After you have played with the waterfall illusion (or motion aftereffect) demo in the lecture (and also see the link below), describe your experience, explain briefly in your own words what a motion after effect is, and say why it occurs. I am not interested in your opinion but in what the science has to say about this.
Click on the above link. Click on Movement and Action in the left hand panel on the page that you come to. Click on demonstration of Motion Aftereffect.