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

Four publications have demonstrated perfect timesharing of 2 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.