To capture and communicate the flow of time, humans represent time using space (Tversky et al., 1991; Lakoff and Johnson, 1999; Boroditsky, 2000; Casasanto and Boroditsky, 2008; Bonato et al., 2012). For example, metaphors like "I left my past behind me" are derived from sensorimotor events linked to forward locomotion (Clark, 1973). Other spatial representations of time stem from sensorimotor experiences related to cultural habits (Núñez and Sweetser, 2006; Casasanto and Bottini, 2014). In left-to-right reading cultures, the past is mentally placed to the left of the future and vice-versa in right-to-left reading cultures (Fuhrman and Boroditsky, 2010; Ouellet et al., 2010; Boroditsky et al., 2011; Callizo-Romero et al., 2020; Pitt and Casasanto, 2020). This phenomenon extends to the representation of time durations. The Spatial Temporal Association of Response Codes effect (STEARC) shows that when visual or acoustic stimuli of various durations are presented, humans belonging to left-to-right reading cultures classify faster short durations with motor responses in the left side of space and long durations with responses in the right side (Space-Time Compatible Condition) rather than vice-versa (Space-Time Incompatible Condition; Vallesi et al., 2008, 2011; Conson et al., 2008; Ishihara et al., 2008). The STEARC is considered the most compelling evidence that the human brain represents time as intrinsically flowing along the spatial direction of reading and scanning habits and that short durations are automatically coded as if they were on the left side of space, while long durations as if they were in the right side.

Spatial compatibility effects between the side of the stimulus and that of the motor response that are found when the position of the stimulus is not relevant to the task, as for the case of duration judgments, are strongest at short RTs and decay at long RTs (De Jong et al., 1994; Rubichi et al., 1997). This has been taken as evidence that, notwithstanding its task irrelevance, the brain automatically codes stimulus position (De Jong et al., 1994; Rubichi et al., 1997). Here, by investigating the relationship between the speed of RTs and the strength of the STEARC, we wished to check whether the brain automatically and intrinsically codes the flow of time in spatial terms so that the STEARC is already found at short RTs or whether the spatial representation of time develops progressively so that the STEARC is found at long RTs though not at short ones. We anticipate that when the relationship between the strength of the STEARC and the speed of motor responses is considered, the spatial representation of time makes its appearance only at slower RTs and that, in contrast, the brain takes fast decisions on the duration of stimuli without resorting to spatial heuristics of time. These results significantly modify the assumption that time is intrinsically and automatically represented in spatial terms and show that the brain needs time to frame time in space.