Oscillatory power and functional connectivity in the speech change detection network

We used passive and active oddball conditions with two types of acoustic contrasts, between speech syllables that cross phonetic boundaries (across-category, AC) and between those that do not cross them (within-category, WC), to explore the effects of meaningful speech contrasts on the dynamics of the neural network underlying the mismatch negativity (MMN) to the speech deviants. We found that easily detected AC deviants evoked a MMN response that lateralized to the left hemisphere, but the very difficult to detect WC deviants did not elicit a MMN response at all. Based on independent component analysis of the continuous EEG, we computed both power changes within, and functional connectivity (phase synchronization) between, brain regional sources comprising the neural network associated with the MMN for these speech stimuli. We found that for acoustic contrasts for which an MMN was generated, power changes suggested whether a particular brain region was more involved with processing standards or deviants. Moreover, we not only replicated the changes in functional connectivity between orbitofrontal cortex and superior temporal gyrus found in previous experiments, but also found significant increases in synchronization between those regions and regions of the left inferior frontal gyrus (Broca's area), which is thought to be involved in the storage and retrieval of phonological and semantic information.