By Anna Riordan
Have you ever wanted to experience the weird, wonderful sensation of synesthesia? Synesthesia is a condition where certain stimuli cause an additional sensory experience beyond what most people experience. For example, some people see sky blue when they hear their favorite song or see emerald green when they see the letter “T.” The associations are effortless, consistent, and specific, and there is a developmental (and possibly genetic) predisposition for the condition. Researchers Roumke Rouw and H. Steven Scholte wanted to find out what causes some people to experience synesthesia while most people don’t. Their hypothesis was that hyperconnectivity between neurons is responsible for the added sensations of synesthesia.
Grapheme-color synesthesia is a condition that causes people to experience specific colors with particular letters or numbers, otherwise known as graphemes. They were also able to differentiate between types of grapheme-color synesthesia—there was more connectivity in the certain parts of the brain for people who see color in the outside world, known as projectors, than people who see the color in their “mind’s eye,” known as associators. All synesthetes had more connectivity in parts of the brain such as the superior parietal or frontal cortex than non-synesthetes, and this did not differ between types of synesthetes. The study found increased structural connectivity in the neurons of the brain was associated with grapheme-color synesthesia, and found evidence that this plays a role in the subjective experience of color.
To test their hypothesis, the researchers used diffusion tensor imaging. This uses a magnetic resonance signal to measure the diffusion properties of water molecules in the brain, allowing the researchers to measure microscopic properties of cerebral white matter, the brain tissue that contains nerve fibers, in vivo—in real, living people! This allowed them to understand the connections of neurons in the brain. They also used a functional MRI to measure brain activity in synesthetes as they viewed pictures of graphemes, so they could see which areas of the brain were active when they were experiencing synesthesia. They compared this to the brain activity of non-synesthetes when shown the same images, so they could locate the exact area of the brain activated in synesthetes that is inactive in most people. They focused on the inferior temporal cortex, which is responsible for recognizing shapes like words. They measured anisotropy in the brain—essentially the strength of connections in relation to direction. They showed participants graphemes that induced strong, weak, and no synesthetic colors. They hypothesized that white matter structures—specifically the strength of connections between neurons—in the temporal lobe would determine whether a synesthete was a projector or associator.
This was the first study to determine the differences in brain structure that are responsible for synesthesia: synesthetes have abnormal white matter structure in their brains. They found evidence that grapheme-color synesthesia is caused by a lack of pruning, which is the cutting away of neuron connections that happens during development for most people, leaving the cross-connections intact between the areas of the brain responsible for grapheme-recognition and color perception, which are next to each other. Grapheme-color synesthetes have more diffusion anisotropy, indicating greater connection between neurons, likely due to small-scale factors like the amount of myelination (the diameter of the axons), the coherence in fiber direction, or the fiber density. Increased diffusion anisotropy in nerve fiber pathways is likely due to the greater presence and strength of connections between neurons. It also suggests this connectivity may be responsible for the variety of color experiences between synesthetes. Projector types, which are closer to “real” color perception, had greater connectivity than associators. This shows that the structural connectivity of neurons, not just the firing of neurons, is important to the nature of sensory experiences, and our subsequent experience of the world.
Rouw, R., Scholte, H. Increased structural connectivity in grapheme-color synesthesia. Nat Neurosci 10, 792–797 (2007). https://doi.org/10.1038/nn1906