In the peripheral fields, however, flicker as slow as 15 to 20 Hz may appear continuous. In addition, the sensitivity to the modulation, or change in amplitude of the flickering light varies between foveal and peripheral vision. The fovea is not very sensitive to changes in light level. Therefore large changes in luminance must occur before an image on the fovea appears to flicker. In the periphery, even small changes in luminance can be detected.
Motion is simply one period of change in a receptor field.
If the object has a similar luminance to the background which will be viewed after the object moves, then the modulation or amplitude of the change is small and more likely to be detectable in the peripheral fields than in the central fovea. Rapid motion, on the other hand, represents high frequency of change and would be more likely to be detectable in the fovea.