Constantly doing two things at once, from texting while walking down the street to checking Facebook while watching a film, our brains are persistently multitasking, having to share processing power between two or more activities. Yet the details of how our brains manage this feat, whether we can genuinely concentrate on two things at once or if our brain is simply switching rapidly between the two, remains unknown.
A new study has, however, detailed how a specific region deep within the brain, known as the thalamic reticular nucleus (TRN), is able to seemingly control how much attention is paid to incoming information from each stimulus. This, according to the researchers, allows the brain to focus specifically on the immediate task at hand, while pushing the other inputs to the back. “Filtering out distracting or irrelevant information is a vital function,” said Michael Halassa in a statement. Halassa coauthored the paper, which is to be published in Nature.
Imagine, for example, walking through Times Square, explains Halassa. Your brain is constantly processing reams and reams of information, and yet if you’re hungry “the McDonald’s sign is going to pop up. So the sensitivity, how you’re sampling the outside world is shaped by your expectation, or what you’re goals are.” To test how the mammalian brain achieves this, the researchers conducted experiments on mice.
They trained the mice to select either a light signal or sound when given particular auditory cues, which would then give the mice a milk reward. Then they distracted the mice with opposing stimuli, so if the mouse was expecting the light to guide it to the milk, they would distract it with a sound. This distraction reduced to the mouse’s accuracy to find the milk by 20% compared to when they had no distraction. At the same time, the researchers were tracking the electrical signals in the TRN neurons, and also inactivating various parts of the mice’s brain with a laser beam.
When the researchers inactivated the prefrontal cortex region of the brain, the area thought to be responsible for decision making, the mice could no longer determine where the milk was upon being presented with a cue, meaning they were now effectively guessing randomly. The researchers think that inactivating this region also disrupted the TRN, as they found exactly the same results when they blocked the TRN while leaving the prefrontal cortex intact. This demonstrates how the prefrontal cortex “stores the knowledge ultimately communicated to the TRN to control how much visual or auditory sensory information or not, and how the brain ultimately multitasks," said Halassa,
So when you’re focusing on a task, your brain will heighten the processing of the stimuli in question, while dampening the other stimuli bombarding the brain. This process, they suggest, is achieved by individual TRN neurons acting in effect like a “switchboard,” filtering the sensory information being delivered by the prefrontal cortex and shifting more or less attention on to one sense over another. They hope that by understanding how this process works, they might be able to develop ways to help people who have attention-deficit disorders, such as ADHD or autism.