A dual mechanism underlying alpha lateralization in attentional orienting to mental representation.

Numerous electrophysiological findings support the notion that selective attention modulates alpha oscillatory activity. Specifically, alpha enhancement and suppression can be dissociated in time and space. It is now accepted that selective attention operates in either the perceptual or the representational environments. Lateralized alpha activity resulting from directing attention to mental representations, might arise from a transient alpha desynchronization, as recent proposals hypothesized. However, the contribution of enhancement vs suppression, as well as their neural correlates to the lateralized alpha modulation remain unstudied. To investigate these questions, we recorded magnetoencephalography while participants performed a retrospective cueing paradigm. Time-frequency analysis revealed a larger transient alpha desynchronization for the sensors contralateral to the relevant items which originated from the ventral lateral occipital cortex. Additionally, greater ipsilateral alpha enhancement in the medial occipital cortex occurred later and was maintained until probe presentation. Based on these differences we reasoned that the former would reflect the allocation of selective attention to relevant items, while the later might signal the inhibition of the irrelevant external hemifield instead of irrelevant WM items. Altogether, our results suggest that alpha lateralization does not arise from a unitary phenomenon. Dissociated anatomical and temporal alpha activity might be signaling different functional roles.