Changing speed along the mental line: A SNARC-like effect for visual acceleration.

Humans tend to map magnitudes onto space, with smaller values associated with the left and larger values with the right. Recent evidence suggests that such spatial mappings extend beyond numbers to include visual speed. Whether a second-order motion property, such as visual acceleration, is also spatially represented remains unclear. We addressed this question in two experiments using a centrally presented random dot kinematogram (RDK). Participants indicated, via lateralised key presses, whether the stimulus decelerated or accelerated relative to an initial constant-speed reference. In Experiment 1, all dots moved coherently in a single direction randomly selected on each trial. In Experiment 2, dot directions were randomised at the element level, creating an isotropic motion field. A distance effect (i.e., faster responses for larger absolute acceleration values) emerged in both experiments, indicating that acceleration was processed as a continuous magnitude. Crucially, a left-to-right spatial mapping emerged only in Experiment 2. These findings provide evidence that visual acceleration can be spatially represented, but that this mapping depends on the structure of the motion signal.