By Matt Wood
Multitasking feels like a required skill in modern life, but as anyone who has tried keeping track of multiple chat windows, an email inbox, and a stream of phone notifications during a video call can tell you, figuring out which messages are important comes at a cost.
New research by neuroscientists at the University of Chicago looked at what happens in the brain when it tries to complete multiple tasks. The study, published in Nature Neuroscience, showed that limits to our ability to juggle multiple tasks come from “interference” between the way our brains represent different inputs, especially when their relevance to a given task is uncertain.
“The brain is good at keeping track of a lot of different things, but it becomes a big job to make sure they don’t get in the way of each other,” said Marlene Cohen, PhD, Professor of Neurobiology at UChicago and senior author of the study. “When you’re under high cognitive load, you’re trying to do too many things at once. Information gets mixed together in your brain even if you know it very well, and that turns out to be an explanation for this behavior.”
Mixing up information
Cheng Xue, PhD, a postdoctoral researcher who led the project, developed a series of experiments to understand why it’s so difficult to juggle multiple tasks. The researchers asked more than 200 human subjects to perform a task where they watched a series of images on a monitor.
First, an image of a circle with stripes would appear, and then after a short amount of time, another one would appear, either rotated in one direction or another or with differently sized stripes. The subjects were then asked to report which feature of the circle they were supposed to be tracking, and the type of change they observed (either the location/direction shift or the different frequency of stripes).
Sometimes they were asked how confident they were with this choice, but with a catch: they were never told which feature they should track. Instead, they needed to infer this from their performance on previous tasks. The “correct” answer would periodically change, introducing uncertainty. Did they choose the wrong feature, rotation over stripes? Or did they get the rotational direction or change of the size of stripes wrong?
While people were usually able to recover after a few more tries, they performed worse on the tasks when they were uncertain, indicating that they weren’t sure which information to use to make their choices.
“What surprised us was that making mistakes under uncertainty isn't just about losing focus or being sloppy,” Xue said. “When we aren’t sure which rule to follow, our brains hold onto visual details we’re supposed to ignore. That extra information bleeds into our decisions, directly contaminating how well we perceive the things that actually matter.”