The effect of temporal asynchrony on the multisensory integration of letters and speech sounds.

Temporal proximity is a critical determinant for cross-modal integration by multisensory neurons. Information content may serve as an additional binding factor for more complex or less natural multisensory information. Letters and speech sounds, which form the basis of literacy acquisition, are not naturally related but associated through explicit learning. We investigated the relative importance of temporal proximity and information content on the integration of letters and speech sounds by manipulating both factors within the same functional magnetic resonance imaging (fMRI) design. The results reveal significant interactions between temporal proximity and content congruency in anterior and posterior auditory association cortex, indicating that temporal synchrony is critical for the integration of letters and speech sounds. The temporal profiles for multisensory integration in the auditory association cortex resemble those demonstrated for single multisensory neurons in different brain structures and animal species. This similarity suggests that basic neural integration rules apply to the binding of multisensory information that is not naturally related but overlearned during literacy acquisition. Furthermore, the present study shows the suitability of fMRI to study temporal aspects of multisensory neural processing.