Neural connections unique in synesthesia
In the perceptual condition known as synesthesia, sensory experiences such as color are triggered by stimuli such as numbers, letters and months of the year. Exactly how and why this happens is poorly understood. 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 unique relationships amongst regions of the synesthetic brain, leading them closer to unlocking 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 in Dr. David Eagleman’s laboratory at BCM where the study took place. Tomson is currently a postdoctoral fellow at the Semel Institute for Neuroscience, University of California.
"For many decades, synesthesia received little scientific attention," said Eagleman, assistant professor of neuroscience and senior author on the study. "It was suspected that perhaps synesthetes were merely malingering, just seeking attention. But the techniques of modern neuroscience allow us to discover measurable differences in the synesthetic brain."
Although synesthesia can come in many different forms, the current study focused on synesthetes who experience colored sequence synesthesia, in which color experiences are triggered by letters or numbers (collectively known as graphemes).
Researchers had synesthetes and non-synesthetes listen to audio clips from Sesame Street while monitoring brain activity using functional magnetic resonance imaging (fMRI). The clips were packed with references to letters and numbers. The researchers found that synesthetes and controls have significantly different neural clustering patterns while listening to graphemes.
"Compared to non-synesthetes, those with synesthesia had more neural connections between the regions of the brain responsible for color perception, and those regions involved in letters and numbers," 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 synesthetes watched the video clips, rather than simply listening to them, they showed similar patterns of brain activity to the non-synesthetes.
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 connectivity 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, for example, that 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."
"Synesthesia provides an inroad into understanding some of the deepest issues of private, individual experience," said Eagleman. "We’re discovering how subtle differences in brain connectivity lead to different experiences of reality."