Understanding the effect of higher and lower order cognitive functions on daily living: the relationship between processing speed, executive function, and functional ability

The literature on age-related cognitive changes suggests that some older adults may begin to experience cognitive declines resulting in difficulty engaging in adaptive behavior necessary for functional independent living. Understanding the relationship between these cognitive and functional changes will be important for guiding researchers and clinicians in addressing this issue. Many theories regarding the underlying causes of cognitive aging have been proposed. Most causes appear to be related to changes in the efficiency and accuracy with which information is processed. Two current, competing hypotheses of cognitive aging include the processing speed theory of aging and the frontal lobe theory of aging; however, these theories propose two different mechanisms of cognitive change. Processing speed theory suggests that cognitive aging is a bottom-up process in which diffuse declines in a foundational cognitive process impact higher order cognitive functions. In contrast, frontal lobe theory suggests a top down process in that frontal lobe functions decline first and influence other processes in the brain. The current project examined data collected from a large multi-site sample during the Staying Keen in Later Life (SKILL) study. Specifically, the current study investigated both processing speed and frontal lobe theories of aging in an effort to determine which hypothesis best fit the data. It was observed that each model fit the data equally well, thus suggesting that both processes play a critical role in daily functioning. Additionally, the constructs overlapped substantially suggesting that the cognitive constructs may not be as separable as traditionally thought. Finally, the model was invariant across age groups and no differences were observed between young-old, middle-old, and old-old groups