What causes synesthesia to occur?
Since research on synesthesia began, scientists have been trying to determine what causes it. One of the difficulties of studying synesthesia is testing to see if people have it. In Personal Profiles of Color Synesthesia: Developing a Testing Method for Artists and Scientists, Campen and Froger (2004) discuss different methods of testing for synesthesia, including self-reporting, diagnostic criteria, brain imaging, and a consistency test. They also introduce their NeCoSyn method of consistency testing, which is less expensive than other forms of testing and can be helpful to artists and musicians by teaching them about their synesthesia. This is important as it has been found that “synesthetes are eight times more likely to work in an artistic profession, and musicians with synesthesia are especially common,” according to a 2015 article by Emily Jupp. In fact, even people without synesthesia associate color with music. In research by Palmer and Schloss (2015) it was shown that people can unconsciously match up color and music based on the emotions they elicit, suggesting that synesthesia may be related to areas in the brain that produce emotional responses.
However, most studies attribute the condition to “an excess of neural connections … possibly due to decreased neural pruning between (typically adjacent) regions [of the brain] that are interconnected in the fetus” (Brang and Ramachandran). Brang and Ramachandran (2011) discuss the different questions surrounding the genetics that cause this cross-talk in the brain to occur. For instance, though David Eagleman (2011) has linked an area on chromosome 16 to colored sequence synesthesia, it is still unknown whether each type of synesthesia is caused by a different gene. It is also possible that everyone is born with a gene for synesthesia and it is only expressed in some people. Synesthesia may also be epiphenomenal: a side effect of a gene meant to do something else.
Though there is much ambiguity surrounding the genetic origin of the condition, most sources, including Ramachandran and Hubbard, agree that the gene or genes are likely X-linked and dominant because the condition usually runs in families. The main contribution of Ramachandran and Hubbard (2001) is their proving that synesthesia is “a genuine perceptual phenomenon, not an effect based on memory associations from childhood or on vague metaphorical speech”. Their research on grapheme-color synesthesia also discovers that there are different ways that the condition (at least this form of it) is experienced, which they propose is on a spectrum from lower synesthetes to higher synesthetes. Lower synesthetes’ synesthesia is completely sensory: the shape, not the concept, of a number or letter will invoke an experience of color. However, for higher synesthetes a color can be activated by a concept such as the days of the week. Ramachandran and Hubbard theorize that higher synesthetes’ synesthesia may be caused by cross-talk in higher areas of the brain than in lower synesthetes, such as the angular gyrus. There has been a lot of progress made in determining what may cause the condition, but more research will have to be done on synesthesia in order to find a concrete answer to this complicated question.