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

Ruthruff, Eric; Pashler, Harold; Hazeltine, Eliot · 2003 · Perception & Psychophysics

DOI: 10.3758/bf03194816

archive: archived pipeline: cataloged verified

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Summary

This study investigates whether dual-task interference arises from a structural central bottleneck or from strategic capacity sharing. Previous research using the psychological refractory period (PRP) design suggests a central bottleneck, where only one central operation occurs at a time. However, these studies typically emphasized Task 1, raising the possibility that the bottleneck is a voluntary strategy rather than a structural limitation. The authors tested the "graded capacity sharing" hypothesis, which posits that central operations can proceed in parallel with limited resources, against the central bottleneck model. They conducted two experiments using equal task emphasis and distinct input/output modalities to isolate central processing. In Experiment 1, 32 participants performed a vocal tone-classification task and a manual character-identification task simultaneously. The design included varied stimulus onset asynchronies (SOAs) and manipulated stimulus-response (S-R) compatibility for the character task. The authors analyzed interresponse intervals (IRIs) and compatibility effects. The central bottleneck model predicted a multimodal IRI distribution with distinct components for serial processing orders and response grouping, along with specific carryover effects of compatibility. Conversely, the capacity-sharing model predicted a broad, unimodal IRI distribution and magnified compatibility effects at short SOAs due to resource division. The results showed that subjects tended to either group responses or respond to one task well before the other, producing an IRI distribution consistent with the central bottleneck model rather than the broad distribution expected under equal capacity sharing. Furthermore, S-R compatibility effects remained roughly constant across SOAs. Crucially, at short SOAs, compatibility effects on the character task carried over onto the response times of the tone task. This pattern of full carryover and constant compatibility effects is difficult to reconcile with graded capacity sharing, which would predict partial carryover and magnified effects. Instead, the findings align with the predictions of a structural central bottleneck, where central operations are strictly serial. The study concludes that dual-task interference is likely due to a structural limitation in central processing rather than a strategic choice or flexible capacity sharing, even when tasks are given equal emphasis. The evidence supports the existence of a central bottleneck that prevents parallel central operations, challenging models that allow for graded resource allocation. This implies that the human cognitive system has inherent structural constraints on processing multiple tasks simultaneously, regardless of instructional emphasis or practice levels in novel tasks.

Key finding

Dual-task performance under equal task emphasis is characterized by serial processing or response grouping rather than parallel capacity sharing, supporting the existence of a structural central bottleneck.

Methodology

lab_experiment

Sample size: 32

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• dual task multitasking
• dual task performance
• capacity resource theory
• task switching