Phase-locked gamma band responses to semantic violation stimuli.

This paper addresses the role of gamma-band activity in semantic networks associated with the processing of words and sentences in humans. Magnetoencephalography (MEG) was used to compare the responses of eleven normal volunteers to semantically congruous and incongruous words at the end of syntactically correct sentences. The averaged low frequency responses evoked by the two word classes were clearly different within the latency range associated with N400 components. The oscillatory characteristics of the evoked responses were analysed using Gabor transform techniques in conjunction with statistical re-sampling. This revealed transient gamma-oscillations ( approximately 35 Hz) that were significantly phase-locked to both types of stimuli but preferentially present at intermediate ( approximately 300 ms) and long (>500 ms) latencies for incongruous words. This stimulus dependent phase locked gamma-activity occurred at latencies that were distinct from the short latency and evoked N400 components. The findings suggest that synchronised oscillations provide independent information about brain dynamics and that some semantic processes may dissociate into distinct functional stages.