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Neural correlates of audiotactile phonetic processing in early-blind readers: an fMRI study

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Abstract

Reading is a multisensory function that relies on arbitrary associations between auditory speech sounds and symbols from a second modality. Studies of bimodal phonetic perception have mostly investigated the integration of visual letters and speech sounds. Blind readers perform an analogous task by using tactile Braille letters instead of visual letters. The neural underpinnings of audiotactile phonetic processing have not been studied before. We used functional magnetic resonance imaging to reveal the neural correlates of audiotactile phonetic processing in 16 early-blind Braille readers. Braille letters and corresponding speech sounds were presented in unimodal, and congruent/incongruent bimodal configurations. We also used a behavioral task to measure the speed of blind readers in identifying letters presented via tactile and/or auditory modalities. Reaction times for tactile stimuli were faster. The reaction times for bimodal stimuli were equal to those for the slower auditory-only stimuli. fMRI analyses revealed the convergence of unimodal auditory and unimodal tactile responses in areas of the right precentral gyrus and bilateral crus I of the cerebellum. The left and right planum temporale fulfilled the ‘max criterion’ for bimodal integration, but activities of these areas were not sensitive to the phonetical congruency between sounds and Braille letters. Nevertheless, congruency effects were found in regions of frontal lobe and cerebellum. Our findings suggest that, unlike sighted readers who are assumed to have amodal phonetic representations, blind readers probably process letters and sounds separately. We discuss that this distinction might be due to mal-development of multisensory neural circuits in early blinds or it might be due to inherent differences between Braille and print reading mechanisms.

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Acknowledgments

We would like to thank the blind participants for their cooperation and patience. We also acknowledge the contributions of Ben Krasnow for design and construction of the Braille device (http://www.magconcept.com/MRI), Mr. Rafi’ee for operating the MR scanner, and Dr. Rouhbakhsh for his insightful ideas in early conceptualization of the current study. We also would like to thank two anonymous reviewers for their thoughtful comments and the suggestion of gathering behavioral reaction time data. This research was supported by a grant from Tehran University of Medical Sciences (Project No. 87-04-30-8111).

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Correspondence to Mohammad Ali Oghabian.

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Pishnamazi, M., Nojaba, Y., Ganjgahi, H. et al. Neural correlates of audiotactile phonetic processing in early-blind readers: an fMRI study. Exp Brain Res 234, 1263–1277 (2016). https://doi.org/10.1007/s00221-015-4515-2

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  • DOI: https://doi.org/10.1007/s00221-015-4515-2

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