What mechanisms mediate prior probability effects on rapid-choice decision-making?

Puri, Rohan; Hinder, Mark R.; Heathcote, Andrew · 2023 · PLoS ONE

DOI: 10.1371/journal.pone.0288085

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Summary

This study investigates the cognitive mechanisms underlying prior probability effects in rapid-choice decision-making, specifically challenging the conventional assumption that prior probability selectively influences response thresholds. While it is traditionally believed that higher prior probability lowers the evidence threshold required for a decision, recent evidence suggests non-selective effects on sensory evidence accumulation and non-decision processes. The authors aimed to determine whether prior probability also affects the rate of evidence accumulation and non-decision time (encompassing stimulus encoding and response execution), and whether these effects vary by biasing procedure (block-wise vs. trial-wise) and age group. The researchers conducted an experiment with 21 healthy young adults and 20 healthy older adults using a choice response-time task. Participants made left- or right-hand responses to imperative stimuli preceded by a warning signal indicating a 70% probability of the upcoming stimulus direction. Prior probability was manipulated in two ways: block-wise bias, where the probable response remained constant across a block of trials, and trial-wise bias, where the probable response varied pseudorandomly from trial to trial. Data were analyzed using the racing diffusion evidence-accumulation model, a sequential sampling model that separates decision time (evidence accumulation) from non-decision time. Hierarchical Bayesian techniques were employed to fit the model, testing seven candidate models to identify which parameters (threshold, accumulation rate, or non-decision time) were influenced by prior probability. The results demonstrated that response times for correct responses were slower on incongruent trials (where the imperative stimulus contradicted the warning signal) compared to congruent trials. Older adults exhibited slower but more accurate responses than young adults. Crucially, evidence-accumulation modeling revealed that prior probability effects were not limited to response thresholds. Instead, the data favored models where prior probability influenced both response thresholds and non-decision time. This indicates that prior probability biases decision-making through multiple mechanisms, including the facilitation of response execution processes, rather than solely by adjusting the amount of evidence required for a decision. These findings cast doubt on the selective threshold influence assumption within the racing diffusion model, suggesting that prior probability effects are mediated by a combination of decision-based and non-decision-based processes. The study highlights that prior probability can prime motor responses, reducing the time required for response execution. By demonstrating non-selectivity in how prior probability influences decision-making, the research contributes to a more nuanced understanding of cognitive control and motor preparation, particularly regarding how these processes are modulated by age and the flexibility of bias information.

Key finding

Evidence-accumulation modeling revealed that prior probability effects on rapid-choice decision-making are mediated by both response thresholds and nondecision times, rather than selectively affecting thresholds alone.

Methodology

lab_experiment

Sample size: 41

Provenance

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

Topics

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• decision making risk perception