A SNARC-like effect for visual speed

Numerical and nonnumerical magnitudes can be represented along a hypothetical left-to-right continuum, where smaller quantities are associated with the left side and larger quantities with the right side. However, these representations are flexible, as their intensity and direction can be modulated by various contextual cues and task demands. In four experiments, we investigated the spatial representation of visual speed. Visual speed is inherently connected to physical space and spatial directions, making it distinct from other magnitudes. With this in mind, we explored whether the spatial representation of visual speed aligns with the typical left-to-right orientation or is influenced dynamically by the movement direction of the stimuli. Participants compared the speed of random dot kinematograms to a reference speed using lateralised response keys. On each trial, all dots moved consistently in one single direction, which varied across the experiments and could also vary from trial to trial in Experiments 2 and 4. The dot movements were left-to-right (Experiment 1), random across a 360° spectrum (Experiment 2), right-to-left (Experiment 3), and random left-to-right or right-to-left (Experiment 4). The results supported a relatively stable left-to-right spatial representation of speed (Experiments 1-3), which was compromised by mutable motion directions along the horizontal axis (Experiment 4). We suggest that representing stimuli as belonging to a single set rather than different sets, may be crucial for the emergence of spatial representations of quantities.