Dissociations and dependencies between speed and accuracy: Evidence for a two-component theory of divided attention in simple tasks

Pashler, Harold · 1989 · Cognitive Psychology

DOI: 10.1016/0010-0285(89)90016-9

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Summary

This paper addresses the theoretical disconnect between two distinct forms of divided attention costs: interference in performing multiple simple tasks simultaneously and capacity limitations in processing multiple stimuli within a single task. Pashler argues that the term "attention" is misleading when applied as a single resource or mechanism. Instead, he proposes a two-component theory positing that divided attention costs arise from two separate sources: (1) lingering mutual interference during perceptual processing of complex visual displays, which has resource-like properties, and (2) discrete queueing at the response selection stage, which has bottleneck-like properties. The study aims to empirically distinguish these components and reject unified accounts such as late-selection theory or general capacity sharing models. To test this theory, Pashler conducted six experiments examining the effects of stimulus onset asynchrony (SOA) on speed and accuracy in dual-choice tasks. Experiments 1 and 2 involved a first task requiring a choice response to a tone and a second task involving complex perceptual decisions (digit identification or conjunction search) with unspeeded responses. Experiments 3 and 4 used the same tasks but required speeded manual or vocal responses for the second task. Experiments 5 and 6 involved both tasks requiring complex visual displays, varying whether the first task response was speeded or unspeeded. The design manipulated SOA to observe how temporal overlap affected performance dependencies and interference patterns. The results revealed distinct dissociations based on task demands. In Experiments 1 and 2, reducing SOA had negligible effects on second-task accuracy, and performance across tasks was virtually independent, indicating no bottleneck at response selection when responses were unspeeded. However, in Experiments 3 and 4, requiring speeded responses produced dramatic interference and strong positive dependencies between reaction times, supporting the response selection bottleneck hypothesis. In Experiments 5 and 6, where both tasks involved complex visual displays, SOA reductions caused dramatic interference but no dependencies between performance metrics, regardless of response speed. This pattern indicates that perceptual processing interference occurs independently of the response selection bottleneck. These findings reject pure late-selection accounts, which deny perceptual capacity limits, and general capacity sharing models, which attribute all interference to a common resource pool. The data support the two-component theory, demonstrating that perceptual capacity limits and response selection postponement are distinct mechanisms. The significance of this work lies in clarifying the architecture of human information processing, suggesting that "attention" is not a unitary entity. Instead, divided attention costs are heterogeneous, arising from either graded resource sharing in perceptual processing or discrete serial processing in central decision-making, depending on the specific demands of the tasks.

Key finding

Divided attention costs arise from two distinct mechanisms: perceptual processing interference that affects accuracy and response selection queueing that affects speed, rather than a single shared resource.

Methodology

lab_experiment

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Topics

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• dual task multitasking
• attention
• dual task performance
• temporal

Information type

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• Empirical Findings: behavioral performance data