A new comparison of brain activity in young and elderly multitaskers suggests an unexpected explanation for why older people frequently lose their trains of thought, and have more trouble juggling multiple tasks.

It's not that elderly people pay more attention to distraction. Instead, they seem to have trouble letting go of distraction, and are slow to regain focus on their original tasks.

In neuroscientific parlance, they experience "an interruption recovery failure, manifest as a deficient ability to dynamically switch between functional brain networks," wrote the authors of the study, published Apr. 12 in the Proceedings of the National Academy of Sciences.

The results are potentially applicable to many forms of distraction, but especially relevant to concerns about a multitasking-intensive modern lifestyle, in which constant cuts between screens and devices are routine – and, in many workplaces, demanded.

Some people think extreme multitasking is harmful; others, that it's beneficial, or simply neutral. Those arguments are complicated, invoking history, sociology and gut feelings. In terms of neurobiology, multitasking's demands and consequences are just beginning to be understood.

In the new study, researchers led by University of California, San Francisco neuroscientist Adam Gazzaley recruited 20 relatively young adults, average age 25, and 20 comparatively elderly people, average age 69.

As their brains were scanned by fMRI, each test subject was shown a landscape picture and asked to keep it in mind. After a few seconds, they were shown a portrait of a face, and had to answer several questions about it. A few seconds after that, they saw another landscape picture, and had to determine if it matched the first.

Earlier research by Gazzaley and others described how elderly people tend to have trouble switching between tasks. Exactly what happened in their minds, however, was murky. By using fMRI in the new study, Gazzaley could get a somewhat clearer picture of the mechanisms.

Gazzaley suspected that elderly people would focus excessively on distractions. Instead, their average brain activity was little different from their younger counterparts when presented with the distracting face. Differences emerged afterward: When the portrait was removed, its activity lingered in elderly brains, while quickly dissipating from younger ones. When the landscape was re-introduced, elderly brains were slow to pick up, and younger brains fast.

The paper "is a snapshot," said Gazzaley. It raises many more questions than answers, and resolves few.

It's not presently clear whether generational differences in multitasking are rooted in culture – people born during the mid-20th century were ostensibly shaped by cognitive environments less fragmented than ours – or deterioration, with brains becoming less flexible with time.

Should multitasking prove age-related, it's an open question when the decline begins. Age groups in the new study are separated by decades; deterioration could start late in life, or early.

Another question, and the one that Gazzaley considers most important, is plasticity: Whether the neurological connections involved in multitasking can be strengthened – or, for that matter, weakened.

In another intriguing study of attention, heavy multitaskers had trouble maintaining focus. It's not clear whether these two studies relate to each other, but it's plausible to think that constant multitasking makes it difficult to ignore distractions.

Gazzaley is continuing to investigate all these questions. "I would really like to figure out how plastic the ability to resolve interference is," he said. "With appropriate training and practice, how good at it can we get?"

Image: Ryan Ritchie/Flickr.

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Citation: "Deficit in switching between functional brain networks underlies the impact of multitasking on working memory in older adults." By Wesley C. Clapp, Michael T. Rubens, Jasdeep Sabharwal and Adam Gazzaley. Proceedings of the National Academy of Sciences, Vol. 108 No. 15, April 12, 2011.