Motor cortex mapping with combined MEG and magnetic stimulationponmlkjihgfedc la. Ruohonen a,b, P. Ravazzani a, R.J. Ilmonierni b, G. Galardi c, J. Nilsson d, M. Panizza d, S. Amadio ", F. Grandori a and G. Corni c aXWVUTSRQPONMLKJIHGFEDCBA Systems Theory Centre (CNR), Polytechnic of Milan, Via Ponzio 34/5, 20133 Milan, 1taly b BioMag Lab., Med. Eng. Centre, Helsinki UnivoCentral Hospital, Tukholmankatu 8 F,00290 Helsinki, Finland c Dept. of Neurophysiology, H.s. Raffaele, Via Olgettina 60, 20133 Milan, 1taly "Lab. Neurophysiology, Salvatore Maugeri Foundation, IRCCS, Via Ospedale 32, 46042 Castel Goffredo (MN), Italy Functional Neuroscience (EEG Suppl. 46) Editors: C. Barber, G. Celesia, G.c. Corni and F. Mauguière © 1996 Elsevier Science B.Y. Ali rights reserved Introduction A well-defined organization characterizes the pri- mary motor cortex (MI) in the precentral gyrus; fine voluntary movements of every muscle of the body are controlled by their representation areas in MI, resulting in the familiar motor hornuncu- lus (Penfield and Boldrey 1937; Woolsey et al. 1979). Likewise, many other cortical functions are organized in a map-like manner and it is one of the most exciting goals in brain research to assess the functional organization of the working human brain. There are several non-invasive methods that can be used to locate and map MI, including mag- netoencephalography (MEG), electroencephalog- raphy (EEG), and transcranial electrical stimula- tion (ES) and magnetic stimulation (MS). The evoked electrical potentials or evoked magnetic fields (EEG and MEG, respectively) can be used to observe physiological activation attributed to motor action. Magnetometers with as many as 122 measurement channels have recently become available (Ahonen et al. 1993), and with their res- olution of 1-2 mm MEG is likely to be the most Correspondence to: Jarmo Ruohonen, BioMag Labora- tory, Helsinki University Centrai Hospital, Tukholmankatu 8 F, 00290 Helsinki, Finland; Te!': 358-0-471 5541; Fax: 358-0-471 5781; E-mail: jarrno@biomag.helsinki.fi. 317 precise non-invasive mapping tool (Hamalainen et al. 1993). In MS, a brief current pulse in an excitation coil induces an electric field in the brain (for a review, see Barker 1991). The electric field causes local depolarization of membranes and activation of neurones. With today's devices, MS can provide centimetre-scale information of MI organization (Brasil-Neto et al. 1992b; Wassermann et al. 1992; Pascual-Leone et al. 1994). Although MS has been used for a decade, there still is no certainty about its cortical site of ac- tion. It is thought that cortical elements parallel to the electric field are preferentially excited, since the stimulation threshold depends on the orienta- tion of the exciting electric field (Boniface et al. 1990; Brasil-Neto et al. 1992a; Pascual-Leone et al. 1994); moreover, comparison of the waveform and latency of single fibre responses to ES and MS has led to a further assumption that MS probably excites neural elements near the surface of the gray matter, possibly presynaptic elements that project on to the principal outflow fibres (Mills 1991). A theoretical connection in mathematical mod- elling of neuromagnetism and magnetic stimula- tion has been established (Heller and van Hulsteyn 1992; Ravazzani et al. 1992, 1994; Ruohonen et al. 1995). Here we present for the first time pre- liminary results of an experimental comparison of MI mapping with MS and MEG; in addition,