Queuing or sharing? A critical evaluation of the single-bottleneck notion.

The model of a single central bottleneck for human information processing is critically examined. Most evidence cited in support of the model has been observed within the overlapping tasks paradigm. It is shown here that most findings obtained within that paradigm and that were used to support the model are also consistent with a simple resource model. The most prominent findings are the millisecond-for-millisecond slope at the left of the RT2-SOA curve, the high RT1-RT2 correlation, the additivity of the effects on RT2 of SOA and of the difficulty of selecting R2, and the washout of the effect of S2 discriminability on RT2 in a dual-task condition. In addition, the asymmetry of the effects of the dual-task requirement on RT1 and RT2 can be accounted for by the resource model provided that it assumes uneven allocation of resources, which is quite reasonable in view of the task asymmetry inherent in the demand characteristics of the paradigm. The same is true for two other findings that appear to support the single-bottleneck model-that in the dual-task condition, the demand of the first task affects equally RT1 and RT2 and that its effect on RT1 is the same as the corresponding effect in the single-task condition. Furthermore, the single-bottleneck model is hard to reconcile with a negative slope at the left of the RT1-SOA curve or a positive slope at the left of the IRI-SOA curve, unless augmented by ancillary assumptions that are yet to be substantiated. Representative data were fit by each of the models using its optimal set of parameters. Both models achieved quite good degrees of fit. It is further argued that since the overlapping tasks paradigm is heavily biased in favor of a speedy reaction to the stimulus that appears first, it is nonoptimal for testing the central bottleneck model. Finally, the bottleneck model is examined in terms of other scientific criteria.