Do response modality effects support multiprocessor models of divided attention?

Some studies have suggested that dual-task interference is greatly reduced when tasks requiring very different types of responses (e.g., manual and vocal) are combined. However, in those studies, the order of stimuli varied unpredictably. In Experiments 1 and 2, variable stimulus order greatly inflated interference between two manual tasks, whereas interference between a manual and a vocal task was only slightly exacerbated. However, central interference (the psychological refractory period) persisted even with the manual/vocal combination. Selection of 2 manual responses with unknown stimulus order may require a special strategy to preclude intertask intrusion errors. Experiment 3 demonstrated that such errors could be provoked with speed stress. Together, these results reconcile response modality effects with the response selection bottleneck model for dual-task interference (once it is suitably amended).