Expectancy Constraints in Degraded Speech Modulate the Language Comprehension Network

In speech comprehension, the processing of auditory information and linguistic context are mutually dependent. This functional magnetic resonance imaging study examines how semantic expectancy ("cloze probability") in variably intelligible sentences ("noise vocoding") modulates the brain bases of comprehension. First, intelligibility-modulated activation along the superior temporal sulci (STS) was extended anteriorly and posteriorly in low-cloze sentences (e.g., "she weighs the flour") but restricted to a mid-superior temporal gyrus/STS area in more predictable high-cloze sentences (e.g., "she sifts the flour"). Second, the degree of left inferior frontal gyrus (IFG) (Brodmann's area 44) involvement in processing low-cloze constructions was proportional to increasing intelligibility. Left inferior parietal cortex (IPC; angular gyrus) activation accompanied successful speech comprehension that derived either from increased signal quality or from semantic facilitation. The results show that successful decoding of speech in auditory cortex areas regulates language-specific computation (left IFG and IPC). In return, semantic expectancy can constrain these speech-decoding processes, with fewer neural resources being allocated to highly predictable sentences. These findings offer an important contribution toward the understanding of the functional neuroanatomy in speech comprehension.