the prefrontal cortical cortex detects that the mouse is close to an important target like a new object.
ADHD and dementia may be related to brain circuit dysfunction.
The team would like to gain a better understanding of the role that this circuit may play in executive function and the consequences for it not being able to perform its duties effectively. Malik believes dysfunction in this pathway might be the cause of cognitive problems related to attention or memory such as dementia, ADHD, or psychiatric disorder.
The next step towards achieving this goal is to examine how the circuit affects behavior. This includes how it works during more complex activities like using information stored within working memory to determine which path to take to find a reward.
Malik believes it’s possible that this connection between the cognitive, higher-order part of the brain and the more ancient and universal wayfinding centre may have broad influence.
“To be able operate in complex environments, to look for food and rewards and then return, you need to be capable of paying attention to specific stimuli in order to arrange them in space in an exact way,” she stated. This circuit is essential in filtering information.
This research was supported in part by NIMH grants R01MH106507 & R01MH117961.
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Journal reference:
Malik, R., et al. (2022). Top-down control by prefrontal long range inhibition of hippocampal signals-to-noise Cell. doi.org/10.1016/j.cell.2022.04.001.
