After artifact attenuation, the EEG traces were used to measure the variations of the P300 response across trials. We used signals from a region of interest (ROI) over the scalp, including electrodes Cz, CP1, CP2, Pz, P3, P4, because they proved to consistently reflect this event-related activity (Picton, 1992). A P300 response time-course was calculated for each subject and each EEG signal, according to the procedure described hereafter. For the i-th trial, with i = 1,…N, the time instant ti corresponding to a maximum in the EEG signal was detected in the 300–400 ms post-stimulus interval; then, a P300 wave template p was calculated averaging the single-trial responses ki across the time window [ti−150 ms; ti+150 ms], selecting only those corresponding to rare stimuli. Then, the template p was compared with each trial response ki, including both rare and frequent events, in the time window [ti−150 ms; ti+150 ms], and the P300 response intensity ai was estimated by means of a least-squares fit of ai·p with respect to ki. After calculating the P300 response time-courses for each of the 6
