When experimenters vary the timing between two intersensory events, and participants judge their simultaneity, an inverse-U-shaped psychometric function is obtained. Typically, this simultaneity function is first fitted with a model for each participant separately, before best-fitting parameters are utilized (e.g., compared across conditions) in the second stage of a two-step inferential procedure. Often, simultaneity-function width is interpreted as representing sensitivity to asynchrony, and/or ascribed theoretical equivalence to a window of multisensory temporal binding. Here, we instead fit a single (principled) multilevel model to data from the entire group and across several conditions at once. By asking 20 participants to sometimes be more conservative in their judgments, we demonstrate how the width of the simultaneity function is prone to strategic change and thus questionable as a measure of either sensitivity to asynchrony or multisensory binding. By repeating our analysis with three different models (two implying a decision based directly on subjective asynchrony, and a third deriving this decision from the correlation between filtered responses to sensory inputs) we find that the first model, which hypothesizes, in particular, Gaussian latency noise and difficulty maintaining the stability of decision criteria across trials, is most plausible for these data.
Public Significance StatementPsychologists have made their competing theories about how humans are able to perceive the relative timing of events concrete by formulating mathematical models that attempt to describe behavior in specific experimental tasks. Here, we focus on one such task and show that people's reports about simultaneity are inherently subjective, as implied by several current models. We also find that the bestperforming of these models explains inconsistencies when responding repeatedly to objectively identical pairs of events by positing inconsistencies in both the time it takes for neural messages to propagate through the brain and how those messages are then interpreted to form a decision.