Using speed to think about space and time

There is a current debate on how time and space are represented in the brain, with some researchers advocating the view that time and space are represented within a generalized magnitude system and others arguing that temporal representations are based on spatial representations. The observation of asymmetric space-time interference, with time perception being more influenced by space than vice versa, has often been interpreted as reflecting a hierarchical representational structure. Here we explore how the factor of speed, which is inherent in many experiments on space-time interference (e.g., growing lines, moving dots), can contribute to the observed asymmetry. This idea is tested in two experiments, directly comparing duration and length judgments for growing and static lines (Experiment 1) and for growing and shrinking lines (Experiment 2). Experiment 1 demonstrates that the introduction of speed generally increases space-time interference, and that this increase is especially pronounced for the space-on-time effect, leading to stronger asymmetry. Moreover, Experiment 2 shows that, when the correlation between line length and speed is reversed (i.e., shorter lengths are coupled with higher stimulus speed), the space-on-time effect reverses as well. We conclude that the observed asymmetric space-time interference in experiments using dynamic stimuli is primarily based on the processing of speed and does not constitute evidence for the idea of a hierarchical representational structure of space and time.