The nature of the influence of speed on adult age differences in cognition.

Two studies were conducted to determine the relations among age, motor speed, perceptual speed, and 3 measures of cognitive performance: study time, decision time, and decision accuracy. Each study involved over 240 adults between I 8 and over 80 years of age who all performed a battery of tests, including computer-administered tests of memory, reasoning, and spatial ability. The results indicated that (a) increased age was associated with lower accuracy as well as with longer study time and decision time and (b) some ofthe relations between age and decision accuracy and between age and decision time appear to be mediated by a slower rate ofexecuting cognitive operations. Considerable evidence now exists indicating that a large proportion ofthe age-related variance in many different cognitive variables is shared with a measure of perceptual speed. Among the most pertinent results are the findings that the age-related diffbrences in various measures of cognitive functioning are greatly reduced when statistical control procedures are used to adjust for differences in perceptual speed. As an illustration of this phenomenon, Thble I in Salthouse (1993b) contains 44 comparisons across a wide range of cognitive variables. Age was associated with a mean of l6.2Vo of the total variance in the variables, but after the variance associated with measures of perceptual speed was held constant, age was associated with only 3.6Vo ofthe variance in the cognitive variables.