What prevents us from handling multiple tasks at once

Scientists have found that the main problem arises due to the so-called “feature interference” — a situation where, under conditions of uncertainty, the brain stops clearly separating different types of information. According to Professor Marlene Cohen, the brain is capable of tracking many details, but it becomes extremely difficult for it to ensure that these details do not interfere with each other. Under high cognitive load, information becomes mixed up, even if it is well known to the person, and this is what explains why we make mistakes more often when multitasking.

To understand the mechanism, researcher Cheng Xue and his team conducted a series of experiments. More than 200 participants viewed images of circles with stripes on a screen and had to track changes: either a rotation of the circle in one direction or another, or a change in the size of the stripes. At the same time, the subjects were not told which specific feature they needed to focus on; they had to guess based on their previous results, and the correct criterion changed periodically. This uncertainty was deliberately designed to create difficulties.

It turned out that during moments of uncertainty, people performed worse on the task not because they lost concentration or became more careless. The brain continued to keep visual details in focus that were supposed to be ignored at that moment, and this unnecessary information directly interfered with decision‑making.

To gain a deeper understanding, the scientists involved two rhesus macaques in similar tasks. While the animals performed the tasks, the researchers recorded the activity of neurons in the primary visual and parietal cortex — areas involved in switching between tasks. Using computer models, the data were analyzed, and the following was discovered. Individual groups of neurons encoded different visual features (for example, the size of the stripes and the direction of movement). However, under conditions of uncertainty, for example, after several errors, this encoding was disrupted, and representations of different features became mixed in the same neural networks. It was this phenomenon that the team called “feature interference.”

As Cheng Xue explains, the problem isn’t that the necessary information disappears from memory: the brain simply loses the ability to keep different features separate from each other. This leads to practical conclusions: if you want to work more efficiently, it’s better to complete one task before switching to another — this makes it easier for the brain.

The authors believe that the discovered mechanism may be a general principle limiting our ability to quickly switch between tasks. Similar interference can also be observed in everyday life. For example, it’s hard to focus on reading if someone is talking nearby. Moreover, understanding this mechanism may also prove useful in medicine. In a recent study by the same group (published in PNAS), similar impairments were observed in a model of Alzheimer’s disease, even without high load and uncertainty.

According to the researchers, studying such basic mechanisms of cognitive behavior opens the way to earlier and more accurate diagnosis of thinking disorders, and, in the long term, to identifying targets for therapeutic intervention.