The decline of cognitive processing speed in old age.

Reviews research on the complexity hypothesis, that asserts that the cognitive slowing evident in older adults is restricted to CNS processing, and is thought to become more apparent as the complexity of the processing demands increases. Findings from research using reaction time (RT) vs ERP-RT analyses of processing speed are contrasted. It is concluded that RT is an aggregate measure that represents the final output of what is very often a complicated series of stimulus- and response-processing (S-R) transactions. As a result, important aspects of processing may not be revealed by using it. However, the latencies of components of the ERP provide on-line indices of the various constituents of S-R processing as they emerge. Thus characterizing the influences of age on the organization and speed of mental processing may best be accomplished by combining a variety of measures of processing speed.