The findings, by scientists at The Rotman Research Institute at Baycrest and the University of Toronto, are reported in the February 2006 issue of the Journal of Cognitive Neuroscience (Vol. 18, No. 2).
"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," says senior Rotman scientist and lead author Dr. Cheryl Grady.
While previous studies have used functional magnetic resonance imaging (fMRI) to look at how brains function differently in young and old adults, and patients with Alzheimer's Disease, this is the first time investigators have used fMRI on normal, healthy middle-aged adults, as well as young and old adults, to understand how brains are changing in the in-between years. Investigators administered a series of memory tasks to the three age groups to assess if age-related changes in brain function are task-specific, or generalized across a number of regions during memory tasks.
The findings add to the growing body of science that implicates two regions in the frontal lobes that gradually shift into a seesaw imbalance - causing older adults to become less efficient in inhibiting distracting information. In younger adults, activity in the dorsolateral prefrontal cortex (associated with tasks that require concentration, such as reading) normally increases during the task, while activity in the medial frontal and parietal regions (associated with non-task related activity in a resting state, such as thinking about yourself, what you did last night, monitoring what's going on around you) normally decreases.
However, starting in middle age (40-60 years), Dr. Grady's team noted that this seesaw pattern begins to break down during performance of memory tasks. Activity in the medial frontal and parietal regions stays turned on while activity in the dorsolateral prefrontal cortex decreases. The imbalance becomes more pronounced in older adults (65+), which could explain their reduced ability to ignore distracting or irrelevant information, she says.
"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."
Twelve young adults (20-30 years), 12 middle-aged adults (40-60 years) and 16 older adults (65-87 years) participated in a series of memory tasks. The groups all had an average of post-secondary education. The first set of memory exercises involved encoding - looking at common nouns and pictures of objects of different sizes. By pushing one of two buttons, participants could decide whether words were printed in capital letters or lower case, whether pictures presented were large or small, and whether the pictures or words corresponded to living or nonliving entities. This tested their semantic and perceptual judgement. Participants were then administered scanned recognition tests made up of words written in lowercase letters, including words to describe the pictures they had seen, to assess semantic and perceptual recall. During all the memory tasks, their brains were scanned using fMRI which constructs computerized images of brain structures and pinpoints the areas being activated during a memory task.
While investigators found that middle-aged adults performed just as accurately as younger adults on encoding and recognition tasks, brain scanning revealed that functional changes (i.e. the seesaw imbalance) are starting to appear as early as middle age, although the changes are gradual and don't translate into a noticeable change in memory performance for that age group. Scanning also revealed that functional brain changes and associated deficits in cognitive performance (i.e. lower accuracy scores particularly in recall tests) become more pronounced after the age of 65. Indeed, the altered activity level in medial frontal and parietal regions in older adults is similar to that seen in patients with Alzheimer's, although less pronounced, says Dr. Grady.
Overall, findings confirmed that age-related changes in brain function are happening across various memory tasks and progress in a linear fashion as adults age.
The study reinforces a cautionary message to aging adults, says Dr. Grady: "Older adults should try to reduce distractions in their environment and concentrate on one key attentional task at a time. It may be as easy as turning down the radio when reading, or staying off the cell phone when driving a car."
Because most of the participants in the study were fairly well educated, the finding of brain changes without accompanying behavioural changes in the middle-aged group may reflect what previous scientific studies have termed the "protective effect" on behaviour.
"Higher education levels may allow for some redundancy of brain function or compensation that leads to preserved performance in middle-aged adults, at least on some tests, despite altered brain activity," says Dr. Grady.
The study was supported by a grant from the Canadian Institutes of Health Research. Dr. Grady's research team included Dr. Randy McIntosh, Dr. Gordon Winocur, Mellanie Springer and Donaya Hongwanishkul.
Baycrest is an internationally-recognized healthcare and research facility for aging adults, with a focus on brain functioning and mental health.