Motor-response generation as a source of aging-related behavioural slowing in choice-reaction tasks

Objective To analyze the effects of stimulus–response (SR) processing modes on different central stages of sensorimotor processing in order to evaluate their contribution to aging-related behavioural slowing. Methods Components of stimulus- and response-related potentials (ERPs/RRPs) and lateralized readiness potentials (LRPs) were analyzed in two groups of young (mean 22.5 years) and older adults (mean 58.3 years) during an auditory and a visual four-choice-reaction task. Results (1) Reaction time (RT) depended on the SR type, indicating SR-specific differences in processing, which did not vary with age. (2) For each SR type, the RT increased with age. RT slowing was not accompanied by significant delays in early stimulus processing (as reflected by P1 and N1 latencies) nor in response selection (as reflected by the onset of stimulus-locked LRP), but resulted from a prolongation of contralateral motor activity during motor response execution indexed by earlier, longer durated and larger motor-related potentials in older adults. These aging effects were observed for each SR type. Conclusion In a four-choice-reaction task, (1) task complexity rather than differences in cognitive strategy or activation patterns subserving SR-specific processing leads to response slowing with aging and (2) the most plausible contributor to this slowing is the cortical response generation system.