Sentence predictability modulates cortical response to phonetic ambiguity.

Phonetic categories have undefined edges, such that individual tokens that belong to different speech sound categories may occupy the same region in acoustic space. In continuous speech, there are multiple sources of top-down information (e.g., lexical, semantic) that help to resolve the identity of an ambiguous phoneme. Of interest is how these top-down constraints interact with ambiguity at the phonetic level. In the current fMRI study, participants passively listened to sentences that varied in semantic predictability and in the amount of naturally-occurring phonetic competition. The left middle frontal gyrus, angular gyrus, and anterior inferior frontal gyrus were sensitive to both semantic predictability and the degree of phonetic competition. Notably, greater phonetic competition within non-predictive contexts resulted in a negatively-graded neural response. We suggest that uncertainty at the phonetic-acoustic level interacts with uncertainty at the semantic level-perhaps due to a failure of the network to construct a coherent meaning.