Non-decision time: the Higg’s boson of decision

Generative models of decision now permeate all subfields of psychology, cognitive and clinical neuroscience. To successfully represent decision mechanisms, it is necessary to also assume the presence of delays for sensory and motor information to travel through the brain; but like the Higg’s boson in particle physics, directly observing this “non-decision time” from behaviour long appeared beyond reach. Here, we describe and apply a set of methods to empirically measure and characterise the properties of non-decision time in fast visually guided decisions (without requiring modelling assumptions). We gather 11 datasets from humans and monkeys from multiple labs and validate the method by showing that visual properties (brightness, colour, size) consistently affect empirically measured non-decision time, as predicted by neurophysiology. We then show that endogenous factors (pro-active slowing, attention) consistently do not affect non-decision time, in contrast to widespread reports based on model fits. Last, contrasting empirically observed non-decision time with estimates from the EZ, DDM and LBA models, we conclude that models cannot be generally trusted to provide valid estimates, either at a group level or for individual differences, and propose a hybrid approach that combines our empirical method with standard modelling.