In the example below, in the left column, you can see two apples—one green, one red—appearing as a single solid color with a black cross over them. Now, if we remove the thick black lines, each apple clearly appears divided into four quarters of different shades—even though they are exactly the same in both columns, with and without the black cross.
Here’s a trickier version: with the black grid in place, the large green square appears uniform. Take the grid away, and it turns into a full-on checkerboard…
This occurs because in our visual system there is a mechanism that enhances the contrast of the outline of an object relative to its background: it is called ‘lateral inhibition’. Thus, even small differences in brightness between adjacent zones or objects are deliberately increased by the brain to better distinguish them. So, when the brightness boundaries of the color zones are concealed, the cues the brain needs to trigger the lateral inhibition mechanism no longer exist and consequently we become “blind” to variations in color brightness. The illustrations above have been taken from my book “Drawing Optical Illusions” who was translated in many languages. The book is still available from Amazon.
This eye-catching, colorful book is designed to inspire those artists interested in optical illusions and as an invaluable reference tool for people who to wish to create them. In clear, easy steps, this book shows people how to design a range of original and classic optical illusions and even how to create their own personalized illusions.