EEG reveals how space acts as a late heuristic of time

To compensate for its sensory intangibility, humans often rely on spatial metaphors, gestures, and visual tools to represent the passage of time. These spatial tools, i.e. heuristics, range from everyday practices--such as directional hand gestures to indicate past or future events--to more abstract scientific conceptualizations, such as the "curving of space-time" in the theory of relativity. Despite this widespread spatialization of time, it remains unclear to what extent space is an inherent component of the brains representation of time and its role in monitoring temporal durations. Here, we combine EEG-behavioral methods and neural network models of optimal decision-making to show that space is a late compensatory mechanism of time representation recruited when faster and non-spatial timekeeping mechanisms are suboptimally engaged. EEG analyses reveal a cascade-like process: spatial engagement in timekeeping follows the insufficient non-spatial encoding of time intervals, leading to delayed decisions on their length and slower response selection. Computational modelling further indicates that trial-by-trial fluctuations in the spatialization of time are explained by stochastic variations in the activity of the dopaminergic/noradrenergic (DA/NE) system and its interaction with the anterior cingulate cortex. These findings provide the first clear evidence of when, why, and how the brain recruits spatial mechanisms in the service of temporal processing and demonstrate that non-spatial and spatial timekeeping systems can be dissociated at both behavioural and electrophysiological levels.