Individual inconsistency across measures of inhibition: an investigation of the construct validity of inhibition in older adults.

Inhibition is a central construct to the frontal lobe theory of ageing, yet its construct validity remains unproven. Furthermore, age effects on measures of inhibition are often reported without adequate control for the effects of global slowing on performance. We investigated inhibitory function in older adults in two experiments. In Experiment 1, 49 people with ages between 59 and 86 (mean=70 years 9 months S.D.=7.54) completed four analogues of the Stroop interference paradigm. To control for global slowing and to enable comparisons across all measures, we used a random effects model based on log-transformed response times. Age did not contribute significantly to the model and the estimated correlation between tasks was not significant. In Experiment 2, 33 people with ages between 62 and 86 (mean=73 years 4 months, S.D.=6.57) were compared on two measures of Stroop-like interference which were very similar in surface task demands. Age did not contribute significantly to the model but the estimated correlation between tasks was robust (r=0.714). We conclude that age may make little contribution to inhibitory function independently of other factors such as speed and intelligence. Second, that the level of individual consistency in the performance of measures of inhibition will depend on the similarity of the tasks used.