Response speed separates nature from nurture in time processing.

To get a concrete representation of its intangible flow, humans frame time in space. How this conceptual superimposition of space over time takes place remains unclear. Here we exploited the STEARC effect (Spatial-Temporal Association of Response Codes), which consists of faster coding of “short“ durations with motor responses in the left side of space and of “long” durations with responses in the right side. The STEARC shows that in western cultures time is mentally represented as flowing from left-to-right. In two experiments with adult participants, we analysed the STEARC as a function of response speed. In both the sub- and supra-second ranges, we found no STEARC effect at fast responses and a significant STEARC only at slow responses. This first demonstrate that space slowly takes over a faster non-spatial processing of time flow. These two mechanisms of time coding co-exist, interact dynamically, and their behavioral effects can be disentangled.