Effect of Speech Intelligibility on the Cortical Entrainment to the Temporal Envelope of Speech

Abstract

The study aimed to evaluate the effects of speech intelligibility on the cortical entrainment to the temporal envelope of speech. Cortical entrainment was evaluated using the cortical envelope following responses (cEFRs). The cEFRs were recorded using sentences with varying levels of intelligibility. Intelligibility was manipulated through chimerization and three types of chimeras; speech-speech chimera, speech-noise chimera and reversed-envelope chimera were used. These chimeras were further divided into three different intelligibility levels based on behavioural speech identification of chimeras using different numbers of frequency bands. The bands that resulted in poor, moderate and good intelligibility of the chimeras were used to record cEFRs. cEFRs were recorded in 29 normal hearing young adults. Entrainment was objectively evaluated based on 1) the similarity of the cEFRs and the temporal envelope of speech, 2) the time-course and cortical sources of the temporal response function that explains the temporal dynamics of the cortical mechanisms aiding in entrainment, and 3) the inter-trial phase coherence, which explains the cortical mechanisms operating at multiple time-scales that aid in encoding of the stimulus acoustics and intelligibility. The results showed that entrainment to the temporal envelope of speech is seen even when the speech is not intelligible. However, manipulation of speech intelligibility modulated this entrainment. The temporal pattern of activation of the cortical sources varied depending on stimulus intelligibility. The findings of the TRFs and their cortical sources gave further insights into the hierarchical and temporal processing mechanisms in the brain that aid in such entrainment and resulting speech perception.