Neural tuning to low-level features of speech throughout the perisylvian cortex

Abstract

Despite a large body of research, we continue to lack a detailed account of how auditory processing of continuous speech unfolds in the human brain. Previous research showed the propagation of low-level acoustic features of speech from posterior superior temporal gyrus towards anterior superior temporal gyrus in the human brain (Hullett et al., 2016). In this study we investigate what happens to these neural representations past the superior temporal gyrus, and how they engage higher-level language processing areas, such as inferior frontal gyrus. We used low-level sound features to model neural responses to speech outside the primary auditory cortex. Two complementary imaging techniques were used with human participants (both males and females): electrocorticography (ECoG) and functional magnetic resonance imaging. Both imaging techniques showed tuning of the perisylvian cortex to low-level speech features. With ECoG, we found evidence of propagation of the temporal features of speech sounds along the ventral pathway of language processing in the brain towards inferior frontal gyrus. Increasingly coarse temporal features of speech spreading from posterior superior temporal cortex towards inferior frontal gyrus were associated with linguistic features, such as voice onset time, duration of the formant transitions, as well as phoneme, syllable and word boundaries. The present findings provide groundwork for a comprehensive bottom-up account of speech comprehension in the human brain.