In The Tell-Tale Brain, V.S. Ramachandran (2011) discusses synesthesia, which he defines as the "surreal blending of sensation, perception, and emotion." (Ramachandran, 2011, p. 75). One form of synesthesia is seeing distinct colors when looking at symbols such as letters or numbers. In the text, Becky, Susan, and Mirabelle have color experiences for symbols which differ from each other, indicating color experiences may differ from synesthete to synesthete. Each time the synesthete views the symbol, the color is the same. For example in the text, Becky experienced seeing both letters and numbers in vivid colors, while Susan only experienced synesthesia when viewing numbers. (Ramachandran, 2011, p. 84). To further explain, Becky views the number 5 as green and 7 as blue. In contrast, Susan views the number 7 as red.
Ramachandran was able to show that synesthesia is not memory association by his experiments with Mirabelle. In the first experiment, a white number five was shown on a black computer screen. Mirabelle viewed the 5 as red. When the number was moved throughout her field of vision, the shade of red altered accordingly. The farther away the 5 appeared to be, the weaker the shade of red was experienced. As Ramachandran noted, "if the number were merely evoking the memory or idea of color, why would it matter where it was placed in the visual field, so long as it is still clearly visible?" (Ramachandran, 2011, p. 87). To elaborate further, a person without synesthesia might views colors as memory associations. They see the idea of color, while not actually experiencing the color. Ramachandran (2011) uses the example of Cinderella, stating that Cinderella is usually associated with "a young girl or of pumpkins or coaches." (Ramachandran, 2011, p. 81). A person with synesthesia actually perceives and experiences the color, not through memory or imagination.
The Ramachandran (2011) text indicates a cross activation between the V4, the fusiform gyrus, and the angular gyrus might explain this type of synesthesia. The V4 processes color and the angular gyrus process arithmetic and logic. The fusiform gyrus also processes the actual shape of numbers.
The video above is pretty interesting and describes several types of synethetes and how they perceive various sensory components. It is fascinating to see how these experiences differ from person to person, while some synesthetes share the same sensory experiences as others.
Ramachandran, V. S. (2011). The tell-tale brain: a neuroscientist's quest for what makes us human. New York: W.W. Norton.