Syntactic processing modulates the theta rhythm of the human EEG.

Changes in oscillatory brain dynamics can be studied by means of induced band power (IBP) analyses, which quantify event-related changes in amplitude of frequency-specific EEG rhythms. Such analyses capture EEG phenomena that are not part of traditional event-related potential measures. The present study investigated whether IBP changes in the delta, theta, and alpha frequency ranges are sensitive to syntactic violations in sentences. Subjects read sentences that either were correct or contained a syntactic violation. The violations were either grammatical gender agreement violations, where a prenominal adjective was not appropriately inflected for the head noun's gender, or number agreement violations, in which a plural quantifier was combined with a singular head noun. IBP changes of the concurrently measured EEG were computed in five frequency bands of 2-Hz width, individually adjusted on the basis of subjects' alpha peak, ranging approximately from 2 to 12 Hz. Words constituting a syntactic violation elicited larger increases in theta power than the same words in a correct sentence context, in an interval of 300-500 ms after word onset. Of all the frequency bands studied, this was true for the theta frequency band only. The scalp topography of this effect was different for different violations: following number violations a left-hemispheric dominance was found, whereas gender violations elicited a right-hemisphere dominance of the theta power increase. Possible interpretations of this effect are considered in closing.