Making two responses to a single object: implications for the central attentional bottleneck.

Experiments with two stimuli and two responses have revealed a central attentional bottleneck and pointed to response selection as its primary locus; however, little has been said about the underlying reasons for this bottleneck. Here we explore these reasons. In the first three experiments, Ss made two separate responses to different aspects of the same object. Interference between selection of the responses persisted, ruling out the possibility that the dual-task bottleneck is caused by the input to the response-selection mechanism being limited to one object at a time. The next four experiments examined what happens when two responses are made to the same attribute of a single object. These experiments show that only one response selection occurred. Hence, the central mechanism is not limited to picking one motor action at a time. Several possible theories about the nature of the bottleneck are discussed.