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Summary: A new method for mental analyses
Review: Written for research neurologists, this handsomely printed book introduces the subject of transcranial magnetic stimulation (TMS), which joins electroencephalography (EEG), event related potentials (ERPs), magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI), and positron emission tomography (PET) as a new tool for studying the dynamics of the human brain. In simplest terms, TMS floods a restricted region of the neocortex with a large magnetic field (about 2 tesla or 20,000 gauss) for a fraction of a millisecond. In order to influence brain dynamics, it is now understood, a magnetic field must be rather large (the earth's magnetic field is about 0.5 gauss), and it must be rapidly established. The broad effect of such a sudden intrusion of magnetic field energy is to introduce computational noise into the neocortical dynamics, interfering with motor activity and causing the perception of spots of light (phosphenes), in addition to more subtly influencing the brain's behavior in a variety of ways.The basic idea of TMS is simple---a steady voltage source (power supply) charges a storage capacitor to some 2 kilojoules of energy, which is suddenly discharged as magnetic field energy through a magnetic stimulating coil by closing a solid-state switch. In an interesting early chapter, the authors of Transcranial Magnetic Stimulation trace the checkered history of related ideas from the discovery of magnetic induction by Michael Faraday in 1831 to the practical realization of TMS by Anthony Barker and his colleagues at Sheffield, England in the mid-1980s. Why ``checkered''? Our brains are relatively insensitive to magnetic fields of ordinary strength (try waving a horseshoe magnet near your temple) so much of the nineteenth-century speculation on mental effects of magnetic fields was pseudoscientific or worse. The brain is insensitive to magnetic fields because it is not an electromagnetic medium. In other words, cortical dynamics are dominated by interacting electric fields and ionic currents, in which magnetic fields play only minor roles. Normal cortical currents generate minute magnetic fields which are observed in MEG measurements, and the very large external magnetic fields that are suddenly turned on under TMS generate small transmembrane voltages that may influence the course of these currents, but this picture is very different from a true electromagnetic medium---such as a propagating radio wave or the output light beam from a laser---in which the total energy remains essentially conserved as it oscillates rapidly back and forth between electric and magnetic energy. In the brain, to the contrary, electric field energy is continually being generated by hydrolysis of adenosine triphosphate, stored in transmembrane potentials, and dissipated through myriad nonlinear electric field interactions with transmembrane ionic currents. Whatever small magnetic fields that are present do not influence normal functioning of the brain. In neuroscience research, TMS has the advantage of acting as a noninvasive ``virtual lesion'' which can be rapidly induced over a region that penetrates an inch or so into the neocortex and is transversely localized to roughly a square inch. Because cortical fibers are somewhat randomly oriented, such a localized and rapidly rising TMS field generates a random spectrum of transmembrane voltages that are either excitatory or inhibitory depending on the local orientation of a particular fiber, thereby introducing the functional equivalent of computational noise. About 20 ms after the end of a TMS pulse, the major effects disappear, and an experiment can be repeated; thus it is feasible to measure changes in timing delays for various motor responses as the stimulating coil is moved over the scalp, a research activity of considerable current interest. In addition to such timing experiments, which the authors of this book describe in some detail, there is also the possibility of using TMS in a ``repetitive'' mode called rTMS. In this mode, a periodic series of magnetic pulses are generated, and the experimenter has the opportunity of introducing steady noise into a restricted volume of the cortical dynamics for a well defined interval of time. This leads to the possibility of observing subsequent effects on a variety of subtle cognitive activities, many of which are reviewed and described in this book. TMS safety is evidently a key concern; the authors mention that no one with a history of epilepsy should be used as a subject. Clearly this is a matter that neuropsychological researchers should consider carefully as they put their subjects through an increasing variety of subjective experiences, including visual suppression and extinction, search interference, geometrical perception, perception of temporal sequence, variations in attention, perceptual learning, and memory inhibition. Considering all of these possible applications, it seems safe to predict that both TMS and rTMS will make important contributions to research in neuropsychology over the next few years. As an introduction, Transcranial Magnetic Stimulation is highly recommended for all who would take up this exciting activity. Alwyn Scott
http://personal.riverusers.com/~rover/
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