For many decades, brain activity before a stimulus has been regarded as non-informative neuronal noise. Only In recent years, it has been recognized that such ongoing brain activity can have substantial influence on perception. In my talk, I will show that this ongoing - or prestimulus - activity influences perception rhythmically. This rhythmic pattern is mainly driven by the power and phase of neuronal oscillations. These results suggest a discrete mode of perception, similar to the operating mechanism of a video camera.