A transcutaneous neural stimulator which is focal would be an important surgical and clinical tool. It would be used over the scalp in spinal cord surgery to produce motor-evoked responses in order to assess problems in the spinal motor tracts. If it is painless, it would also be used in the awake patient for diagnostic tests of both the central and peripheral nervous systems. Although magneto-electric stimulators are now available commercially which are both transcutaneous and relatively painless, when placed over the head they all induce extended currents over too large an area of the brain and may produce unwanted effects in the extended regions. This is because the stimulating coils used at the head are large (9-14 cm in diameter), in order to minimize engineering problems. A small coil would produce a more focal stimulating current in the brain; however, because it is inefficient, the pulsed current through the coil must be very large, which presents special problems. It is here proposed to develop this small coil. This will be done in a national laboratory which is experienced in developing pulsed electromagnets, in consultation with neurologists working in neural stimulation. The engineering problems in a small coil are of two types: How to produce a large current through the coils, and how to accommodate the large stresses due to this current. A novel design of a coil is suggested which minimizes both problems, where the coil is 2.5 cm in diameter. The development will take place in two stages. First, a 5-cm diameter coil will be built and tested as an intermediate step to evaluate necessary parameters; from these parameters, then, the final 2.5-cm coil will be built. The testing of both coils will consist of both engineering measurements and neural stimulation. The engineering measurements will be of mechanical and electrical quantities, and field distributions. The neural stimulation will be only of the ulnar nerve at the human wrist, where the stimulator will elicit a thumb twitch. By varying the coil location over the wrist and measuring the strength of twitch, the focal properties will be measured and compared with commercial units. From these measurements, the improvement in focal properties can be deduced for stimulating the human brain. Actual human brain stimulation is planned as the next, separate project.

National Institute of Health (NIH)
National Institute of Neurological Disorders and Stroke (NINDS)
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Special Emphasis Panel (SSS (L))
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Massachusetts Institute of Technology
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Cohen, D; Cuffin, B N (1991) Developing a more focal magnetic stimulator. Part I: Some basic principles. J Clin Neurophysiol 8:102-11
Yunokuchi, K; Cohen, D (1991) Developing a more focal magnetic stimulator. Part II: Fabricating coils and measuring induced current distributions. J Clin Neurophysiol 8:112-20
Chiappa, K H; Cros, D; Cohen, D (1991) Magnetic stimulation: determination of coil current flow direction. Neurology 41:1154-5