When you think about the all of the decisions we make and actions we take during an average day, the ordinary things don’t seem like much. Finding our way around, participating in a hobby or sport, socializing with friends are all actions that just seem to happen naturally.
However, there is a tremendous amount of work being done by the brain to manage all of these tasks. While there have been many advances in neuroscience research, there is still much scientists don’t know about how the brain organizes the millions of cells and neurotransmitters that keep us up and running. When something goes wrong, like a brain injury or neurological disease, some understanding can be gained on how it all works.
Researchers at the Salk Institute in La Jolla, CA have published their recent study on how behavior is organized in the brain. Behaviors and actions are like a list of tasks for the brain, but how they get parsed out and completed wasn’t fully understood until recently. Using optogenetics, where beams of light activate certain cells, the team at Salk found that there is a hierarchy in the brain, with several levels of control for actions. There is much debate in neuroscience on whether the brain organizes itself in a line of one task after another, like a domino effect or in a more pyramid shape, with control at the top, that filters down to the different levels. Using mice that were trained to perform a task, the scientists looked at the brain with optogenetic technology to see what was happening during the task.
Xin Jin, an Associate Professor at Salk’s Molecular Neurobiology Laboratory, led the work. He explained, “For many decades, scientists have been debating how the brain organizes behavior. Using optogenetics, a technique that uses light to manipulate brain cells’ activity, we were able to change individual actions that the animals were planning to perform, revealing this precise level of neural control.”
The team in Jin’s lab spent several weeks teaching mice to complete a series of tasks with a lever. The mice were placed in a custom box and taught to push a series of levers in a specific order to gain a treat. The sequence of left-left-right-right would result in the mice getting a piece of chow that was sweet and preferred over their regular food.
So what was going on in the brain while the mice were doing the lever two-step? Two specific neurons were targeted for observation and manipulation. The D1 and D2 neurons in the striatum of the brain are known to be involved in memory and learning how to perform certain tasks. Using laser light to activate the neurons and diphtheria toxin to de-activate them, the researchers could see exactly how they were part of organizing the task. By isolating D1 or D2, the researchers found that the mice would skip a step in the process or add a step, depending on if the cells were turned on or off.
The team found that the structure had three levels. At the top, the process was started and stopped by cells in that layer. The middle layer was where the neurons were dedicated to the process of switching between actions. The third level is like an assembly line of workers each doing a specific part of the task. For instance, if a person is tying their shoes, the top level would signal the person to begin the process. The middle layer would be busy switching between making two loops and pulling the laces tight. The neurons at the lowest level were engaged in the specifics of the task, and getting each part just right.
It wasn’t just that simple, however. There were some patterns unique to each cell, but there was also a complex network process of the cells working together. Claire Geddes, the paper’s first author is a grad student at UC San Diego. She explained, “Neurons are like snowflakes. D1 and D2 neurons have certain similar patterns, but they don’t all do exactly the same thing. There’s a complexity to how they work together to control movement.” In the video below, see how the Salk team uncovered the organizational chart of the brain. It’s not unlike some offices, with leaders, middle managers and workers, check it out.
Sources: Salk Institute, Cell Press