The Effects of Task Probability on Processing Mode in Dual-task Contexts

One of the major concerns about multitasking performance is whether dual-tasks are performed in parallel or in a strict serial sequence. The Response Selection Bottleneck (RSB) model assumes a structural bottleneck which can only process one task at a time, whereas the Central Capacity Sharing (CCS) model argues for parallel central processing between tasks. The present study manipulated the relative probability of responding to different tasks to investigate whether more parallel processing would be promoted when more background task responses were needed, using a Prioritized-Processing paradigm in which participants first responded to the primary task and responded to the background task only when no action was required for the primary task. Four experiments were conducted with one condition requiring mostly primary task responses and one condition requiring mostly background task responses. The task probability imbalance utilised for Experiments 1 and 3 was 80% vs. 20%, and for Experiments 2 and 4 was 60% vs. 40%. The task order presented in Experiments 1 and 2 was reversed in Experiments 3 and 4. The results indicated that more parallel processing was exhibited when more background task responses were needed, as reflected by a shallower slope function (compared to -1 slope) and a larger backward compatibility effect in the conditions with high probability of responding to the background task. These findings were found in four experiments and appear to support the CCS model. More generally, the results demonstrate that information processing is not limited by a structural bottleneck and highlight the incentive value of relative task probability in promoting parallel processing