Research Article Summary:
Neural Networks of Colored Sequence Synesthesia
This study investigates the neural mechanisms underlying colored sequence synesthesia, a phenomenon in which individuals perceive ordered sequences such as numbers, letters, or days of the week as having a specific color. The study used functional magnetic resonance imaging (fMRI) to investigate the neural correlates of this phenomenon in a group of individuals with colored sequence synesthesia.
Tomson, S. N., Narayan, M., Allen, G. I., & Eagleman, D. M. (2013). Neural networks of colored sequence synesthesia. Journal of Neuroscience, 33(35), 14098–14106. https://doi.org/10.1523/jneurosci.5131-12.2013
The participants in the study were six individuals with colored sequence synesthesia, five of whom experienced colors for numbers and one for letters. The control group consisted of six individuals without synesthesia, matched for age and gender. All participants underwent fMRI scans while performing a task that involved viewing colored letters, colored numbers, and colored weekdays presented in a randomized order.
The Study Hypothesis
The neural basis of CSS is not well understood. The general hypothesis is that synesthesia arises from hyperconnectivity between brain regions, and that this connectivity could arise from insufficient pruning of connections during youth or incomplete inhibitory circuitry.
Findings and Conclusions
During the fMRI scans, the researchers found that the individuals with colored sequence synesthesia showed significantly increased activation in several brain regions compared to the control group. Specifically, the individuals with synesthesia showed increased activation in the left fusiform gyrus, which is involved in processing visual information, as well as in the inferior parietal lobule, which is associated with spatial perception and attention. The researchers also found increased activation in the precuneus, which is involved in self-referential processing and mental imagery.
In addition, the researchers found that the individuals with colored sequence synesthesia showed greater functional connectivity between the left fusiform gyrus and the inferior parietal lobule, as well as between the left fusiform gyrus and the precuneus. These findings suggest that the neural networks involved in colored sequence synesthesia involve increased communication and integration between brain regions involved in visual processing, spatial attention, and mental imagery.
The study also investigated the relationship between the colors experienced by the individuals with synesthesia and the physical properties of the stimuli. The researchers found that the colors experienced by the synesthetes were influenced by several factors, including the luminance and saturation of the stimuli as well as their semantic meaning. For example, the number 5 was consistently experienced as green, regardless of its actual color, whereas the number 1 was experienced as white when presented on a dark background but as black when presented on a light background.
"Synesthesia is a condition in which normal stimuli can trigger anomalous associations. In this study, we exploit synesthesia to understand how the synesthetic experience can be explained by subtle changes in network properties. Of the many forms of synesthesia, we focus on colored sequence synesthesia, a form in which colors are associated with overlearned sequences, such as numbers and letters (graphemes). Previous studies have characterized synesthesia using resting-state connectivity or stimulus-driven analyses, but it remains unclear how network properties change as synesthetes move from one condition to another. To address this gap, we used functional MRI in humans to identify grapheme-specific brain regions, thereby constructing a functional “synesthetic” network. We then explored functional connectivity of color and grapheme regions during a synesthesia-inducing fMRI paradigm involving rest, auditory grapheme stimulation, and audiovisual grapheme stimulation. Using Markov networks to represent direct relationships between regions, we found that synesthetes had more connections during rest and auditory conditions. We then expanded the network space to include 90 anatomical regions, revealing that synesthetes tightly cluster in visual regions, whereas controls cluster in parietal and frontal regions. Together, these results suggest that synesthetes have increased connectivity between grapheme and color regions, and that synesthetes use visual regions to a greater extent than controls when presented with dynamic grapheme stimulation. These data suggest that synesthesia is better characterized by studying global network dynamics than by individual properties of a single brain region".
The Short Conclusion:
Overall, the results of this study provide insight into the neural mechanisms underlying colored sequence synesthesia and suggest that this phenomenon involves increased communication and integration between brain regions involved in visual processing, spatial attention, and mental imagery. The findings also suggest that the colors experienced by individuals with synesthesia are influenced by both the physical properties of the stimuli and their semantic meaning.
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