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."
The researchers also trained two rhesus monkeys to perform similar tasks, and they were also able to get back on track after a few incorrect, unrewarded tries. While the animals performed these tasks, the researchers also recorded brain cell activity in their primary visual cortex and parietal cortex, which helps with task switching.
Using computer models that simulated the tasks and choices of the animals, Xue and his teammates found clues that helped them look deeper into the brain activity data recorded from the animals. They saw that there were separate groups of neurons that encoded visual features of the stimuli on the screen, either the size of the stripes or where they moved. When the test subjects were less certain, like after a couple of failed tasks, this information was no longer encoded independently—instead, the visual representations were mixed up in the same sets of neurons. The researchers call this "feature interference."
"It's not that the visual information for one feature or another isn't there and you can't remember, it's that you can't keep the different features separate anymore," Cohen said.
"It's comforting to know that when we struggle to multitask, it isn't a failure of willpower or discipline. The brain's hardware is prone to rely on the wrong information when switching gears," Xue added. "Accepting that leaves us with a simple solution: tackle one task until it's finished and give our brain enough time to turn to another."
Explanations for complex cognitive behaviors
There are many examples in life where we see this kind of interference among other senses, like how it's hard to focus on reading if someone is talking nearby. Cohen said this suggests that it might be a general mechanism that limits our ability to switch tasks. It may also provide clues as to what goes wrong in cognitive diseases. Her group recently published another study showing that they saw similar interference in an animal model of Alzheimer's disease, even when the subjects weren't under high cognitive load or conditions of uncertainty.
"We can understand much more complicated aspects of our behaviors, our thoughts, and our perceptions than we ever could before with the technological and modeling improvements that give us so much insight," she said. "There are so many people who have some sort of disorder that affects their cognition. These are not conditions that are going to have simple answers, but I think as we start to understand mechanisms for complex cognitive behaviors, we can also start to use that knowledge to diagnose people earlier and more accurately, and perhaps even identify targets for treatments."
The study, "Feature interference underlies a neuronal basis for the behavioral cost of task uncertainty," was supported by the Simons Foundation and the National Institutes of Health. Additional authors include Sol K. Markman from UChicago and the Massachusetts Institute of Technology, Ruoyi Chen from the California Institute of Technology, and Lily E. Kramer from UChicago.
