In the examples on your right hand, you can perceive two apples with four different zones. But what happens if we conceal the brightness boundaries of the color zones with black thick lines? You may see the apples as having a uniform color!
Below is an advanced version of the visual puzzle. Covered with the black grid the green color looks uniform, without the grid, it looks like a 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.