Cross-sensory modulation of alpha oscillatory activity: suppression, idling, and default resource allocation.

Alpha oscillations (~10 Hz) have been suggested to have an inhibitory influence on stimulus processing within the context of attention being coordinated across space, with an increase in the power of alpha activity occurring in spatially irrelevant regions. However, with respect to cross-sensory attention (distribution of attention to different sensory modalities) an increase in alpha activity from baseline has not yet been definitively linked to active inhibition of distraction. In the current study, we examined the role of top-down alpha modulation in facilitating and suppressing stimulus processing between the visual and auditory domain. We utilized two cross-sensory paradigms, one containing distractors while the other paradigm only contained targets, in order to separate distractor related activity. We found a cue induced increase from baseline for the power of occipital alpha activity in posterior cortex when participants anticipated the occurrence of auditory targets combined with visual distractors. Interestingly, there was no increase in alpha power observed in the condition where auditory targets occurred without distractors. These findings suggest that the increase in alpha activity from baseline reflects top-down drive processes serving to inhibit distracting input. However, we found that there was no significant difference in the absolute occipital alpha power between when participants were expecting a visual distractor, and the session where no visual distractors were present. We account for these findings by suggesting that an increase in alpha power in the anticipation of visual distractors, rather than being exclusively inhibitory, could also signal the re-allocation of resources in the sensory systems.