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 brain’s representation of time and its role in monitoring temporal durations. Here, in a combined EEG and behavioral study we have examined this issue by testing a sample of 30 young healthy adult volunteers (mean age: 23.3 y) in a task that required discriminating between short 1 sec. and long 3 sec. time intervals. In a compatible condition, “short” decisions were associated with left-side button presses and “long” decisions with right-side ones. In a incompatible condition the association between time durations and the side of lateral motor responses was reversed. Faster RTs in the compatible vs incompatible condition, i.e. a signature of the spatial representation of time, were only observed at slower RTs. This results point outs that space is a late compensatory mechanism of time representation that is recruited when faster and non-spatial timekeeping mechanisms are suboptimally engaged. EEG analyses reveal a cascade-like process: spatial engagement in timekeeping followed the insufficient non-spatial encoding of time intervals, leading to delayed decisions on their length and slower response selection. Computational models of meta-reinforcement learning suggest that trial-by-trial fluctuations in the speed of RTs and the corresponding spatialization of time durations could be 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.