Radial frequency (RF) trajectories are a new class of stimuli that have been developed to study the visual perception of periodic motion (Or et al., 2011). These stimuli are described by a moving dot that traces a distorted path through space with periodic radial deformations whose frequency, amplitude, and phase can be independently specified. Here, we extend Or et al.'s findings by investigating how the discrimination of RF amplitude changes as a function of different reference amplitudes in a two-interval forced choice task. Using an RF3 trajectory (a pattern with three cycles of deformation along its trajectory), increment thresholds were measured at six different reference amplitudes: Detection (discriminating a circle from RF3), 1X (discriminating a pair of RF3 patterns, with the amplitude of one member of this pair set to (1X) threshold obtained from the detection condition), 2.5X, 5X, 10X, and 15X. Data show that sensitivity to changes in amplitude improves at 2.5X by a factor of about 2, recovers to detection threshold levels at 5X, and continues to rise at 10X and 15X. These results generalize across both radial frequency and the angular speed of the trajectory, and persist with low contrast trajectories. Our findings point to the existence of a neural mechanism that is sensitive to deviations from circular motion trajectories.