When one race is not enough: a relay model explains multisensory response times

Humans typically respond faster to multisensory signals than to their unisensory components, a phenomenon known as the redundant signal effect (RSE). One of the earliest and most influential accounts, the race model, attributes the RSE to statistical facilitation, which arises from parallel, independent processing across sensory modalities. While this model captures some key features of the RSE, it frequently underestimates the observed speed-up leading to violations of the race model inequality (RMI), a benchmark used to test the models validity. To reconcile this discrepancy, we introduce the relay model, a minimal extension of the race architecture that incorporates cross-modal initiation. In this model, responses result from two sequential race processes, allowing a signal in one modality to trigger the onset of perceptual decision processing in another. This structure retains statistical facilitation as a core principle while introducing a single free model parameter that divides unisensory processing into gating and decision stages. Through simulations and fits to foundational empirical datasets, we show that the relay model captures both the magnitude and distributional shape of the RSE, including RMI violations. It also accounts for changes in the RSE under asynchronous stimulus onsets, a critical test in multisensory integration research. By extending the classical race model with minimal added complexity, the relay model offers a mechanistically explicit and biologically plausible framework for explaining the dynamics of multisensory decision-making. Author SummaryHow does the brain combine information from different senses, like hearing and seeing? For decades, researchers have found that people respond faster when signals from multiple senses occur together than when signals come from a single sense alone. Two main ideas have been proposed to explain this effect: one suggests that the senses operate independently, racing to trigger a response, while the other suggests that they cooperate to enhance processing. In this study, we introduce a new model of multisensory decision-making, called the relay model, which combines elements of both competition and cooperation. We propose that the brain uses a two-stage process: first, the faster sense initiates a second decision process. Then, in that second stage, the faster signal triggers a behavioural response. This relay-like mechanism accelerates responses through competition, while still allowing both senses to cooperate. By capturing key findings from classic experiments, the model offers a new perspective on how the brain combines multisensory information to guide behaviour.