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Synesthesia and Lateral Inhibition: A Case Study

33.346, Sunday, 18-May, 8:30 am - 12:30 pm, Jacaranda Hall
Session: Multisensory processing: Visuo-auditory interactions

Diana Arias1,2, Mathieu Simard1,2, Dave Saint-Amour1,2,3; 1Département de psychologie, Université du Québec à Montréal (UQÀM),, 2Centre de recherche en neurosciences de l'UQÀM (NeuroQÀM),, 3Centre Hospitalier Universitaire Sainte-Justine

Introduction: Synesthesia is a perceptual phenomenon when a stimulus in one sensory modality elicits an additional percept in the same or in another modality (e.g., when black letters and numbers are perceived as color graphemes or when sounds inducing color percepts). Although the underlying mechanisms of synesthesia are not known, it is largely thought that the synesthetic brain is characterized by a hyperconnectivity between different regions including the visual areas. Lateral inhibition (LI), which involves inhibitory local interactions between neighboring neurons in the visual brain, might be a new approach to explore the nature of the putative abnormal connectivity in synesthesia. Objective: The purpose of this study was to assess brain responses associated with lateral inhibition in synesthesia using scalp-recorded EEG. Methods: Steady-state visual evoked potentials (ssVEPs) were recorded from Oz in a color-grapheme synesthete female (27 years old) and in 12 control non-synesthetes using the windmill/dartboard paradigm (Zemon and Ratliff, PNAS, 1982). Stimuli consisted in four radial contiguous patterns, two static and two dynamic that were presented at a reversal-contrast frequency of 4.28 Hz and 30% contrast. Fast Fourier Transform (FFT) analysis was conducted to extract the fundamental and the second harmonic of ssVEPs in order to calculate the suppression and facilitation LI indices. Then, z-scores were computed for the synesthete subject in relation with the non-synesthete subjects. Results: No significant difference in the suppression or facilitation index was found. Conclusion: Lateral inhibition processing in the visual cortex is not abnormal in synesthesia. This case-study finding challenges the nature of the hyperconnectivity hypothesis in synesthesia.

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