Skirting the Limits of Speech Intelligibility The Role of Theta-Oscillations in Decoding Degraded Speech: A Behavioral and Electrocorticographical Study of the Auditory Pop-Out Effect

Abstract

Oscillations are present both in natural speech and in the brain. This may be more than a mere coincidence. Re-instating information in the theta-frequency band has been shown to remarkably improve intelligibility. Moreover, a recent theory has proposed the existence of an internal tracking mechanism that parses and decodes incoming speech at a theta rhythm. This study sought to clarify the importance of theta-frequency band oscillations for speech comprehension as well as to establish their significance as a speech processing mechanism in the human auditory cortex. Here, it is shown that exposure to information in the theta-frequency range can restore intelligibility to a degraded, previously unintelligible stimulus, producing an auditory pop-out effect. This effect was observed regardless of whether participants were exposed to the intact sentence in the auditory or the visual domain. Compressing or extending the presentation speed of the intact sentence reduced the size of the effect, except for an extension rate of 1.5 times the original speed. At a neural level, it was previously unknown whether theta oscillations in auditory regions are internally generated or merely reflect stimulus driven evoked responses. Electrocorticographical recordings from one clinical patient provide evidence for the existence of theta-frequency oscillations in auditory regions, specifically the superior temporal gyrus, which are internally generated and effectively track incoming speech.