Perceptual and motor factors mediate the bowed spatial position effect in aging

27 young, 40 middle-aged, and 25 older participants (mean ages 30, 49, and 68 years, respectively) performed a 4-choice reaction time task requiring discrete button-press responses to visual stimuli. Two levels of stimulus discriminability were factorially combined with two levels of response discriminability. Results indicated that advancing age was associated with increasingly longer reaction times to inner than to outer stimulus-response positions, and that stimulus and response discriminability factors independently modulated this effect to the same degree, with the more difficult discriminations showing earlier age-related impairments. Hence, the results implicate both perceptual and motor processes as loci for the bowed spatial position effect in ageing. The implications of these findings for theories of ageing and motor control are discussed.