Abstract
In grapheme-color synesthesia, letters and numbers evoke abnormal colored perceptions. Although the underlying mechanisms are not known, it is largely thought that the synesthetic brain is characterized by atypical connectivity throughout various brain regions, including the visual areas. To study the putative impact of synesthesia on the visual brain, we assessed lateral interactions (i.e., local functional connectivity between neighboring neurons in the visual cortex) by recording steady-state visual evoked potentials (ssVEPs) over the occipital region in color-grapheme synesthetes (n = 6) and controls (n = 21) using the windmill/dartboard paradigm. Discrete Fourier Transform analysis was conducted to extract the fundamental frequency and the second harmonics of ssVEP responses from contrast-reversing stimuli presented at 4.27 Hz. Lateral interactions were assessed using two amplitude-based indices: Short-range and long-range lateral interactions. Results indicated that synesthetes had a statistically weaker signal coherence of the fundamental frequency component compared to the controls, but no group differences were observed on lateral interaction indices. However, a significant correlation was found between long-range lateral interactions and the type of synesthesia experience (projector versus associator). We conclude that the occipital activity related to lateral interactions in synesthetes does not substantially differ from that observed in controls. Further investigation is needed to understand the impact of synesthesia on visual processing, specifically in relation to subjective experiences of synesthete individuals.
Highlights
Synesthesia occurs when a stimulus in one sensory modality elicits an additional and abnormal perception in the same or a different sensory modality
Because of the putative atypical connectivity in the primary visual cortex in synesthesia, we hypothesized that the brain responses evoked by the windmill/dartboard paradigm will differ in synesthetes compared to controls, reflecting the altered lateral interaction processing in V1
We examined brain responses to visual lateral interactions in synesthetes using
Summary
Synesthesia occurs when a stimulus in one sensory modality elicits an additional and abnormal perception in the same or a different sensory modality. Synesthetes showed an increased intrinsic internetwork connectivity compared to the controls, in that stronger functional connectivity was observed between the frontoparietal and the lateral visual network, which encompasses many associative regions including the color area V4 Such connectivity was elicited in the medial visual network, which includes the calcarine gyrus and the primary visual cortex (V1) [20]. Using psychophysical tasks that preferentially target V1 computation, Terhune et al [27] reported no difference in performance between controls and synesthetes, neither for the tilt-after effect task nor the orientation-specific surround suppression task The latter task is thought to rely on local lateral interactions, which are dependent on the local connectivity networks. Because of the putative atypical connectivity in the primary visual cortex in synesthesia, we hypothesized that the brain responses evoked by the windmill/dartboard paradigm will differ in synesthetes compared to controls, reflecting the altered lateral interaction processing in V1
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