The picture on the left depicts “Fraser’s Spiral”. If you place the mouse* over it, it becomes obvious that there are no spirals at all…
Fraser J (1908) A New Visual Illusion of Direction. Brit J Psych 2:307–320
Morgan MJ, Moulden B (1986) The Münsterberg figure and twisted cords. Vision Res 26:1793–1800
Popple AV, Sagi D (2000) A Fraser illusion without local cues? Vision Res 40:873–878
Parts of Fraser’s original paper have been made available by Hervé Varenneere.
*On a touch device, tap the image. To “untap”, tap on the icon top right.