High levels of magnesium in the diet may lower a woman's risk of If you're midway through life and wondering why your powers of concentration aren't are sharp as they used to be, a new study could help explain why.

Gradual brain changes beginning in middle age cause older adults to be more easily distracted by irrelevant information, and to lose focus in busy environments, Canadian researchers report.

"It's known that older adults are more easily distracted. We think we've found a mechanism in the brain to explain this and generated new insight into when in the lifespan these brain changes begin to occur," study author Dr. Cheryl Grady, a senior scientist at the Rotman Research Institute in Toronto, said in a prepared statement.

She and her colleagues used functional magnetic resonance (fMRI) to study brain function in healthy middle-aged adults. They then compared them to younger and older adults. The study volunteers were assigned a series of memory tasks while their brain function was recorded.

In younger adults, activity in the dorsolateral prefrontal cortex (associated with tasks that require concentration) increases when they're doing memory tasks. At the same time, there's decreased activity in the medial frontal and parietal regions (associated with non-task related activity in a resting state -- such as thinking about yourself or monitoring your surroundings).

As reported in the February issue of the Journal of Cognitive Neuroscience, this pattern begins to break down in people aged 40 to 60. When middle-aged people do memory tasks, activity in the "daydreaming" medial frontal and parietal regions stays turned on, while activity in the concentration-linked dorsolateral prefrontal cortex declines.

This imbalance in brain activity is even more pronounced in people aged 65 and older, the researchers said, which may explain why older adults are less able to tune out irrelevant or distracting information.

"Our fMRI scanning reveals that middle age represents the transition between the patterns observed in youth to that found in old age. The seesaw imbalance in the two frontal lobe areas is not as significant as in older adults, but the functional changes are detectable by middle age," Grady said.