fMRI measures neuronal activity indirectly; overestimates age differences in brain activity

From BBSCR News:

Copyright: iStock

Human brains age less than previously thought

Older brains may be more similar to younger brains than previously thought. In a new paper published in Human Brain Mapping, BBSRC-funded researchers at the University of Cambridge and Medical Research Council’s Cognition and Brain Sciences Unit demonstrate that previously reported changes in the ageing brain using functional magnetic resonance imaging (fMRI) may be due to vascular (or blood vessels) changes, rather than changes in neuronal activity itself. Given the large number of fMRI studies used to assess the ageing brain, this has important consequences for understanding how the brain changes with age and challenges current theories of ageing.

A fundamental problem of fMRI is that it measures neural activity indirectly through changes in regional blood flow. Thus, without careful correction for age differences in vasculature reactivity, differences in fMRI signals can be erroneously regarded as neuronal differences. An important line of research focuses on controlling for noise in fMRI signals using additional baseline measures of vascular function. However, such methods have not been widely used, possibly because they are impractical to implement in studies of ageing.

An alternative candidate for correction makes use of resting state fMRI measurements, which is easy to acquire in most fMRI experiments. While this method has been difficult to validate in the past, the unique combination of an impressive data set across 335 healthy volunteers over the lifespan, as part of the CamCAN project ( www.cam-can.org), allowed Dr. Kamen Tsvetanov and colleagues to probe the true nature of ageing effects on resting state fMRI signal amplitude. Their research showed that age differences in signal amplitude during a task are of a vascular, not neuronal, origin. They propose that their method can be used as a robust correction factor to control for vascular differences in fMRI studies of ageing.

The study also challenged previous demonstrations of reduced brain activity in visual and auditory areas during simple sensorimotor tasks. Using conventional methods, the current study replicated these findings. However, after correction, Tsvetanov et al. results show that it might be vascular health, not brain function, that accounts for most age-related differences in fMRI signal in sensory areas. Their results suggest that the age differences in brain activity may be overestimated in previous fMRI studies of ageing.

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