Older brains slow due to greater experience, rather than cognitive decline
What happens to our cognitive abilities as we age? Traditionally it is thought that age leads to a steady deterioration of brain function, but new research in Topics in Cognitive Science argues that older brains may take longer to process ever increasing amounts of knowledge, and this has often been misidentified as declining capacity.
The study, led by Dr. Michael Ramscar of the University of Tuebingen, takes a critical look at the measures that are usually thought to show that our cognitive abilities decline across adulthood. Instead of finding evidence of decline, the team discovered that most standard cognitive measures are flawed, confusing increased knowledge for declining capacity.
Dr. Ramscar’s team used computers, programmed to act as though they were humans, to read a certain amount each day, learning new things along the way. When the researchers let a computer ‘read’ a limited amount, its performance on cognitive tests resembled that of a young adult.
However, if the same computer was exposed data which represented a lifetime of experiences its performance looked like that of an older adult. Often it was slower, not because its processing capacity had declined, but because increased “experience” had caused the computer’s database to grow, giving it more data to process, and that processing takes time.
“What does this finding mean for our understanding of our ageing minds, for example older adults’ increased difficulties with word recall? These are traditionally thought to reveal how our memory for words deteriorates with age, but Big Data adds a twist to this idea,” said Dr. Ramscar. “Technology now allows researchers to make quantitative estimates about the number of words an adult can be expected to learn across a lifetime, enabling the team to separate the challenge that increasing knowledge poses to memory from the actual performance of memory itself.”
Age Related Cognitive Decline
All aging humans will develop some degree of decline in cognitive capacity as time progresses. Data indicates that deterioration of the biological framework that underlies the ability to think and reason begins as early as the mid twenties and includes a drop in regional brain volume, loss of myelin integrity, cortical thinning, impaired serotonin, acetylcholine, and dopamine receptor binding and signaling, accumulation of neurofibrillary tangles, and altered concentrations of various brain metabolites. Cumulatively these changes give rise to a variety of symptoms associated with aging, such as forgetfulness, decreased ability to maintain focus, and decreased problem solving capability. If left unchecked, symptoms oftentimes progress into more serious conditions, such as dementia and depression, or even Alzheimer’s disease.
Cognitive decline does not affect all individuals equally; clear associations exist between the rate and severity of cognitive decline and a variety of factors, including oxidative stress and free radical damage, chronic low-level inflammation, declining hormone levels, endothelial dysfunction, excess body weight, suboptimal nutrition, lifestyle, social network, other medical conditions, and various biomarkers. Fortunately, many of these factors are modifiable to a significant extent, and proactive lifestyle changes, cognitive training, and nutritional interventions have been shown to decrease the rate of intellectual decay and potentially reverse age-related cognitive decline.
The Aging Brain
The aging process profoundly impacts the brain in ways that can be observed on multiple levels, ranging from sub-cellularly to macro-structurally. On a diminutive scale, aging causes deterioration of neuronal and mitochondrial membranes, which leads to the loss of cellular integrity and impaired neuronal function. Steep age-related declines in neurotransmitter synthesis and signaling,coupled with reductions in synaptic density and plasticity (adaptability), and loss of as much as 50% of the length of myelinated axons make the brain increasingly less efficient as we age.
In a broader sense, the physical structure of the brain as a whole also deteriorates with age. Shrinkage and death of neurons, and reductions in the number of synaptic spines and functional synapses contribute to annual reductions of as much as 0.5% to 1.0% in cortical thickness (the cortex is the outermost layer of the brain) and sub-cortical volume in some regions of the brain. Specifically, even in healthy individuals, aging accounts for volume variances of 37% in the thalamus, which is involved in sight, hearing, and the sleep-wake cycle; 36% in the nucleus accumbens, which plays a major role in mood regulation (e.g. pleasure, fear, reward); and 33% in the hippocampus, a critical site for consolidation of short-term to long-term memory. Taken together, age related neuroanatomical changes account for an estimated 25% to 100% of the variance in cognitive ability between young and aged individuals. In other words, age related cognitive decline occurs in tandem with the physical degradation of brain structure. Thus, conserving cognitive vigilance into late life requires early and aggressive intervention to preserve the brain in its youthful physical and functional state.
“Imagine someone who knows two people’s birthdays and can recall them almost perfectly. Would you really want to say that person has a better memory than a person who knows the birthdays of 2000 people, but can ‘only’ match the right person to the right birthday nine times out of ten?” asks Ramscar.
“It is time we rethink what we mean by the aging mind before our false assumptions result in decisions and policies that marginalize the old or waste precious public resources to remediate problems that do not exist,” said Topics in Cognitive Science, Editors Wayne Gray and Thomas Hills.
Biological Risk Factors Contributing to Cognitive Decline
Various biological systems work in conjunction to maintain optimal brain function and cognitive ability. Perturbations in the harmony of these systems, caused by such age-associated insults as chronic inflammation oxidative stress, insulin resistance, declining hormone levels, and endothelial dysfunction, result in physical deterioration of the brain and subsequent cognitive decline.
Hypertension
Small, delicate capillaries, like those that perpetuate the flow of blood throughout the brain, are particularly susceptible to damage caused by elevated blood pressure. Chronic hypertension leads to the breakdown of cerebrocapillaries, a condition associated with the development neurodegenerative diseases and cognitive impairment.
A case-control study of over 700 patients found a statistically significant correlation between blood pressure and rate of cognitive decline over a six-month period for subjects younger than 65 years. Accordingly, an observational study of more than 1,800 people revealed that individuals taking an antihypertensive medication were less likely to have dementia at the study onset, and were also less likely to develop dementia over the following three year period. Significantly, subjects who did have dementia at baseline and were not taking blood pressure medication exhibited a two-fold faster rate of cognitive decline than demented individuals with medication-controlled hypertension.
In a study which followed 717 individuals for 38 years starting from age 45, researchers found that subjects with systolic blood pressure ≥ 140 mmHg throughout the study period “performed consistently less well than the normal systolic blood pressure subgroups on a composite measure of verbal learning and memory.”
Evidence suggests that blood pressure of 115/75 mmHg significantly reduces the risk for cardiovascular disease, and thus may be an ideal target for those who wish to maintain optimal cognitive performance as well.
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Ben Norman
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Wiley