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

Recent models disagree on bottleneck localization in overlapping tasks in w hich participants make 2 responses to 2 stimuli presented with varying stim ulus onset asynchrony (SOA). Key evidence for a peripheral bottleneck is th at response selection difficulty and SOA underadditively affect reaction ti mes in Task 2 if response selection difficulty is varied by increasing the number of alternatives from I to 2. Three experiments examined an alternati ve explanation that suggests that differences in response preparation and s timulus anticipation between 1 and 2 alternative conditions are responsible for this effect. A manipulation of preparational state and number of antic ipatory reactions directly affected the likelihood of obtaining the underad ditive interaction. In the absence of advance response preparation and stim ulus anticipation, an additive interaction was found, providing key evidenc e for a central bottleneck in overlapping-task performance.