The effects of periodic interruptions on cortical entrainment to speech

Speech is perceived as a continuous stream of words despite consisting of a discontinuous, quasi-periodic signal of interleaved sounds and silences. Speech perception is surprisingly robust to interference by interruption, however speech that is replaced by gaps of silence is difficult to understand. When those silences are filled with noise, the speech is once again perceived as continuous even when the underlying speech sounds are removed completely. This is a phenomenon known as phonemic restoration. Perception of normal speech is accompanied by robust phase-locking of EEG signals to acoustic and linguistic features of speech. In this study we test the theory that interrupting speech with silence impairs perception by interfering with neural speech tracking. Further, we test the theory that we can restore perception and phase-tracking of the original acoustics by inserting noise in the interruptions. We find that disruptions of the acoustic envelope reduce the tracking of both acoustic and phonemic features. By inserting amplitude modulated noise such that the original broadband envelope is restored, we improved perception of the degraded speech and restored the magnitude of the speech tracking response; however, topographic analysis suggests that the neural response to noise-interrupted speech may recruit systematically different brain areas. The acoustic envelope seems to be an important physical component of speech that facilitates the dynamic neural mechanisms for perception of spoken language, particularly in adverse listening conditions.