Pashler, H. & Christian, C. (Technical Report). Bottlenecks in planning and producing vocal, manual and foot responses. Center for Human Information Processing Technical Report 1994.
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When subjects perform two sensorimotor tasks (T1 & T2) close together in time, T2 response selection is often delayed. Five experiments examined whether T2 response selection waits for both selection and execution of the response in T1. In the first two experiments, T1 required a sequence of vocal responses (R1). When the sequence length in R1 was increased, R1 took longer to complete (unsurprisingly); however, the (manual) second response (R2) was little affected, and R2 usually occurred while the R1 sequence was underway. Similar results were found when T1 involved a sequence of vocal responses and T2 required a foot movement (Experiment 3). However, when R1 was a manual sequence, and R2 involved either manual or foot movements, R2 was substantially delayed, and usually occurred after R1 was completed. When T1 required arm reaching, variability in reaction time (but not movement time) was associated with slowing of R2. The results argue for (1) a central bottleneck that does not include response production and (2) a separate response- production bottleneck specific to production of manual or foot responses (and likely to play no role in usual dual-task laboratory experiments).