Synesthesia
The Brain and Synesthesia
Synesthesia cannot be traced to any specific area of the brain, instead it is linked to increased neural "cross talk". Multiple areas of the brain, responsible for smell, color, touch, vision, pain, etc., are connected through neural networks in the brain. There are two main theories of how this increased neural "cross talk" came about: more connections or less inhibition (Cytowic & Eagleman, 2009).
1. More Connections:
In Crytowic and Eagleman's book Wednesday is Indigo Blue (2009), they relate the newborn brain to a garden, wherein a garden must be pruned, or weeded. Just like a garden, the brain must be "pruned" of weeds, or in this case, excess neural connections. For this theory, a synesthetes brain is an overgrown garden, containing more synaptic connections than a non-synesthete brain. These excess connections are what cause the synesthetic experience (Cytowic & Eagleman, 2009).
2. Less Inhibition:
For this theory, the concept of faulty inhibition is thought to cause synesthesia (pictured left). The non-synesthete brain's neurons' excitation response is balanced by the inhibition response (a). For synesthetes, the inhibition response is much weaker, causing activity in one area to lead to activity in another (b). This theory says that everyone has the same basic neural framework but only those with less inhibition experience synesthesia (Cytowic & Eagleman, 2009).
Genetics
Synesthesia is thought to be the only genetic perceptual condition, as in, there may be a gene or genes that can be traced to cause synesthesia. This gene(s) have yet to be discovered, but evidence of synesthesia being passed down through genes is plentiful.
Synesthesia is proposed to be transmitted as a dominant trait on the X chromosome, as in, mothers pass the gene to sons and daughters, and fathers pass it to daughters, but not commonly fathers to sons (Cytowic & Eagleman, 2009).
However, not every carrier of the gene will express the trait (Cytowic & Eagleman, 2009). Synesthesia has been noted to skip generations, where the grandparents and grandchildren might have the condition, but it skips over the generation in between (Ward, 2008).
Even though synesthesia may be genetic, receivers of the gene typically do not have the same form of synesthesia as the giver (Duffy, 2001).
Squares represent males and circles females. Color intensity reflects probability of synesthesia pedigree taken from the numbers at the bottom, representing incidence of each case (Brang & Ramachandran, 2011).
Brain Study
A 2002 study by Julia Nunn helped strengthen the idea that the synesthetic brain is different than the average brain. In her results, she found the V4 color area of the brain (pictured below) was activated when synesthetes with word-color synesthesia heard spoken words. This was an astonishing finding considering, normally, the V4 area is only activated by viewing real colors in the world. Potential criticisms such as "what if they were simply thinking of the color?" led Nunn to add a control group of non-synesthetes to her study. She had the non-synesthetes memorize word-color associations and tested them later while they were under MRI scanning. There was no activation of the V4 color area at all. She also had the original word-color synesthetes and the non-synesthetes imagine colors, but this still resulted in no activation of the V4. Her study helped confirm that the synesthetic color experience is not like imagination but more like actual color perception and she helped confirm the theory of neural cross talk in the synesthetic brain (Crytowic & Eagleman, 2009).
