Perceptual cycles and waves
Various pieces of experimental evidence using both psychophysical and physiological (EEG) measurements have lead us (and others) to conclude that at least certain aspects of visual perception and attention are intrinsically rhythmic. For example, in a variety of perceptual and attentional tasks, the trial-by-trial outcome was found to depend on the precise phase of pre-stimulus EEG oscillations in specific frequency bands (between 7 and 15Hz). This suggests that there are “good” and “bad” phases for perception and attention; in other words, perception and attention proceed as a succession of cycles. These cycles are normally invisible, but in specific situations they can be directly experienced as an illusory flicker superimposed on the static scene. The brain oscillations that drive these perceptual cycles are not strictly spontaneous, but can also be modulated by visual stimulation, giving rise to perceptual "echoes" of the stimulation sequence. Further, these echoes propagate across the visual and cortical space with specific and robust phase relations. In other words, the perceptual cycles are actually perceptual waves.
Attention-driven discrete sampling of motion perception, PNAS, 2005 VanRullen-etal-2005.pdf
Visual Attention: A Rhythmic Process? Current Biology, 2013 VanRullen-CB2013.pdf