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

Rapid-choice decision-making is influenced by various factors, one of which is the prior probability of various response alternatives (‘bias’). Conventionally, prior probability effects are assumed to selectively manifest in terms of the amount of evidence required to trigger a decision (‘response threshold’). However, the generalizability of this assumption remains untested.Healthy young (n = 21) and older (n = 20) adults completed a choice response-time task requiring left- or right-hand responses to imperative stimuli (IS). Prior probability was manipulated using a warning signal (WS) that informed participants that a particular response was 70% likely (i.e., the IS was either congruent or incongruent with the WS). In addition, prior probability was either fixed for blocks of trials (block-wise bias) or varied from trial-to-trial (trial-wise bias). Response-time and accuracy data were analysed using evidence accumulation modelling to test the selective influence assumption.Response-times were slower on correct incongruent, than correct congruent, trials and older adults were slower but more accurate than young adults. From an evidence accumulation perspective, a) models that allowed bias to vary over response threshold and nondecision time had the best fit, and b) removing response threshold and nondecision time bias effects lead to significant and consistent model misfit whereas removing the evidence accumulation rate bias effect lead to relatively lesser and inconsistent model misfit.Overall, the current results cast doubt on the selective influence assumption of prior probability by demonstrating a robust effect on nondecision time, having implications for future theoretical and applied research.