Scientists have found that alternative brain regions or networks can take on the roles and functions of impaired areas to compensate for age-related cognitive decline.
Their study, published today as a Reviewed Preprint in eLife, is described by the editors as an important advancement in our understanding of neural functional compensation, supported by compelling evidence. The work delivers methods for researchers to quantify compensation in future studies relating to the neuroscience of healthy ageing.
Functional compensation is a common notion in neuroscience, whereby the brains of older adults are proposed to recruit additional activity from an alternative brain region to compensate for reduced cognitive function. Whether this compensation actually results in improved cognitive performance remains unclear.
“Preventing cognitive decline in old age is a major public health priority, which demands a better understanding of the neurophysiological changes that preserve cognitive function,” says lead author Ethan Knights, Research Associate at the Medical Research Council (MRC) Cognition and Brain Sciences Unit, University of Cambridge, UK. “Fluid intelligence – solving new and abstract problems – is a cognitive function that shows one of the largest decreases in older age. We know that fluid intelligence engages a network in the brain called the multiple demand network (MDN), and that activation of the MDN tends to decrease with age.”
To explore the concept of functional compensation during fluid intelligence tasks, Knights and colleagues enlisted 223 adult participants, between 19 and 87 years of age, from Stage 3 of the Cambridge Centre for Ageing and Neuroscience project. They designed a modified version of the odd-one-out subtest of the standardised Cattell Culture Fair Intelligence Test and used functional magnetic resonance imaging (fMRI) to monitor changes in brain activity during the trials. In each trial, four display panels were presented to the participants in a horizontal row in the centre of a screen. Participants were then asked to press a button when they identified the odd-one-out panel, which differed visually to the other three. These trials ranged in difficulty, from easily solvable, to hard-to-solve puzzles.
During the trials, the team noticed through the fMRI scans an activation in brain regions generally considered to form part of the MDN. So, to determine whether any of these regions could be involved in functional compensation, they looked for areas that showed positive effects of both age and performance, which would indicate an age-related compensatory response. This revealed two brain regions of interest: the cuneus, which is situated towards the back of the brain and is typically involved in processing visual information, and the bilateral frontal cortex, which is typically responsible for decision-making, problem-solving and social interactions.
The team knew that, if these areas were involved in functional compensation, they would possess additional information about the task the participants were completing. So, they used a neuroimaging analysis approach called multivariate Bayesian decoding, which considers patterns of activity across multiple brain regions simultaneously and uses statistical probabilities to make inferences about the data. This confirmed that both areas did indeed possess information about the task at hand. However, in the frontal cortex, the amount of information did not change with the age of the participant, while, the amount of information in the cuneus was found to be greater in older adults. These findings suggest that, in fluid intelligence tasks, greater activation of the cuneus in older adults allows the maintenance of proper cognitive performance – which is indicative of functional compensation by this region of the brain.
eLife’s editors note that the study would be strengthened by including more demographic information for the cohort of participants, to ensure that the sample is truly representative of the wider population.
“Our results represent the most compelling evidence to date for functional compensation in healthy ageing. Since the cuneus has a well-established role in visual attention, we hypothesise that the additional recruitment of this brain region allowed older participants to concurrently assess multiple features of the visual panels, to correctly select the odd-one-out,” concludes senior author Kamen Tsvetanov, an Alzheimer's Society Dementia Research Leader Fellow in the Department of Clinical Neurosciences, University of Cambridge. “Further work is needed to determine the precise function of the cuneus in problem-solving tasks like the one we designed, and to fully explain how age, cuneus activation and fluid intelligence are connected, considering things like education and lifestyle choices.”
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