Illusory distance modulates perceived size of afterimage despite the disappearance of depth cues. Laterality effects in the spinning dancer illusion: The viewing-from-above bias is only part of the story. Decoding the subjective rotation direction of the spinning dancer from fMRI data. Using a contrast illusion to teach principles of neural processing. Making sense of the Hermann Grid illusion. S1 simple-cell theory proposes that the illusion is caused by how S1-type simple cells in the primary visual cortex respond to certain visual stimuli. Evidence supporting this theory includes the fact that the illusion is not dependent upon the size of the grid and that the illusion still occurs when the contrast of the image is reversed.

S1 simple-cell theory may be more helpful for understanding the illusion.

Lateral inhibition happens when the excitation of surrounding neurons inhibits a neuron's response to a stimulus. This theory suggests that the brightest at the intersections forces retinal cells to adjust the intensity.

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.

Like many optical illusions, different theories have been proposed to explain exactly why this happens. Notice how the dots at the center of each intersection seem to shift between white and gray? It's a very interesting grid that's a puzzle in itself.Sometimes we see things that aren't there, and the Hermann Grid illusion is a great example. Kreuzworträtsel: How's your German? Put it to the test by playing the daily Kreuzworträtsel from Hamburg's evening newspaper, Hamburger Abendblatts.This Spanish version of the crossword is fun and definitely a challenge, no matter how fluent you are. Crucigramas: El Pais is Spain's most-read newspaper and their website offers a daily crucigrama.If you want to test your French, try a daily mots-croisés puzzle from LCI, a TV station in France. Mots-Croisés: French crosswords are not only in a different language, but they also change the grid.Plus, you have to remember to type the words in the language given or the answer's wrong. The translating tool won't affect the puzzle itself. Use that to read directions and get a feel for the puzzle if you're not very fluent yet.

What is nice about these online puzzles is that you can use a tool such as Google Translate to transform the page into English. These free crossword games are also good vocabulary builders for language students. Finally, for those whose mother tongue is something other than English or those learning a second language, there's a challenging crossword puzzle for you.

Optical illusion games puzzles