Motorcycle-Related Fatalities: A Systematic Review and Structured Narrative Synthesis.

Structural and functional changes of the brain during normal aging lead to reduction of cognitive functions, such as executive function, working memory, processing speed and cognitive plasticity (). Functional neuroimaging data provide important insight into brain functional changes associated with aging (). Spontaneous fluctuations of blood oxygenation level-dependent (BOLD) signal, as measured by functional magnetic resonance imaging (fMRI), has revealed a number of functional networks, such as the default mode network (DMN) and the central executive network (CEN), which are involved in cognitive processes (; ; ). The DMN is a widely investigated network and important in aging, because it is vulnerable to early stages of Alzheimer's disease (AD) pathology () and has been associated with memory functions, mind-wandering and self-monitoring (). It includes regions, such as posterior cingulate cortex, lateral parietal cortex and medial prefrontal cortex (). The CEN is a wide network, often also referred to as fronto-parietal network (FPN), which includes frontal and parietal regions and has been linked to higher cognitive processes, such as working memory and cognitive control (; ). Cross-sectional studies indicated decreased DMN activity in aging (; ; ; ; ; ). Moreover, some studies found age-related decreases in higher order networks, such as the CEN (), and others found increased activation in the CEN at rest (). Data from longitudinal functional imaging studies provided mixed findings. For instance, they demonstrated functional changes on DMN and CEN connectivity over a period of four years of healthy aging (). In contrast, other longitudinal studies showed a stable average DMN connectivity over a 4 year () or even 7 year period () in healthy older adults. Additionally, they demonstrated that some DMN components, such as posterior cingulate cortex (PCC) and parietal cortex showed decreased functional connectivity (FC), and in contrast prefrontal regions showed increased FC over time (). These findings indicate a complex pattern of functional changes in DMN during aging. Moreover, they support previous evidence of, possibly compensatory high resting state FC in prefrontal regions in healthy aging (; ). These age-related changes in DMN and CEN connectivity provide the neural context for examining whether cognitive training can modulate functional network organisation, and whether individual differences in baseline FC predict who will benefit most from training.