On doing two things at once: IV. Necessary and sufficient conditions: Rejoinder to Lien, Proctor, and Ruthruff (2003).

Four publications have demonstrated perfect timesharing of 1 simultaneous decisions. In all of these, (a). subjects were motivated to respond as quickly as possible, and with 1 exception that involved unusually extensive practice (E. Hazeltine, D. Teague, & R. B. Ivry, 2002, Experiment 4), (b). at least 1 of the 2 tasks was characterized by ideomotor (IM) compatibility, meaning that each task stimulus incorporated a component of the sensory feedback from its required response. Conclusions justified by these reports are that (a). the use of speed-stress instructions is necessary for perfect timesharing of 2 simultaneous decision tasks: (b). when both tasks are IM compatible, perfect timesharing requires little or no practice: (c). extensive practice is needed to achieve perfect timesharing when only 1 of the 2 tasks is IM compatible: and (d). very large amounts of practice can yield perfect timesharing of 2 decision tasks when neither is IM compatible.