September 3, 2013

Neural Connections Unique In Synesthesia

In the perceptual condition known as synesthesia, unrelated sensory experiences such as color are triggered by ordinal stimuli such as numbers, letters and months of the year. Exactly how and why this happens is unknown. Researchers at Baylor College of Medicine, Rice University and the Jan and Dan Duncan Neurological Research Institute at Texas Children’s Hospital took a closer look at these connections and found a special relationship between the color and graphemes regions of the brain during certain stimuli, leading them closer to unlocking some of the mystery.

“People who have synesthesia experience it automatically as a normal part of their lives, so we wanted to focus on how the connections of the brain are structured differently from those who are non-synesthetes,” said Dr. Steffie N. Tomson, formerly a Ph.D. student at BCM where the study took place and currently a postdoctoral fellow at the Semel Institute for Neuroscience, University of California.

Perceputal modes

During synesthesia there can be a blend between two senses or perceptual modes. For the current study, researchers focused on those who experience colored sequence synesthesia, a form in which colors are associated with letters or numbers (graphemes).

Researchers allowed synesthetes and nonsynesthetes to listen to grapheme-rich clips from Sesame Street while monitoring brain activity using functional MRI (fMRI). Synesthetes and controls have significantly different neural clustering patterns during audio presentations of graphemes.

“Compared to nonsynesthetes, those with synesthesia had more neural connections between the regions of the brain responsible for recognizing color and letters and numbers during auditory stimuli,” said Steffie, first author on the study. “During a rest period where there were no stimuli, these connections were still greater in synesthetes.”

However, when they were allowed to hear the grapheme clips with video, both had similar connections.

New path to study

“Something important is happening in visual regions of the brain while synesthetes are hearing the graphemes spoken to them. There is a greater connectvity between much of the visual cortex, including color and grapheme brain regions, during this auditory exposure,” Steffie said. “These findings are helping us formulate theories, such as synesthetes may be better visualizers or perhaps the synesthetic experience depends on the way in which graphemes are presented. It gives us a new path to begin further studies.”

Others who took part in the study include: Manjari Marayan (coauthor), of Rice University; Genevera I. Allen, of Rice University and the Jan And Dan Duncan Neurological Research Institute at Texas Children’s Hospital; and David Eagleman, of Baylor College of Medicine.

Funding for the study is from Grant NSF-DMS 1209017.


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