Alpha-band lateralization and microsaccades elicited by exogenous cues do not track attentional orienting

Abstract

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. We addressed it by leveraging the phenomenon of Inhibition of Return (IOR), consisting of an initial facilitation in response times (RTs) for stimuli appearing at an exogenously cued location, followed by a suppression of that location. Our behavioral data from human participants showed a typical IOR effect with both early facilitation and subsequent inhibition. By contrast, alpha lateralized in the cued direction after the behavioral facilitation effect, and never re-lateralized compatibly with the behavioral inhibition. Furthermore, we analyzed the interaction between alpha lateralization and microsaccades: while alpha was lateralized toward the cued location, microsaccades were mostly oriented away from it. 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.Significance StatementOur exploration of the surrounding visual environment relies on constant attentional shifts towards different spatial locations, tightly coupled to moment-to-moment changes in electrical brain activity. One of the most prominent signatures of this activity, the alpha oscillations, have long been considered a central substrate of these attentional shifts. Here we show that alpha oscillations, albeit being correlated to the direction of attention toward a salient spatial position, do not track the disengagement from this position. Moreover, we found that alpha oscillations are tightly related to miniature eye movements. Altogether, these findings challenge the notion that alpha oscillations are a fundamental substrate of attentional shifts, and suggest that alpha oscillations subserve a preparatory function for eye movements.