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Even with highly extensive training, the human brain is not really capable of performing two tasks simultaneously. Moreover, even the smallest deviations from trained routines can have a significant impact on how quickly and successfully people complete tasks simultaneously. This is shown by a new study conducted by Martin Luther University Halle-Wittenberg (MLU), the FernUniversität in Hagen and the Medical School Hamburg. It was recently published in the “Quarterly Journal of Experimental Psychology”.
In three experiments, the researchers investigated how people perform two tasks simultaneously that involve different senses: participants were asked to indicate the size of a circle that was briefly displayed with their right hand and simultaneously to say whether a sound played at the same time was high, medium or low. The speed at which the participants completed the tasks and the number of mistakes they made were measured. The tasks were repeated over a period of up to twelve days. The results showed that the more often the test subjects completed the test, the faster they solved both tasks without errors. Earlier studies with similar findings had therefore suggested that so-called dual-task costs, i.e. performance losses when working on two tasks simultaneously, could almost completely disappear after extensive practice. "This phenomenon, known as virtually perfect time sharing, has long been considered evidence of true parallel processing in the brain and proof that our brain is capable of unlimited multitasking. The results of our study clearly contradict this assumption," says psychologist Professor Torsten Schubert from MLU.
The new study shows that the underlying cognitive processes still do not run completely in parallel. What's more, even the smallest changes to the tasks caused the error rate to rise and the participants to take longer to complete the tasks. "Our brain is very adept at sequencing processes so that they no longer interfere with each other. However, this optimisation has its limits. In particularly challenging situations, our cognitive apparatus therefore tires very quickly and becomes error prone," Schubert continues.
The study also provides new impetus for safety research. "Our results show why multitasking can often be risky in everyday life, despite routine, for example when driving and talking on the phone at the same time. This is also relevant for professions with complex activities where several tasks have to be performed in parallel, such as air traffic controllers or simultaneous translators," says Professor Tilo Strobach from the Medical School Hamburg. Professor Roman Liepelt from the FernUniversität in Hagen adds: "Our study sheds new light on the limits of human information processing. Understanding such cognitive bottlenecks is crucial for improving work processes, learning environments and safety measures in everyday life."
Study: Schubert, T., Liepelt, R., & Strobach, T. Evidence for a latent bottleneck after extensive dual-task practice of a visual-manual and an auditory-verbal task. Quarterly Journal of Experimental Psychology (2025). doi: 10.1177/17470218251396870
https://doi.org/10.1177/17470218251396870
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