Time-dependent cortical asymmetries induced by emotional arousal: EEG analysis of event-related synchronization and desynchronization in individually defined frequency bands.

Event-related desynchronization (ERD) and synchronization (ERS) in the individually defined theta, alpha-1, alpha-2 and alpha-3 frequency bands were measured in 20 healthy subjects in response to International Affective Picture System (IAPS) stimuli with low, moderate and high arousal (LA, MA and HA) content. The 62-channel EEG, skin conductance response (SCR) and heart rate (HR) were simultaneously recorded while subjects viewed sequentially presented pictures and subjectively rated them after each presentation. In the theta band, both MA and HA vs. LA stimuli induced larger synchronization over the left anterior and bilaterally over posterior cortical leads. However, rather unexpectedly, both MA and HA vs. LA stimuli yielded larger alpha-1 synchronization, predominantly over occipital leads. In both theta and alpha-1 bands, affectively salient stimuli prompted larger ERS against the background of the overall dominance in power synchronization of posterior regions of the right hemisphere, irrespective of stimulus category. Finally, in the alpha-3 band, HA stimuli induce a lateralized time-dependent power increase over anterior leads of the left hemisphere. The hemispheric asymmetries revealed point to recruitment of not only posterior regions of the right hemisphere (theta and alpha-1 bands), but also of anterior regions of the left hemisphere (theta and alpha-3 bands) in affect analysis beyond valence dimension. In terms of affective chronometry, the significant arousalxtime interactions clearly indicate that in the theta frequency band discrimination of affective stimuli has already started at 200 ms post-stimulus, whereas in the alpha-1 and alpha-3 bands this process is delayed by up to 800-1200 ms.