Based on these data, two major theories have been proposed concerning the neural basis of synesthesia. Both theories start from the observation that there are dedicated regions of the brain that are specialized for certain functions. 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.