trials together, and for rare and frequent events separately (Figs. 3a and b, respectively). The latter clearly revealed differences among the two groups in terms of event-related electric activity at the latency of about 350 ms, which corresponded to the P300 component. The P300 response across trials, obtained comparing the single epochs with the ERP (Supplementary Fig. 3), showed significantly larger intensity for rare than for frequent events (Fig. 3c). A P300 reference time-course for EEG–fMRI correlation analysis could be obtained from the P300 response variations by convolution with a canonical HRF (Fig. 3d). The P300 response time-course calculated using a selection of electrodes over the centro-parietal region showed a large correspondence with those separately extracted from the constituting electrodes (Supplementary Fig. 4), with an average correlation across subjects ranging between 0.90 and 0.94.
