Fundamentals on Neuromagnetism

Although the first attempt to detect a magnetic signal associated with bioelectric activity in the human brain traces back to the late sixties, and was performed by means of a non-superconducting sensor (Cohen, 1968), it was only four years later than a SQUID was successfully used to record a human magnetic alpha rhythm with a satisfactory signal-to-noise ratio (Cohen, 1972). Some years later (Brenner et al., 1975), magnetic signals associated with brain activity evoked by peripheral sensory stimulation were also detected. Hughes and co-workers (1977) examined some cases of generalized epilepsy, but the identification of the most promising application of the neuromagnetic method in the clinical field, namely the study of focal epilepsy, was achieved by the independent work of two groups at the beginning of the nineteeneighties (Barth et al., 1982; Barth et al., 1984; and Modena et al., 1982; Chapman et al., 1983). During the last decade impressive progress have been achieved in the field of neuromagnetism. Fundamental understanding on the structural organization of primary areas in the human brain (Romani et al., 1982a; Pantev et al., 1988, Hari and Kaukoranta, 1985) has been accompanied by important findings on higher levels of brain functions, such as different sounds processing (Hari and Lounasmaa, 1989), or the effect of memory (Lounasmaa et al., 1989). The generators of some of the natural rhythms, like alpha and mu, have been found to be partly localized in specific cortical areas (Chapman et al., 1984) and, even more fascinating, they are likely to be synchronized in an impressive “resonant”, macroscopic response (Narici et al., 1987). The noninvasive investigation of focal epilepsy has proved to be a new, powerful tool for preoperative diagnosis in a widespread disease (Ricci et al., 1985; Rose et al., 1987). But also in more complex pathologies, such the photoconvulsive response in generalized epilepsy, the neuromagnetic investigation has provided significant new information (Ricci et al., 1989).