As the moving ramp waveforms move back and forth they appear to change in brightness. As they move to the right, the upper field seems to brighten and the lower field seems to dim. A fixed retinal receptor viewing the upper half will see continuous brightening ramps punctuated by sudden drops. Probably, visual nonlinearities reduced the effectiveness of the sudden drops, so the ramping brightness predominates. (Cavanagh & Anstis, Vision Research 1986)
When the stripes move steadily to the right, the upper field looks apparently brighter. Adapt for ~20s, then click the Pause button. You will see a leftward motion aftereffect, plus a ‘ramp aftereffect’ of apparent dimming in the upper half and apparent brightening in the lower half.