ORIGINAL PAPER Spatial Correspondence Between Functional MRI (fMRI) Activations and Cortical Current Density Maps of Event-Related Potentials (ERP): A Study with Four Tasks Ludovico Minati Æ Cristina Rosazza Æ Ileana Zucca Æ Ludovico D’Incerti Æ Vidmer Scaioli Æ Maria Grazia Bruzzone Accepted: 15 August 2008 / Published online: 29 August 2008 Ó Springer Science+Business Media, LLC 2008 Abstract We investigated the spatial correspondence between functional MRI (fMRI) activations and cortical current density maps of event-related potentials (ERPs) reconstructed without fMRI priors. The presence of a sig- nificant spatial correspondence is a prerequisite for direct integration of the two modalities, enabling to combine the high spatial resolution of fMRI with the high temporal resolution of ERPs. Four separate tasks were employed: visual stimulation with a pattern-reversal chequerboard, recognition of images of nameable objects, recognition of written words, and auditory stimulation with a piano note. ERPs were acquired with 19 recording channels, and source localisation was performed using a realistic head model, a standard cortical mesh and the multiple sparse priors method. Spatial correspondence was evaluated at group level over 10 subjects, by means of a voxel-by-voxel test and a test on the distribution of local maxima. Although not complete, it was significant for the visual stimulation task, image and word recognition tasks (P \ 0.001 for both types of test), but not for the auditory stimulation task. These findings indicate that partial but significant spatial correspondence between the two modalities can be found even with a small number of channels, for three of the four tasks employed. Absence of correspondence for the auditory stimulation task was caused by the unfavourable situation of the activated cortex being perpendicular to the overlying scalp, whose conse- quences were exacerbated by the small number of channels. The present study corroborates existing literature in this field, and may be of particular relevance to those interested in combining fMRI with ERPs acquired with the standard 10-20 system. Keywords Event-related potentials Á Functional MRI Á Spatial correspondence Á Visual stimulation Á Image recognition Á Word recognition Á Auditory stimulation Introduction Electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) are very different techniques: the former records the electric potentials generated by synchronous activity of large populations of neurons, while the latter records a haemodynamic response which follows changes in the intensity of neural activity. Nevertheless, the two techniques share a common neurophysiological basis, since both the EEG signal and the blood-oxygen level dependent (BOLD) signal are largely determined by post- synaptic activity (Regan 1989; Logothetis and Pfeuffer 2004). Electroencephalography, in the form of event-related potentials (ERPs), and fMRI have complementary roles in L. Minati (&) Á C. Rosazza Á I. Zucca Science Direction Unit, Fondazione IRCCS Istituto Neurologico ‘‘Carlo Besta’’, Via Celoria 11, 20133 Milano, MI, Italy e-mail: lminati@istituto-besta.it L. Minati Á C. Rosazza Á I. Zucca Á L. D’Incerti Á M. G. Bruzzone Neuroradiology Department, Fondazione IRCCS Istituto Neurologico ‘‘Carlo Besta’’, Milan, Italy C. Rosazza Institut des Sciences Cognitives, CNRS, Lyon, France V. Scaioli Neurophysiopathology Department, Fondazione IRCCS Istituto Neurologico ‘‘Carlo Besta’’, Milan, Italy 123 Brain Topogr (2008) 21:112–127 DOI 10.1007/s10548-008-0064-3