Distinct roles of forward and backward alpha-band waves in spatial visual attention

Previous research has associated alpha-band [8-12Hz] oscillations with inhibitory functions (Klimesch et al., 2007; Jensen and Mazaheri, 2010): for instance, several studies showed that visual attention increases alpha-band power in the hemisphere ipsilateral to the attended location (Thut et al., 2006; Handel et al., 2011). However, other studies demonstrated that alpha oscillations positively correlate with visual perception (Busch et al., 2009; Vanrullen and MacDonald, 2012), hinting at different processes underlying their dynamics. Here, using an approach based on traveling waves, we demonstrate that there are two functionally distinct alpha-band oscillations propagating in different directions. We analyzed EEG recordings from two datasets of human participants (N=32, both genders) performing a covert visual attention task. Participants were instructed to detect a brief target by covertly attending to the screens left or right side. Our analysis reveals two distinct processes: allocating attention to one hemifield increases top-down alpha-band waves propagating from frontal to occipital regions ipsilateral to the attended location, both with or without visual stimulation. These top-down oscillatory waves correlate positively with alpha-band power in frontal and occipital regions. Yet, different alpha-band waves propagate from occipital to frontal regions and contralateral to the attended location. Crucially, these forward waves were present only during visual stimulation, suggesting a separate mechanism related to visual processing. Together, these results reveal two distinct processes reflected by different propagation directions, demonstrating the importance of considering oscillations as traveling waves when characterizing their functional role.