Are processing speed tasks biomarkers of cognitive aging?

We examined the association between 5 processing speed measures and general cognitive ability in a large (>900) sample of relatively healthy men and women at age 70. The processing speed tests were the Wechsler Digit Symbol-Coding and Symbol Search, simple reaction time, 4-choice reaction time, and inspection time. To inquire whether the processing speed tasks might be biomarkers of cognitive aging, we examined the attenuations in their associations with general cognitive ability after adjusting for cognitive ability measured almost 60 years earlier. With the exception of inspection time, the attenuations were substantial. Inspection time was the only processing speed measure--all of which were measured at age 70--whose correlation with cognitive ability at age 70 was significantly greater than the correlation with cognitive ability at age 11. In old age, individual differences in most commonly used measures of processing speed are largely dependent on childhood cognitive ability. For all processing speed tasks, a little variance is left that appears to be related to aging differences. Inspection time, the marker that was least dependent on childhood intelligence, should be considered further as one biomarker of cognitive aging.