Functional MRI (fMRI) combined with the paired-stimuli paradigms (referred as dynamic fMRI) was used to study the "illusory double-flash" effect on brain activity in the human visual cortex. Three experiments were designed. The first two experiments aimed to examine the cross-modal neural interaction between the visual and auditory sensory systems caused by the illusory double-flash effect using combined auditory (beep sound) and visual (light flash) stimuli. The fMRI signal in the visual cortex was significantly increased in response to the illusory double flashes compared to the physical single flash when the inter-stimuli delay between the auditory and visual stimuli was 25 ms. This increase disappeared when the delay was prolonged to ∼300 ms. These results reveal that the illusory double-flash effect can significantly affect the brain activity in the visual cortex, and the degree of this effect is dynamically sensitive to the inter-stimuli delay. The third experiment was to address the spatial differentiation of brain activation in the visual cortex in response to the illusory double-flash stimulation. It was found that the illusory double-flash effect in the human visual cortex is much stronger in the periphery than the fovea. This finding suggests that the periphery may be involved in high-level brain processing beyond the retinotopic visual perception. The behavioral measures conducted in this study indicate an excellent correlation between the fMRI results and behavioral performance. Finally, this work demonstrates a unique merit of fMRI for providing both temporal and spatial information regarding cross-modal neural interaction between different sensory systems.
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Acknowledgments We thank Drs. Xiao-Hong Zhu, Xiaoliang Zhang, Peter Andersen, Yukiyasu Kamitani, Seiji Ogawa and Kamil Ugurbil, and Mr. Shuangyi Tang and Mr. John Strupp for their insightful discussion, support and technical assistance. This work was supported in part by NIH RO1 grants of EB00329, NS41262, NS38070, NS39043, EB00513 and P41 RR08079 (a National Research Resource grant from NIH), the W.M. Keck Foundation and the MIND institute.
- Cross-modal neural interaction
- Dynamic fMRI
- Functional MRI
- Human brain
- Illusory double-flash effect
- Paired-stimuli paradigm
- Visual cortex