Many people believe they can effectively perform several tasks at the same time—replying to messages while working, talking on the phone while driving, or monitoring multiple streams of information simultaneously.

However, a new study by researchers from Martin Luther University Halle-Wittenberg, the University of Hagen, and the Medical School Hamburg suggests that the human brain has fundamental limitations that cannot be completely overcome, even through extensive practice. The findings were published in the Quarterly Journal of Experimental Psychology.

During the experiment, participants were asked to perform two different tasks simultaneously, each involving a different sensory modality. They had to judge the size of a circle displayed on a screen while also determining the pitch of a sound played at the same time. The trials continued for up to twelve consecutive days, allowing researchers to examine how intensive training affects brain performance.

As expected, participants became faster and made fewer mistakes with practice. At first glance, this appeared to suggest that the brain was gradually learning to process information in parallel. However, a more detailed analysis revealed a different picture. Even after significant improvements, a cognitive “bottleneck” remained—a stage of information processing that cannot occur simultaneously for multiple tasks.

The second phase of the experiment provided particularly compelling evidence. When researchers introduced even minor changes to the familiar sequence of tasks, error rates increased sharply and performance slowed down. This indicated that the brain had not learned to process both tasks in parallel; instead, it had developed a highly efficient system for rapidly switching between them.

The authors emphasize that the findings have important real-world implications. They help explain why even experienced drivers are more likely to make mistakes while talking on the phone, and why professionals working in high-demand environments—such as air traffic controllers, simultaneous interpreters, and operators of complex systems—can become fatigued quickly and more prone to errors.

According to the researchers, the human brain excels at organizing sequences of actions, but the idea of “limitless multitasking” remains a myth. The more complex the tasks and the more frequently attention must be shifted, the greater the cognitive load and the higher the risk of mistakes.