Top–down signal transmission and global hyperconnectivity in auditory‐visual synesthesia: Evidence from a functional EEG resting‐state study |
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| Authors: | Christian Brauchli Stefan Elmer Lars Rogenmoser Anja Burkhard Lutz Jäncke |
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| Affiliation: | 1. Department of Neuropsychology, Institute of Psychology, University of Zurich, Zurich, Switzerland;2. Laboratory of Integrative Neuroscience and Cognition, Department of Neuroscience, Georgetown, University Medical Center, Washington DC;3. Neuroimaging and Stroke Recovery Laboratory, Department of Neurology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts;4. Center for Integrative Human Physiology (ZIHP), University of Zurich, Zurich, Switzerland;5. International Normal Aging and Plasticity Imaging Center (INAPIC), University of Zurich, Zurich, Switzerland;6. University Research Priority Program (URPP) “Dynamic of Healthy Aging”, University of Zurich, Zurich, Switzerland;7. Department of Special Education, King Abdulaziz University, Jeddah, Saudi Arabia |
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| Abstract: | Auditory‐visual (AV) synesthesia is a rare phenomenon in which an auditory stimulus induces a “concurrent” color sensation. Current neurophysiological models of synesthesia mainly hypothesize “hyperconnected” and “hyperactivated” brains, but differ in the directionality of signal transmission. The two‐stage model proposes bottom–up signal transmission from inducer‐ to concurrent‐ to higher‐order brain areas, whereas the disinhibited feedback model postulates top–down signal transmission from inducer‐ to higher‐order‐ to concurrent brain areas. To test the different models of synesthesia, we estimated local current density, directed and undirected connectivity patterns in the intracranial space during 2 min of resting‐state (RS) EEG in 11 AV synesthetes and 11 nonsynesthetes. AV synesthetes demonstrated increased parietal theta, alpha, and lower beta current density compared to nonsynesthetes. Furthermore, AV synesthetes were characterized by increased top–down signal transmission from the superior parietal lobe to the left color processing area V4 in the upper beta frequency band. Analyses of undirected connectivity revealed a global, synesthesia‐specific hyperconnectivity in the alpha frequency band. The involvement of the superior parietal lobe even during rest is a strong indicator for its key role in AV synesthesia. By demonstrating top–down signal transmission in AV synesthetes, we provide direct support for the disinhibited feedback model of synesthesia. Finally, we suggest that synesthesia is a consequence of global hyperconnectivity. Hum Brain Mapp 39:522–531, 2018. © 2017 Wiley Periodicals, Inc. |
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| Keywords: | synesthesia auditory‐visual resting‐state EEG connectivity top– down |
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