 For example, the part of the human brain involved in processing visual input, called the visual cortex can be further subdivided into regions that are preferentially involved in color-processing (the fourth visual area, V4) or with motion processing, called V5 or MT.Based on this notion of specialized regions, some researchers have suggested that increased cross-talk between different regions specialized for different functions may account for different types of synesthesia.Since regions involved in the identification of letters and numbers lie adjacent to a region involved in color-processing (V4), the additional experience of seeing colors when looking at graphemes might be due to "cross-activation" of V4 .