Electromagnetic Functional Imaging of the Brain

　Risto Ilmoniemi,

　　BioMag Laboratory, Medical Engineering Centre, Helsinki

University

　Central　Hospital, Finland

At the BioMag Laboratory of the Helsinki University Central Hospital, we　are combining several techniques for functional imaging of the brain. For　simultaneous MEG and measurements, we have integrated to our 122-channel　MEG (Neuromag Ltd.) array a 64-channel EEG system (Virtanen et al.　Electroenceph. Clin. Neurophysiol. 99, 568, 1996). This technology is now　available with the new 306-channel MEG system as well. In an effort to　combine fMRI to help solve the MEG inverse problem, we have compared MEG　and fMRI localization (Korvenoja et al., Human Brain Mapping 8, 13, 1999;　Ahlfors et al., J. Neurophysiol., in press). We have compared motor cortex　localization with MEG and transcranial magnetic stimulation (TMS). With　TMS-compatible 60-channel EEG (Ilmoniemi et al., NeuroReport 8, 3537,1997), we can now record TMS-evoked cortical activation. The local response　provides an index of cortical reactivity while activity elicited elsewhere　in the brain implies a neural pathway connecting the stimulated site and　the activated site. We are also studying how TMS affects sensory evoked　potentials and spontaneous EEG. In stroke, we have studied somatosensory cortices with MEG, finding changes　in cortical responses that accompany the recovery of the patient's symptoms　(Wikstrom et al., Clin. Neurophysiol. 110, 916, 1999). We are extending　these studies, using fMRI and TMS.　We study sensory memory with the mismatch negativity (MMN) paradigm, in　which a neuronal memory trace is formed by repeated presentation of the　same sound to the subject. When a deviant sound is presented, an automatic　recognition mechanism gives rise to strong electrical activation (Naatanen,　Ilmoniemi, Alho, Trends Neurosci. Trends Neurosci. 17, 389, 1994). The MMN　technique has been able to locate phoneme-memory traces and to demonstrate　the existence of long-term phoneme memory specific to mother tongue　(Naatanen et al., Nature 385, 432, 1997).