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How the brain distinguishes important and unimportant sensations

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How the brain distinguishes important and unimportant sensations

Imagine you are playing the guitar. You sit, maintaining the weight of the tool on your lap. One hand rattles; another presses the strings to the neck of the guitar to play chords. Your eyesight tracks the notes on the page, and your hearing allows you to listen to the sound. In addition, two more sense organs allow you to play this instrument. One of them, Touch, talks about your interaction with the guitar. Another, proprioception, talks about the position and movement of your hands and arms during play. Together, these two abilities combine into what scientists call self-sensation, or body perception.

Our skin and muscles have millions of sensors that promote somatosensation. However, our brains are not overwhelmed by the barrage of these inputs – or from any of our other senses, for that matter. You won’t be distracted by squeezing your shoes or pulling your guitar strap while playing; you focus only on sensory data that matter. The brain skillfully amplifies some signals and filters out others so we can ignore distractions and focus on the most important details.

How does the brain perform these feats of attention? In recent research at Northwestern University, the University of Chicago, and the Salk Institute for Biological Research in La Jolla, California, we highlighted a new answer to this question. Through several studies, we have found that a small, largely ignored structure at the very bottom of the brainstem plays an important role in the brain’s choice of sensory signals. The area is called the wedge-shaped core, or CN. Our CN research is changing not only the scientific understanding of sensory processing, but can also lay the groundwork for medical interventions to restore sensitivity in patients with trauma or disease.

To understand what’s new, we need to consider a few basics of how somatosensation works. Every time we move or touch something, specialized cells in our skin and muscles respond. Their electrochemical signals go through nerve fibers to the spinal cord and brain. The brain uses these messages to track the posture and movement of the body, as well as the location, time, and strength with which we interact with objects. Experiments have shown that the conscious experience of our body and its interaction with objects depends on these signals that reach the cerebral cortex, the outermost layer of the brain. Scientists have long believed that this area of ​​the brain was one of the main players involved in selective amplification or filtering of sensory signals. They believed that CN, on the other hand, was just a passive relay station that moved signals from the body up to the cerebral cortex.

But we were skeptical. Why exist CN if it does not somehow change the signals? We decided to look at wedge-shaped neurons in action to find out. The problem has historically been that CN is small and very inaccessible. It is located at a very flexible junction of the head and neck, which means that the movement of the animal can make it difficult to reach. What’s worse, the wedge-shaped nucleus is in the brain stem, surrounded by vital areas of the brain that, if damaged, can lead to death.

Fortunately, modern neuroscience tools allow us to stably observe CN in awake animals without harming nearby areas. The monkeys were implanted with tiny arrays of electrodes that were used to control individual wedge nucleus neurons. For the first time, we were able to study how individual brain cells react in this area when a monkey moved and touched things. This method allowed us to answer a few questions about what CN does. First, we studied how these neurons respond to touch signals by exposing monkey skin to a variety of stimuli, including vibrations and embossed Braille-like dot patterns. We then compared the reactions in CN with the activity in the nerve fibers that feed this brain structure. If this area simply transmitted information collected by sensory skin cells, nerve activity in CN would essentially respond with activity in nerve fibers. Instead, we found that CN neurons don’t just transmit their input but transform them. In fact, wedge-shaped neurons showed patterns of activity that were more similar to those found in neurons in the cerebral cortex than in models in nerve fibers.

But the link between CN and bark is not a one-way street. In addition to the sensory nerves going up, there are pathways from the sensory and motor areas of the cerebral cortex that go down to the wedge-shaped nucleus. We wondered if CN promotes some form of sensory filtering based on planned arbitrary animal movements. To this end, we observed CN activity as the monkeys approached the target, and compared these signals with the CN signals generated when the robot moved the monkey’s arm in a similar manner. We found that the activity of wedge-shaped neurons has really changed, depending on what the animals were doing and whether the movements were arbitrary or involuntary. As just one example, we know that signals from the arm muscles can help an animal determine that movement is going as planned. Consistent with this idea, we found that many signals from the arm muscles were amplified in the CN when the monkey voluntarily moved the hand, compared to when the robot moved it.

These studies have shown that the processing of signals coming from our body has already begun when the signals reach the wedge-shaped nucleus. But what are the brain cells and pathways that allow CN to selectively amplify signals that matter and suppress those that don’t matter? In the third study, we used genetic and viral techniques to study the nervous system of mice. With these tools we were able to manipulate certain types of cells by turning them on or off by illuminating them with a laser. We combined these techniques with behavioral tasks: by teaching mice to pull a string or respond to different textures for a reward, we tested how activating or inactivating certain neurons could affect a mouse’s ability to perform dexterous tasks. This approach allowed us to first study the function of cells in the CN by discovering a specific set of neurons that surround it that can suppress or enhance the passage of sensory signals when they enter the brain. We then applied similar techniques to examine how other higher parts of the brain may affect CN activity. We found out two different paths from the bark all the way to the CNs, which regulate how much information the clients can transmit. In other words, CN receives not only information from the body but also guidance from the cortex to help determine which signals are most important or important to a person at any given time.

Obviously, the wedge-shaped nucleus is a much more interesting part of the brain than was attributed to it. Our work helps to explain its function: to emit certain signals and suppress others before transmitting them to the areas of the brain responsible for perception, motor movement and higher cognitive functions. This important role may help explain why CN appears in a wide variety of mammals, including mice and primates.

Although our work is far from complete, our results already have important implications for rehabilitation. In addition to the active tactile and muscular signals we were able to study, the data suggest that CN receives much more “sleeping” inputs, which may be important for recovery after neurological damage. Millions of people around the world suffer from some form of limb dysfunction, such as paralysis or loss of feelings. With a better understanding of how sensory and motor signals support movement, physicians can ultimately improve the diagnosis and treatment of these conditions. For example, implanted electrodes can once electrically activate a wedge-shaped nucleus in people who have lost sensitivity in their limbs, potentially restoring their body’s ability to perceive.

Are you a scientist specializing in neuroscience, cognitive science or psychology? Have you read a recent peer-reviewed paper you would like to write about for Mind Matters? Please send suggestions to Scientific American Mind Matters editor Daisy Juhas at pitchmindmatters@gmail.com.

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