Gaze at the central fixation spot. This will adapt your eyes to a repetitively brightening patch (above) and a repetitively dimming patch (below). Watch until both rectangles turn it into a steady gray. You should see an aftereffect of apparent dimming (above) and apparent brightening (below).Keep watching to build up the aftereffect.
This shows the existence of transient visual pathways selective for gradual changes of luminance.