Beta oscillations decrease in left inferior frontal gyrus when a word violates semantic expectations formed by preceding words
The event-related potential N400 response is elicited when a word violates semantic expectations formed by preceding words in a sentence, for example, the word “river” in sentence “Yesterday, I went to the grocery store to buy a river” would elicit an N400 response. Event-related potentials such as the N400 are obtained by averaging tens or even hundreds of electroencephalogram (EEG) or magnetoencephalogram (MEG) epochs time-locked to onset of stimuli (such as words that violate semantic expectations). The rationale behind this is that the much larger background EEG/MEG activity (that has been presumed to be non-interesting noise) is averaged out, thus leaving the signal of interest for scrutiny. Over the last couple of decades, alternative ways to inspect stimulus-related EEG and MEG activity have been steadily gaining popularity; in one of such approaches, instead of averaging the EEG/MEG trials, power as a function of time and frequency is calculated over the trials, thus allowing inspection of EEG/MEG oscillatory activity that is related to, yet not precisely time-locked to onset of, the stimuli.
In their recent study, Lin Wang and colleagues (Wang et al. 2012) inspected oscillatory activity elicited by semantic violations that resulted in generation of the N400 event-related response. A correlation was observed between the N400 response (that was predominantly generated in auditory cortical areas) and decreased power in the beta frequency band within left-hemisphere inferior frontal gyrus. The authors suggested that beta suppression reflects larger effort within the task-relevant network (consisting of the left inferior frontal gyrus and that is part of the speech motor system and auditory cortical areas in the left superior temporal region) for attempting to integrate the incongruent word with the preceding context. These findings also add to the pool of evidence indicating that speech motor system plays an important role in speech perception, and further stress the importance of inspecting electromagnetic activity not strictly time-locked to onset of stimuli, in addition to assessing event-related responses, in EEG and MEG studies.
Reference: Wang L, Jensen O, van den Brink D, Weder N, Schoffelen J-M, Magyari L, Hagoort P, Bastiaansen M. Beta oscillations relate to the N400m during language comprehension. Human Brain Mapping (2012), e-publication ahead of print. http://dx.doi.org/10.1002/hbm.21410