Dual-task interference with equal task emphasis: Graded capacity sharing or central postponement?

When people are presented with multiple tasks at the same time, each involving a rapid choice of actions, substantial interference usually occurs. In the laboratory, this interference between tasks is often measured using the psychological refractory period (PRP) design, where two different stimuli requiring speeded responses are separated in time by a variable stimulus onset asynchrony (SOA). As the SOA becomes shorter, responses to the second stimulus are slowed dramatically, even with seemingly very easy tasks (e.g., choice response time [RT] tasks involving trivial perceptual discriminations). This slowing, known as the PRP effect, is very robust; it has been found in numerous experiments using various combinations of tasks, input modalities, and output modalities (see Welford, 1952). Several different converging measures point toward the conclusion that the PRP effect is due largely to a central bottleneck; that is, as shown in Figure 1, only one central operation takes place at a time (for recent reviews, see Lien & Proctor, 2002, and Pashler & Johnston, 1998; for a contrary view, see Meyer et al., 1995). Central operations include response selection and perhaps also memory retrieval (Carrier & Pashler, 1995), memory consolidation (Jolicœur & Dell’Acqua, 1998), mental rotation (Ruthruff, Miller, & Lachmann, 1995), and lexical processing (McCann, Remington, & Van Selst, 2000), but generally do not include input/output operations such as stimulus identification and response execution. Although the evidence for a central bottleneck appears to be strong, it has primarily come from PRP experiments that emphasized Task 1 over Task 2. There are two ways in which PRP studies emphasize Task 1. First, the Task 1 stimulus is almost always presented before the Task 2 stimulus (hence the labels Task 1 and Task 2). Subjects might therefore infer that Task 1 is more important than Task 2. Second, PRP instructions often place explicit emphasis on the speed of Task 1. For example, subjects might be told to “respond to Task 1 as fast as possible.” These instructions are intended to encourage subjects to emit their Task 1 response as soon as it has been selected. Absent these instructions, subjects often withhold their Task 1 response until their Task 2 response is also ready. This responsegrouping strategy (Borger, 1963) is undesirable from the experimenter’s point of view because it makes it difficult to determine when the critical Task 1 operations actually finished. The emphasis on Task 1 raises the question of whether the central bottleneck revealed by PRP studies is obligatory (i.e., due to some structural limitation) or merely voluntary. Subjects might have the latent ability to overlap