 It may be that both mechanisms are possible causes of synesthesia, but that one or the other is present in differing degrees between different synesthetes, or for different types of synesthesia.Functional neuroimaging studies using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have demonstrated significant differences between the brains of synesthetes and non-synesthetes (although some studies failed to find such differences).The first such study used PET to demonstrate that some regions of the visual cortex (but not V4) were more active when auditory word → color synesthetes listened to words compared to tones .