Despite the large number of studies on ongoing and task-related brain activity in different behavioral conditions, little is known on how intrinsic variability relates to and is comparable with event-related responses, both in EEG (Arieli et al., 1996; Makeig et al., 2002, 2004) and fMRI (Fair et al., 2007; Fox et al., 2006, 2007; Fox and Raichle, 2007). Nonetheless, recent oddball studies suggested that a better understanding of the cerebral processes underlying target detection could be achieved by means of the single-trial analysis of the P300 responses (Benar et al., 2007; Eichele et al., 2005, 2008). On the basis of the above findings, we propose that this variability could be related to the distinct cortical networks in which coherent intrinsic activity is organized, and particularly to the ventral and dorsal attention networks. In order to understand their different contribution to target detection, as well as their link to the P300 response fluctuations, we investigated the ongoing cerebral responses during a visual oddball paradigm by means of simultaneous EEG–fMRI.
