Chronometric Evidence for Central Postponement in Temporally Overlapping Tasks

When the stimuli from two tasks arrive in rapid succession (the overlapping tasks paradigm), response delays are typically observed. Two general types of models have been proposed to account for these delays. Postponement models suppose that processing stages in the second task are delayed due to a single-channel bottleneck. Capacity-sharing models suppose that processing on both tasks occurs at reduced rates because of sharing of common resources. Postponement models make strong and distinctive predictions for the behaviour of variables slowing particular second-task stages, when assessed in single- and dual-task conditions. In Experiment 1, subjects were required to make manual classification responses to a tone (S1) and a letter (S2), presented at stimulus onset asynchronies of 50, 100, and 400 msec, making R1 responses to S1 as promptly as possible. The second response, R2, but not R1, was delayed in the dual task condition, and the effects of two S2 variables (degradation and repetition) on R2 response times in dual- and single-task conditions closely matched the predictions of a postponement model with a processing bottleneck at the decision/response-selection stage. In Experiment 2, subjects were encouraged to emit both responses close together in time. Use of this response grouping procedure had little effect on the magnitude of R2 response times, or on the pattern of stimulus factor effects on R2, supporting the hypothesis that the same underlying postponement process was operating. R1 response times were, however, dramatically delayed, and were now affected by S2 difficulty variables. The results provide strong support for postponement models of dual-task interference in the overlapping tasks paradigm, even when response times are delayed on both tasks.