Dual-task slowing and the effects of cross-task compatibility.

Recent studies have shown that visual encoding requires central processing, evidenced in the form of dual-task interference on response processes of a reaction time (RT) task. We report two experiments examining such dual-task interference: (1) at global level, to establish that visual encoding indeed requires central processing; and (2) at code level, to see whether two tasks may share the same representational codes. To this end, we manipulated the compatibility relation between the response codes for an auditory choice RTtask and the stimulus codes for a logically independent visual motion detection task. The results showed significant dual-task slowing of the response for the auditory task, demonstrating the interference at a global level. The effects of cross-task compatibility (CTC) were obtained at the short stimulus onset asynchrony (SOA), showing that the RTs were faster for compatible trials. Furthermore, the CTC effect remained even when the visual stimulus did not need to be "consolidated" (Jolicoeur & Dell'Acqua, 1998) for reporting, while the response postponement effect was greatly reduced. We interpret these results as indicating that the present two tasks share both "central" processes and common representational codes, but that these two levels of dual-task interference can be dissociated.