If you've ever walked to the kitchen to get a snack, been interrupted by a phone call, and then forgotten what you wanted from the fridge, you may have seen a glimpse of your future. A wealth of studies have found that as we age we become more distracted by interruptions and less able to juggle multiple tasks at once. Now neuroscientists have identified a potential explanation: a sticky switch in the neural circuits that coordinate memory and attention.
In the new study, published online today in the Proceedings of the National Academy of Sciences, Wesley Clapp and Adam Gazzaley of the University of California, San Francisco, and colleagues recruited 20 healthy adults in their 60s and 70s to play a simple memory game inside a brain scanner. An image of a scene, such as a field or a forest, flashed on a monitor inside the scanner, and volunteers had to keep it in mind for about 15 seconds before indicating whether a second image was a match. In this version of the test, the older volunteers did extremely well, getting 96% correct.
But when the researchers added an interruption, the older subjects struggled. When they had to perform a second task while keeping the scene in mind—for example, judging the age and gender of a face flashed on the monitor—their performance on the memory test for the scenes dropped to 88%. A control group of mostly 20-something volunteers was less fazed by the interruption; their performance did not drop significantly.
Functional magnetic resonance imaging of activity in the subjects' brains hints at a possible explanation. Gazzaley says the team focused on two networks in the brain, one that revs up when volunteers are trying to keep the scene in mind and another that becomes active as they evaluate a face. In subjects of all ages, the scans suggested that when people are trying to keep the scene in mind, there is a lot of communication between the parahippocampal place area, a region that specializes in analyzing scenes, and the right middle frontal gyrus (MFG), a region of prefrontal cortex thought to be involved in focusing attention on what's needed to accomplish a particular goal. In contrast, during the interrupting task with the faces, the scans in all subjects suggested increased communication between the MFG and the fusiform face area, a region involved in, you guessed it, analyzing faces. What distinguishes older folks from the youngsters, the researchers found, is that they were slower to switch off the face-processing network and reactivate the scene-memory network after an interruption.
"The news here is that one reason older folks have difficulty is that they might have trouble disengaging from what's intrusive and getting back to what's important," says Arthur Kramer, a neuroscientist who studies aging at the University of Illinois, Urbana-Champaign. "It really does move us forward in terms of understanding the mechanisms that might underlie these age-related deficits." The findings mesh with other research, Kramer says, including a recent study from his group  that examined the effects of listening to an iPod or talking on a cell phone while crossing the street in a virtual reality environment. Cell phone conversations were especially dangerous for the older subjects in the experiment, Kramer says. "They have a tough time getting safely across the street, and they get run over more often," he says. Fortunately, no one actually gets hurt in virtual reality experiments, but Kramer says it's a lesson worth taking seriously in a world increasingly filled with high-tech distractions.