Lateral inhibition is often used to explain the Hermann grid illusion, but more recent evidence suggests that this might not be why the illusion happens. This theory suggests that the brightest at the intersections forces retinal cells to adjust the intensity. Lateral inhibition happens when the excitation of surrounding neurons inhibits a neuron's response to a stimulus. Glover EM, Lauzon O. Using a contrast illusion to teach principles of neural processing. J Undergrad Neurosci Educ. 2018;17(1):A81–A88. Ninio J. Geometrical illusions are not always where you think they are: a review of some classical and less classical illusions, and ways to describe them. Front Hum Neurosci. 2014;8:856. doi:10.3389/fnhum.2014.00856 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

Kreiner WA. Algebraic functions describing the Zöllner illusion. Open Access Repositorium der Universität Ulm. 2012. doi:10.18725/OPARU-2597 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.

Two Faces

Bachy R, Zaidi Q. Troxler fading, eye movements, and retinal ganglion cell properties. Iperception. 2014;5(7):611-612. doi:10.1068/i0679sas 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. Wagemans J, Elder JH, Kubovy M, et al. A century of Gestalt psychology in visual perception: I. Perceptual grouping and figure-ground organization. Psychol Bull. 2012;138(6):1172–1217. doi:10.1037/a0029333 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 Like many optical illusions, different theories have been proposed to explain exactly why this happens.

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?

Candle

Bertamini, M. Lilac chaser illusion. In: Programming Visual Illusions for Everyone. Vol 2. Springer International Publishing; 2017: 153-161. 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. 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. 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