NONINVASIVE STIMULATION OF HUMAN CENTRAL NERVOUS SYSTEM

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Recently, techniques have become available for the noninvasive stimulation of the human cortex and deep proximal peripheral nerves. Stimulation can be with a high-voltage, extremely brief electrical pulse or with magnetic stimulation. One purpose is to use these methods for noninvasive localization of different parts of the human cortex including motor cortex, sensory cortex and language cortex. Another purpose is to study cortical physiology in different disease states. We have continued to make advances in understanding the technical aspects of magnetic stimulation, defining the optimal method to map different body part representations in the motor cortex. We have been able to compare the site of stimulation with the area of the brain activated in positron emission tomography (PET) scans by mapping the results of both studies onto the same magnetic resonance imaging scan; correlation is excellent. Extensive effort has been devoted to the study of the inhibitory effects of brain stimulation. We have shown, for example, that the ipsilateral silent period does not appear to be transmitted by the corpus callosum. We have also studied the response of the brain to pairs and trains of stimuli. The brain undergoes a complex response of excitation and inhibition. With long and strong trains, epileptic seizures can be precipitated, and we have developed safety guidelines to avoid this. We have studied reaction time to brain stimulation and found that magnetic stimulation can speed the response to any other stimulus. Magnetic stimulation can also speed the responses in patients with Parkinson's disease, and this effect may be useful therapeutically. Following-up our previous studies that showed reorganization of motor cortex pathways following anesthetic block of the forearm and hand, we have shown that this effect is probably mediated largely at the cortical level. In relation to sensory effects, we have demonstrated that the sites of stimulation that block somatosensory sensation differ from those responsible for motor activation. We have continued to study the visual cortex demonstrating that it was possible to produce brief visual images in blind patients. This technique may identify patients suitable for a visual prosthesis that uses direct brain stimulation.