Synesthesia is a human neurological variant, a perceptual difference in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. Individuals who have a history of such experiences over the course of their lifetime are known as synaesthetes. Increased cross-talk between dedicated regions of the brain specialized for different functions may account for the many types of synesthesia. For example, grapheme-color synesthesia might be due to cross-activation of the grapheme-recognition area of the brain and the color recognition area. In support of this attribute are cognitive assays showing that people with grapheme-color synaesthesia are able to identify the color of a grapheme in their peripheral vision even when they cannot consciously identify the shape of the grapheme.
Another explanation for synesthesia is focused on disinhibited feedback, or reduced inhibition along existing feedback pathways. Typically, excitation and inhibition are balanced, but if normal feedback is not inhibited (as expected), then signals feeding back from late stages of multi-sensory processing might influence earlier stages. Therefore, an aroma could instigate a visual perception. Dr. Richard Cytowic, and Dr. David Eagleman find support for the disinhibition idea in the so-called acquired forms of synesthesia that occur in non-synaesthetes under certain conditions such as traumatic brain injury and cerebral vascular accident.
Some forms synesthesia can be scientifically validated via the Synaesthesia Battery, which can be found here.