Processing speed and executive functions in cognitive aging: how to disentangle their mutual relationship?

The processing-speedtheory and the prefrontal-executivetheory are competing theories of cognitive aging. Here we used a theoretically and methodologically-driven framework to investigate the relationships among measures classically used to assess these two theoretical constructs. Twenty-eight young adults (18-32 years) and 39 healthy older adults (65-80 years) performed a battery of nine neuropsychological and experimental tasks assessing three executive function (EF) components: Inhibition, Updating, and Shifting. Rate of information processing was evaluated via three different experimental and psychometric tests. Partial correlations analyses suggested that 2-Choice Reaction Time (CRT) performance is a more pure measure of processing speed than Digit Symbol Substitution Test (DSST) performance in the elderly. Hierarchical regression analyses showed that, although measures of processing speed and EF components share mutual variance, each measure was independently affected by chronological age. The unique adverse effect of age was more important for processing speed than for EF. The processing-speed theory and the prefrontal-executive theory of cognitive aging were shown not to be mutually exclusive but share mutual variance. This implies the need to control for their mutual relationship before examining their unique potential role in the explanation of age-related cognitive declines. Caution has still to be taken concerning the tasks used to evaluate these theoretical constructs.