Giving that presentation on the thalamus got me really interested in a lot of different neurological conditions I came across during my research. Although I could probably talk about thalamic pain syndrome for days, I thought I’d delve a little bit more into one of the weirdest results of crossed wires in the brain: synesthesia.
Synesthesia is a phenomenon where the sensory perception gets all jumbled up. People with this condition (synesthetes) can sometimes hear smells, or taste colors. It’s kind of like when the classic sitcom character tries to rewire their house, and the doorbell ends up turning on the porch lights. The most common form of synesthesia is color-grapheme synesthesia, where achromatic symbols, like letters and numbers, are perceived to have an associated color. How full words are perceived (as one color or as a jumble of colored letters) is different from person to person, although words that are part of an ordinal collection (likes days of the week) usually have their own color. What color is associated to each symbol varies from person to person, although some studies have shown a link between rule-based linguistic mechanisms and perceived color (Hung, Simner, Shillcock, & Eagleman, 2014).
There are many different types of synesthesia, and just as many different causes. As mentioned above, lesions in the ventral lateral nucleus of the thalamus (usually caused by strokes) can result in many different kinds of synesthesia due to the thalamus’ role as the brain’s sensory switchboard (John Wiley & Sons, Inc., 2007). For grapheme-color synesthesia, the explanation seems to be in the layout of the brain. The area of the brain that processes graphemes (the fusiform gyrus in the posterior temporal lobe) and one of the areas that processes color (V4 in the visual cortex) are adjacent to one another, as shown in the picture below. In most people, a lot of the neural pathways between these two areas are pruned during early development, but in grapheme-color synesthetes, more of the pathways are left over (Yokoyama, et al., 2014). In most brains, the neural pathway is a sleek highway leading directly from the eyes to the grapheme area, while in synesthetes, there are a bunch of extra exits, and the visual information gets lost for a while before it reaches its destination.
Most other forms of synesthesia are still poorly understood, because they’re often hard to categorize. What’s most interesting to me about synesthesia is that there are so many different causes for very similar symptoms. Intuitively, you would think that lesions in the thalamus and extra neurons in the temporal lobe would cause vastly different outcomes, but they both can result in this crazy neurological phenomenon. The brain is a crazy place.
Hung, W., Simner, J., Shillcock, R., & Eagleman, D. (2014, Jan). Synaesthesia in Chinese characters: The role of radical function and position. Conscious Cognition , n/a.
John Wiley & Sons, Inc. (2007, September 24). Explanation for Synesthesia? Area Deep Wtihin Brain Plays a Role in Sensory Perception. Retrieved Febuary 21, 2014, from SceinceDaily: http://www.sciencedaily.com/releases/2007/09/070924072449.htm
Yokoyama, T., Noguchi, Y., Koga, H., Tachibana, R., Saiki, J., Kakigi, R., et al. (2014, January 29). Multiple Neural Mechanisms for Coloring Words in Synesthesia. NeuroImage , 1-12.