It is not known for certain how synesthesia occurs. Brain imaging studies have confirmed that in people with color synesthesia (synesthetes) some black and white images tend to activate an area of the brain called V4 that is involved in the perception of color. Also some studies have found anatomical difference in the brains of synesthetes, leading to the theory that their sensory systems are not isolated from each other as in most people, but rather that there is some degree of interconnectivity among the neural pathways that generate sensation. Other theories posit that everyone is capable of synesthesia, but only a few individuals develop it to a significant extent. It is known, for example, that people under the influence of certain drugs can experience synesthesia. Also people who have lost a sensory function such as sight can experience flashes of color in response to touch.
Whatever causes it, synesthesia tends to run in families, although not all family members will have the same type. More interestingly, however, there seems to be a higher prevalence of synesthesia among creative people like artists. There is also some evidence that synesthetes tend to have better sensory processing and cognitive abilities compared to regular people.
An interesting question is whether synesthesia is an exclusively human trait or whether it is also found among animals. A couple of years ago researchers identified a gene in mice called α2δ3 (alpha-2, gamma-3) that is involved in the perception of pain. Researchers created a line of mice missing this gene and found the animals had decreased pain perception because the pain stimuli were not relayed appropriately to the higher processing areas of the brain. However, when the researchers did brain imaging on these mice they found that stimuli that would cause pain in normal mice, activated the visual, olfactory, and auditory centers of the brain of the mice missing the gene. The absence of this gene made the mice see, smell, and hear things in response to pain, in other words: synesthesia!
Research into synesthesia is ongoing and in the future it may yield information that may allow us to better understand the inner working of the human mind. On the meantime, if you belong to that 2-4 % of the population that has synesthesia, I hope that reading this post has been a colorful reading experience!
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