|Photo Credit : The Triple Helix @ UChicago
There are many theories as to why this condition occurs such as retaining low-level, cross-sensory connections (that would typically sever during the later stages of infancy), or the lack of inhibiting processes that eliminate certain types of signals from flowing into or across sensory brain areas. It is interesting to note that "specific associations experienced by synesthetes may be based on their experience" (Rosenblum 284); An example of this association by experience can be found in the colorful letter and number magnets for the refrigerator that children use. A child sees that the letter "D" on the refrigerator is the color red - if the child is a synesthete, then as they grow older they will always see the letter D as red! This may differ from someone else who had a different set of fridge magnets, or no magnets at all, that instead see the color "D" as light blue.
A great video example that explains and allows you to experience synesthesia can be found by clicking the following link : https://www.youtube.com/watch?v=rkRbebvoYqI
Rosenblum, L. D. (2010). See What I'm Saying: The Extraordinary Powers of Our Five Senses. W.W. Norton & Company
TED-Ed. (2013, June 10). What Color is Tuesday? Exploring Synesthesia [Video]. Youtube. https://www.youtube.com/watch?v=rkRbebvoYqI