What your brain interprets isn't always exactly what your eyes see. The visual cortex is the area of the brain responsible for interpreting the 12 different tracks of information we receive from our eyes. Because our visual cortex is constantly interpreting data, the brain sometimes adds information and makes shortcuts based on what it expects to be there based on past experience. Optical illusions like the ones to the left help reveal some of the errors our brains make while interpreting data.
As designers, we can influence this phenomenon through use of shapes and colors.