"<a href=""https://www.youtube.com/watch?v=tueAcSiiqYA&amp;t=16m20s"">16:20</a> Perceptual Hysteresis means  Perception depends on the recent past. Or in the case of morphing 1 into 2", your seeing a 1 longer and when watching the same sequence backward, so morphing 2 into 1, you see a 2 longer, there is no single frame where it switches.