Non-decision time: the Higgs boson of decision

Generative models of decision now permeate all subfields of psychology, cognitive and clinical neuroscience. To successfully investigate decision mechanisms from behaviour, it is necessary to assume the presence of delays prior and after the decision process itself. However, directly observing this “non-decision time” from behaviour long appeared beyond reach, the field mainly
relying on models to estimate it. Here, we propose a biological definition of decision that includes perceptual discrimination and action selection, and in turn explicitly equates non-decision time with the minimum sensorimotor delay, or “deadtime”. We show how this delay is directly observable in behavioural data, without modelling assumptions, using the visual interference
approach. We apply this approach to 11 novel and archival datasets from humans and monkeys gathered from multiple labs. We validate the method by showing that visual properties (brightness, colour, size) consistently affect empirically measured visuomotor deadtime, as predicted by neurophysiology. We then show that endogenous factors (strategic slowing, attention) do not affect visuomotor deadtime. Therefore, visuomotor deadtime consistently satisfies widespread selective influence assumptions, in contrast to non-decision time parameters from model fits. Last, contrasting empirically observed visuomotor deadtime with non-decision
time estimates from the EZ, DDM and LBA models, we conclude that non-decision time parameter from these models is unlikely to consistently reflect visuomotor delays, neither at a group level nor for individual differences, in contrast to a widely held assumption.