One of the more fascinating illusions (in my opinion) involves color. It's a grid of squares with dots at the corner that look green... or yellow, to some people. I saw yellow in my peripheral vision but green when looking direct at the spots. Sakiyama T., Sasaki A., Gunji YP. Origin of Kanizsa triangle illusion. In: Rhee SY., Park J., Inoue A. (eds) Soft Computing in Machine Learning. Advances in Intelligent Systems and Computing, vol 273. Springer, Cham; 2014. doi:10.1007/978-3-319-05533-6_10 Weidner R, Boers F, Mathiak K, Dammers J, Fink R. The temporal dynamics of the Müller-Lyer illusion. Cerebral Cortex. 2010;20(7):1586-1595. doi:10.1093/cercor/bhp217 s winning entry is a novel take on the 19th Century Ebbinghaus Illusion. This new version is dynamic, which makes the effect much stronger. “It’s like the Ebbinghaus effect on steroids,” says Martinez-Conde. Just like the original, the illusion highlights that the brain always perceives the size of objects in the context of those that surround them. But if you continually vary this context, then the effect gets even stronger, she explains. Bertamini M. Ponzo illusion. In: Programming Visual Illusions for Everyone. Vision, Illusion and Perception, vol 2. Springer, Cham; 2018. doi:10.1007/978-3-319-64066-2_5

Song S, Liu Y, Zhang J. Decoding the subjective rotation direction of the spinning dancer from fMRI data. Proceedings. 2015;94171. doi:10.1117/12.2081570

Two Faces

Glover EM, Lauzon O. Using a contrast illusion to teach principles of neural processing. J Undergrad Neurosci Educ. 2018;17(1):A81–A88. Qian L, Liu S, Lei Q. Illusory distance modulates perceived size of afterimage despite the disappearance of depth cues. PLOS ONE. 2016;11(7):e0159228. doi:10.1371/journal.pone.0159228

Like many optical illusions, different theories have been proposed to explain exactly why this happens. Kreiner WA. Algebraic functions describing the Zöllner illusion. Open Access Repositorium der Universität Ulm. 2012. doi:10.18725/OPARU-2597 While we know that different areas of the brain deal with colour, form, motion and texture, how the brain encodes and combines this information into a coherent picture remains poorly understood. Sometimes we see things that aren't there, and the Hermann Grid illusion is a great example. Notice how the dots at the center of each intersection seem to shift between white and gray? My favorite puzzle is How to Become a Giant. This puzzle shows a man at one end of a hallway with column's and it looks like the man is extremely small in comparison with the column's. The next picture shows the man at the end of the hallway and he looks like a giant in comparison to the columns. The book shows you the illusion and then at the end of the book, it gives you the answer of why this illusion is possible.

Powell G, Bompas A, Sumner P. Making the incredible credible: afterimages are modulated by contextual edges more than real stimuli. J Vis. 2012;12(10). doi:10.1167/12.10.17 The chapter on ambiguous impossible pictures is great fun. This is the sort of thing where you look at a picture one way and see one image, or you might see a completely different image with a change in perception. It also has images where lines become ambiguous and trying to count how many of something are in the picture becomes impossible. The sort of impossible Physics in Escher drawings are much like some of these examples. Bachy R, Zaidi Q. Troxler fading, eye movements, and retinal ganglion cell properties. Iperception. 2014;5(7):611-612. doi:10.1068/i0679sas