When a wheel rolls along a flat surface, a point on its perimeter traces a cycloid trajectory. However, subjects perceive the point's path not as the cycloid, but as the curtate cycloid, containing loops where the point contacts the surface. This is the curtate cycloid illusion. I hypothesize that the illusion occurs because the cognitive system does not have sufficient activation, or capacity, to both maintain an updated representation of the wheel's translation and compute its instant centers, the point about which the wheel is rotating at a given instant. This hypothesis is supported by showing that illusion susceptibility is decreased when the competing instant center demand is reduced, either by giving subjects practice at instant center computation (Experiment 1) or by eliminating the contour containing the instant centers subjects are most likely to compute (Experiment 2). Experiment 3 demonstrates that heightened instant center demands have less effect on illusion susceptibility when they are confined to irrelevant portions of the wheel's contour. A general form of the capacity account may explain illusions in the perception of many kinematic systems and point the way toward theoretical unity in the study of the perception of motion and events.

When a wheel rolls along a flat surface, a point on the wheel's perimeter follows a cycloid trajectory. Subjects, however draw the curtate cycloid, characterized by bottom loops, rather than the cycloid to depict the path that a point on a static wheel's perimeter would trace if the wheel were rolling. This is the curtate cycloid illusion. In Experiment 1, we show that animating the wheel does not dispel the illusion and that subjects high in spatial ability are less susceptible to the illusion than are low-spatials. Experiments 2, 3a, and 3b supported the hypothesis that the illusion occurs when subjects reallocate cognitive resources from processing a rolling wheel's translation to computing its instant centers, the point about which the wheel is rotating at a given instant in time. This reallocation occurs only when a reference point on the wheel's perimeter contacts and leaves the surface. We conclude that the illusion does not reflect fundamental perceptual biases, but rather stems from transient shortages of cognitive resources during the higher-level processing of the wheel's translation and rotation.