Locating information-processing deficits of mildly mentally retarded young adults

When 2 nonoperative lights were added to each of the ends of a stimulus display in a 4-choice reaction time (RT) task, the RTs of mentally retarded and nonretarded subjects were slowed to about the same extent. When 2 nonoperative keys were added to each of the ends of the response display, retarded subjects' RTs were slowed more than those of the nonretarded subjects. The combined effects of the conditions involving nonoperative lights and keys were not found to be additive for either group of subjects, suggesting that, separately, each condition resulted in an increase in stimulus-response incompatibility in addition to an increase in the difficulty of stimulus identification or response selection.