Abstract : During atrial fibrillation (AF), atrial activity (AA) on the surface ECG consists of a pattern of quasi-periodic oscillations (f-waves), which are related to the electrical activation of the atrial substrate. However, to date no direct comparison between the extracted f-wave pattern in surface recordings and specific activation sites within the atria has been carried out. In the present study, one reference intracardiac modality consisting of a bipolar electrogram (EGM) recorded from the left atrial appendage (LAA) is exploited for the first time to guide the extraction of LAA electrical activity from standard 12-lead ECG recordings. A periodic component analysis (πCA) technique is employed for this task. The performance of the proposed multimodal extraction technique is compared to that obtained employing a noninvasive, fully blind approach, namely, independent component analysis (ICA). On a database of 31 AF patients, results suggest that the estimation of LAA activity is indeed possible, even though its contribution to the ECG total power is relatively low. Interestingly, ICA seems to provide a slightly better estimation of LAA activation rate, expressed in terms of dominant frequency (DF). On the other hand, the multimodal invasive approach performs better QRST complex suppression and provides AA waveforms with narrower spectra.