The effect of aging on the stages of processing in a choice reaction time task.

Two experiments were conducted to determine the effect of aging on the encoding and response selection stages of a choice reaction time task. In Experiment I, the Sternberg Additive Factor Method was used. Young (average age 20) and old (average age 73) subjects pressed a left- or right-hand key in response to the onset of an X or an O. The discriminability of these characters was manipulated so as to affect primarily the stimulus encoding stage. A tone presented simultaneously with the characters provided an irrelevant directional cue which previous research had indicated affects the response selection stage. The pattern of interactions of these manipulated variables with age suggested that aging affected stimulus encoding but not response selection. Results of Experiment II suggested that reducing stimulus discriminability may affect information processing prior to the encoding stage but confirmed the major conclusion of Experiment I that the encoding stage was the primary locus of the slowing which accompanied aging.