Age-related losses in cognitive control efficiency in the face of response conflict are commonly reported in ageing research. However, it is unclear to what extent this effect reflects changes in actual inhibitory control, or the well-known age-related slowing of processing speed.

We compared young (n = 42; 29 women; mean age = 19.6 years; mean formal education = 13 years) and older adults (n = 42; 27 women; mean age = 68.7 years; mean formal education = 12.8 years) using a spatial Stroop task. Participants responded to the direction of an arrow, ignoring its position. Direction and position could be congruent, incongruent or neutral (respectively low, high and no conflict trials). The level of conflict in trial n-1 (high or no conflict) modulated the level of adaptive control in the nth trial. We used multivariate analyses of variance to probe age-group effects on inhibitory efficiency, adaptive (high conflict n-1 trial) and momentary (no conflict n-1 trial). We analysed accuracy and direct as well as proportional reaction times, which respectively integrate and control for differences in general processing speed.

Older participants showed a larger overall Stroop effect in both direct [Wilks’ λ = .61, F(2,81) = 25.99, p<.001] and proportional reaction times [Wilks’ λ = .79, F(2, 81) = 10.55, p<.001]. Controlling for differences in general processing speed did not impact age-group effects on momentary inhibitory efficiency [F(1,82) = 17.78, p<.001], but eliminated a trend for poorer adaptive inhibitory control in the older group [F(1, 82)=.198, p = .657]. As for accuracy, we unexpectedly found a larger Stroop effect for the younger group [Wilks’ λ = .79, F(2, 81) = 11.07, p=.001].

Older and younger adults are as effective in using previous response conflicts to prepare for current conflict resolution. Older adults' lower momentary inhibitory effectiveness likely reflects age-related slowing of processing speed as well as, to a degree to be determined in future research, larger strategic reaction times investment in accuracy enhancement.

BIAL Foundation (Grant 234/14)