Alpha-band lateralization and microsaccades elicited by exogenous cues do not track attentional orienting
We explore the world by constantly shifting our focus of attention towards salient stimuli, and then disengaging from them in search of new ones. The alpha rhythm (8-13 Hz) has been suggested as a pivotal neural substrate of these attentional shifts, due to its local synchronization and desynchronization that suppresses irrelevant cortical areas and facilitates relevant areas, a phenomenon called alpha lateralization. Whether alpha lateralization tracks the focus of attention from orienting toward a salient stimulus to disengaging from it is still an open question. In this study, we addressed this question by leveraging the well-established phenomenon of Inhibition of Return (IOR), consisting of an initial facilitation in response times (RTs) for target stimuli appearing at an exogenously cued location, followed by a suppression of that location. Our behavioral data showed a typical IOR effect with both early facilitation and subsequent inhibition. By contrast, alpha was lateralized only in the cued direction, but never re-lateralized in a manner compatible with the behavioral inhibition effect. Importantly, also the initial lateralization towards the cue ocurred too late to account for the behavioral facilitation effect. Furthermore, we analyzed the interaction between alpha lateralization and microsaccades: at the same time when alpha was lateralized towards the cued location, microsaccades were mostly oriented away from the cued location. Crucially, the two phenomena showed a significant positive correlation. These results indicate that alpha lateralization reflects primarily the processing of salient stimuli, challenging the view that alpha lateralization is directly involved in exogenous attentional orienting per se. We discuss the relevance of the present findings for an oculomotor account of alpha lateralization as a modulator of cortical excitability in preparation of a saccade.