Abstract
Combined auditory and tactile stimuli have been found to enhance speech-in-noise perception both in individuals with normal hearing and in those with hearing loss. While behavioral benefits of audio-tactile enhancements in speech understanding have been repeatedly demonstrated, the impact of vibrotactile cues on cortical auditory speech processing remains unknown. Using functional near-infrared spectroscopy (fNIRS) with a dense montage setup, we first identified a region-of-interest highly sensitive to auditory-only speech-in-quiet. In the same region, we then assessed the change in activity ('audio-tactile gains') when presenting speech-in-noise together with a single-channel vibratory signal to the fingertip, congruent with the speech envelope's rate of change. In data from 21 participants with normal hearing, audio-tactile speech elicited on average 20% greater hemodynamic oxygenation changes than auditory-only speech-in-noise within bilateral middle and superior temporal gyri. However, audio-tactile gains did not exceed the sum of the unisensory responses, providing no conclusive evidence of true multisensory integration. Our results support a metamodal theory for the processing of temporal speech features in the middle and superior temporal gyri, providing the first evidence of audio-tactile speech processing in auditory areas using fNIRS. Top-down modulations from somatosensory areas or attention networks likely contributed to the observed audio-tactile gains through temporal entrainment with the speech envelope's rate of change. Further research is needed to understand the neural responses in concordance with their behavioral relevance for speech perception, offering future directions for developing tactile aids for individuals with hearing impairments.