The initial stages of age-related cognitive decline manifest themselves through fairly innocuous, fleeting moments of everyday life: moments of forgetfulness, reduced ability to multitask, and an increased susceptibility to distraction. However, as we age, these symptoms become progressively worse and potentially debilitating. What began as innocently as going to the refrigerator only to have forgotten the item we wished to retrieve can quickly progress to events that have serious health and societal consequences. As the global population is rapidly aging, this problem takes on particular urgency. In the studies below, we set out to examine the age-related inability to successful allocate attentional resources, in particular we were concerned with the global neural networks that may drive these processes, how they change with age, how they influence behavior, and if there is a neuro-structural underpinning for this decline. In effect, we wanted to know why and how does the brain age.
In our first experiment, we set out to determine how the brain allocates attentional resources and what neural networks are involved. We utilized a selective attention, working memory paradigm while recording whole-brain neural activity using functional magnetic resonance imaging (fMRI) in healthy, young controls. We found evidence that there exist separate neural networks associated with the attentional processes of enhancement (increasing the neural response to task-relevant information) and suppression (decreasing the neural response to task-irrelevant stimuli) of visual cortices. Specifically, we found that neural enhancement was associated with the frontal-parietal network (a network thought to be associated with the process of orienting attention to external stimuli) and suppression was associated with the default network (a network thought to be associated with various internal cognitive processes such as introspection and mind-wandering). Interestingly, the frontal-parietal network is traditionally associated with allocation of attention while the default network is suppressed during attentionally demanding tasks. These network activity changes co-vary with the effects of neural enhancement and suppression within the visual cortex (Chapter 2).
The finding that two separate networks are involved in enhancement and suppression presented an attractive target for studying age-related functional and cognitive changes. It has been shown that 1) older adults have deficits in suppression of irrelevant information compared to younger participants, i.e. an age-related selective suppression deficit and 2) there is an age-related dysfunction in the default network. Thus, since the results above suggest a link between suppression and the default network, our hypothesis was that we would find corresponding age-related changes in a healthy, older population. Indeed, this was the case; older participants showed a reduced ability to suppress task-irrelevant information and exhibited a corresponding reduction in default network function. Most significantly, however, we found that gray-matter atrophy within a node of the default network (the medial prefrontal cortex, mPFC) was correlated with declines in both suppression and cognitive performance (Chapter 3). These findings support our hypothesis that age-related neuro-structural changes underlie the selective suppression deficit and, more generally, the effects of cognitive aging.
