Warning: this is a somewhat subtle effect.
Gaze for at least 10 s at the fixation cross in the neighbouring dynamic image. This will adapt your eyes to a brightening patch (above) and a repetitively dimming patch (below). Then click the mouse, or press the spacebar or tap the panel to turn both patches into a steady gray. You should see an aftereffect of apparent dimming (above) and apparent brightening (below) for a few seconds.
This demonstrates the existence of transient visual pathways selective for gradual changes of luminance.
Based on Steward Anstis’s Demos with kind permission.
Anstis SM (1967) Visual adaptation to gradual change of intensity. Science 155:710–712
Arnold K, Anstis S (1993) Properties of the visual channels that underlie adaptation to gradual change of luminance. Vision Res 33:47–54