Effects of aging, skill modification, and demand alternation on multiple-task performance.

The magnitude of age effects in single- and dual-tasks may be affected by the degree to which performance depends on well-learned skills that were previously developed. In addition, age-effects may be affected by the requirement to modify these skills and by attentional requirements emerging from the mutual relation of subtasks. Effects of skill modification and emergent attentional processes were examined in an experiment in which experienced subjects performed two perceptual-motor tasks, a vehicle steering task and car-following task in a driving simulator. Car-following was performed under two conditions of familiarity, determining whether or not a normal psychomotor routine had to be modified. In dual-task performance, the demand of subtasks was constant or alternating in counterphase. In general, the older subjects' performance did not differ from that of their younger counterparts, except when the single- or dual-task involved routine modification in car-following. Dual-task costs were basically manifested in the car-following task. Post hoc interpretations of the data indicated that the results were not completely consistent with the complexity hypothesis.