Processing differences between simple and choice reactions affect bottleneck localization in overlapping tasks

Recent models disagree on bottleneck localization in overlapping tasks in which participants make 2 responses to 2 stimuli presented with varying stimulus onset asynchrony (SOA). Key evidence for a peripheral bottleneck is that response selection difficulty and SOA underadditively affect reaction times in Task 2 if response selection difficulty is varied by increasing the number of alternatives from I to 2. Three experiments examined an alternative explanation that suggests that differences in response preparation and stimulus anticipation between 1 and 2 alternative conditions are responsible for this effect. A manipulation of preparational state and number of anticipatory reactions directly affected the likelihood of obtaining the underadditive interaction. In the absence of advance response preparation and stimulus anticipation, an additive interaction was found, providing key evidence for a central bottleneck in overlapping-task performance.