The elements in a central rectangular region changed in synchrony according to a random sequence, while the elements in the background region changed independently. We downloaded several such movies from their web site, and simulated the effects of temporal lowpass and bandpass filtering. In the filtered movies, the target region's contrast fluctuated noticeably above and below that of the background. If a Gabor element undergoes a run of multiple shifts in one direction, its effective contrast is low due to the temporal averaging. Since the Gabor elements in the target region are synchronized, the effective contrast of the entire region fluctuates en masse, and from one moment to the next can be noticeably different than the background.
Similar results hold for bandpass temporal filters.
Lee and Blake's stimuli were cleverly designed to remove form cues from single frames and frame pairs. However, when one considers the full sequence, strong contrast cues can emerge due to the spatio-temporal filtering present in early vision. These cues may well explain the perception of form in these displays, thus obviating the need to posit special grouping mechanisms based on temporal synchrony.