Microsaccades strongly modulate but do not directly cause the EEG N2pc marker of spatial attention

Abstract

The N2pc is a popular human-neuroscience marker of covert and internal spatial attention that occurs 200–300 ms after being prompted to shift attention—a time window also characterized by the spatial biasing of small fixational eye movements known as microsaccades. Here, we show how co-occurring microsaccades profoundly modulate N2pc amplitude during top-down shifts of spatial attention in both perception and working memory. At the same time, we show that a significant—albeit severely weakened—N2pc can still be established in the absence of co-occurring microsaccades. Moreover, despite the strong modulation of the N2pc by microsaccade presence and direction, the N2pc does not align to the precise timing of microsaccades, ruling out that the observed N2pc modulations by microsaccades are a direct artifact of microsaccade-related eye-muscle activity, corneo-retinal dipole movement, or visual inputs moving over the retina. Thus, while microsaccades strongly modulate N2pc amplitude, microsaccades themselves are not a prerequisite for, nor a direct cause of, the N2pc.