This interesting open access paper reports on one of a number of efforts to map the details of the association between cardiovascular fitness and memory function over the course of aging. The brain is that the mercy of the vascular system in many ways. There is the age-related reduction in capillary formation cutting down the supply of nutrients and oxygen to the brain. Stiffening of blood vessels results in hypertension, and raised blood pressure pummels delicate tissue structures in the brain, kidney, and elsewhere. The structural decline of the vascular system, the weakening of blood vessels due to atherosclerotic lesions, combines with raised blood pressure to produce many ruptures of lesser blood vessels in the brain over the years, destroying small areas of functional tissue in silent, tiny strokes.
All of these processes are to some degree slowed by maintaining cardiovascular fitness - even capillary formation. Physical activity also adjusts many signaling processes related to tissue maintenance, such as the pace of neurogenesis in the brain, the production and integration of new neurons. To look at it the other way, in our modern age of comfort and indolence, most people fail to put in the necessary maintenance activity required to keep the declines of aging to the slowest possible rate. No-one can reliably add decades to life through exercise, but it is certainly possible to find oneself in one's sixties, unfit, overweight, encountering the first earnestly troubling signs of mental decline, and all the while regretting the path not taken.
Aging is associated with progressive changes in brain structure that gradually impair essential cognitive functions, including memory. Numerous mechanisms have been proposed to underlie memory decline, such as altered plasticity, connectivity, and excitability. One change of particular interest is the reduction of hippocampal neurogenesis and the observation that age-associated declines in neurogenesis and memory can be partly rescued in animals that engage in aerobic exercise. Aerobic exercise also increases hippocampal blood volume in humans. Critically, this suggests that physical activity in older humans may be associated with better memory performance. However, some aspects of memory processing may be more associated with aerobic fitness than others. The present study compared age-related differences for two critical memory processes: high-interference memory and general recognition memory.
High-interference memory represents the ability to discriminate between highly similar yet distinct items. In animal models, high-interference memory can be improved by stimulating neurogenesis and is impeded by interfering with neurogenesis. Younger adults (YA) have better high-interference memory than older adults (OA), therefore, the decline in hippocampal neurogenesis that occurs with aging may impact high-interference memory. General recognition is another aspect of memory that represents the ability to discriminate novel stimuli from those previously encountered. Unlike high-interference memory, general recognition memory may not be as dependent on hippocampal neurogenesis. The recruitment of a more distributed network for processing by general recognition may mean that this aspect of memory is less affected by the age-related decline. Indeed, YA and OA have been shown to have similar performance on general recognition memory tasks.
Individual differences influencing age-related decline of memory extend beyond biological aging to include lifestyle factors, such as physical activity. The present study investigated the effects of aging on high-interference memory and general recognition memory. Ninety-five YA and eighty-one OA performed the Mnemonic Similarity Task (MST). Age-related differences in high-interference memory were observed across the lifespan, with performance progressively worsening from young to old. In contrast, age-related differences in general recognition memory were not observed until after 60 years of age. Furthermore, OA with higher aerobic fitness had better high-interference memory, suggesting that exercise may be an important lifestyle factor influencing this aspect of memory. Overall, these findings suggest different trajectories of decline for high-interference and general recognition memory, with a selective role for physical activity in promoting high-interference memory.