Temporal dynamics of TMS interference over preparatory alpha activity during semantic decisions.

The mean amplitude of the EEG alpha (8-12 Hz) power de-synchronization (ERD) is a robust electrophysiological correlate of task anticipation. Furthermore, in paradigms using a fixed period between warning and target stimuli, such alpha de-synchronization tends to increase and to peak just before target presentation. Previous studies from our group showed that the anticipatory alpha ERD can be modulated when magnetic stimulation is delivered over specific cortical regions during a variety of cognitive tasks. In this study we investigate the temporal dynamics of the anticipatory alpha ERD and test whether the magnetic stimulation produces either a general attenuation or an interruption of the typical development of alpha ERD. We report that, during a semantic decision task, rTMS over left AG, a region previously associated to semantic memory retrieval, shortened the peak latency and decreased the peak amplitude of the anticipatory alpha de-synchronization as compared to both active (left IPS) and non-active (Sham) TMS conditions. These results, while supporting the causal role of the left AG in the anticipation of a semantic decision task, suggest that magnetic interference not simply reduces the mean amplitude of anticipatory alpha ERD but also interrupts its typical temporal evolution in paradigms employing fixed cue-target intervals.