Effects of aging on slowing of motor-response generation.

The aim of the present study was to analyze different stages of central processing mechanisms during a choice reaction task and to evaluate their contribution to aging-related response slowing. Event-related potentials (ERPs) were recorded from two groups of subjects, young (mean 22 years) and older adults (mean 58 years), who performed a four-alternative choice-reaction task. The results showed the expected reaction time slowing in the older subjects. This behavioural slowing was not due to delays in stimulus processing (as reflected by latencies of early ERP components), or in response selection (as reflected by the onset of the lateralized readiness potential). Instead, this slowing was due to an alteration of movement-related components, particularly an amplitude enhancement and prolongation of the motor-related potential at the cortex contralateral to the responding hand. This alteration was reliable and of general nature since it was also found in a second study using a different choice-reaction task with a more direct stimulus-response relation. The results suggest that the overt response requires a higher activation level in older vs. young subjects; this extra-activation needs time and hence prolongs reaction time with aging.