Audiotactile stimulation can improve syllable discrimination through multisensory integration in the theta frequency band

Syllables are an essential building block of speech. We recently showed that tactile stimuli linked to the perceptual centers of syllables in continuous speech can improve speech comprehension. The rate of syllables lies in the theta frequency range, between 4 and 8 Hz, and the behavioural effect appears linked to multisensory integration in this frequency band. Because this neural activity may be oscillatory, we hypothesized that a behavioural effect may also occur not only while but also after this activity has been evoked or entrained through vibrotactile pulses. Here we show that audiotactile integration regarding the perception of single syllables, both on the neural and on the behavioural level, is consistent with this hypothesis. We first stimulated subjects with a series of vibrotactile pulses and then presented them with a syllable in background noise. We show that, at a delay of 200 ms after the last vibrotactile pulse, audiotactile integration still occurred in the theta band and syllable discrimination was enhanced. Moreover, the dependence of both the neural multisensory integration as well as of the behavioral discrimination on the delay of the audio signal with respect to the last tactile pulse was consistent with a damped oscillation. In addition, the multisensory gain is correlated with the syllable discrimination score. Our results therefore evidence the role of the theta band in audiotactile integration and provide evidence that these effect may involve oscillatory activity that still persists after the tactile stimulation.