[1] 8w Magnetic stimulation of the brain: starting or stopping

[1] 136w A very brief magnetic pulse is able to stimulate cortical neurons through the intact scalp. It can elicit movement of a limb by activation of pyramidal cells either directly or via interneurons. When making a voluntary movement, a corresponding program of the task is thought to be compiled beforehand. In patients with ideomotor apraxia who can not perform a task, a lesion of the left hemisphere is frequently found. A magnetic cortical stimulus given directly before a movement is executed is known to delay the onset without altering the pattern. When it will become feasible to deliver a localized stimulus to one hemisphere, this will induce a temporary pause in that area and thus simulating a lesion in intact brain. In this way information can be gathered on the localization of voluntary action in intact brains.

[1] 153w The concept of motor programs is essential when trying to understand the coordination of movement. Ideational apraxia is often seen with lesions in both parietal lobes as whereas ideomotor apraxia seems to be more related to left hemisphere lesions. Looking at current concepts of anatomical localization of the processing of voluntary action, based on lesions in the central nervous system, and the new opportunities to disrupt the process involved, new possibilities to study movement in intact man appear. One can instruct a person to make a bilateral movement and then selectively stimulate each hemisphere. This paradigm would require a small coil with which one can selectively stimulate one hemisphere. There might be a delay in the execution of the task for both hands if the left hemisphere is stimulated, since the running of the program is postponed. A delay on the left side only will be expected on stimulation of the right hemisphere.

[2] 26w The technique of cortical magnetic stimulation is improving so quickly, that such highly localized focal stimulation may be expected to become possible in the near future.

[1] 74w Stimulation of the descending motor pathways of the brain can be performed by an electromagnetic pulse, A very briefly lasting magnetic field is generated by current through a coil. The coil is placed over the motor area of the brain. A small counter-current is induced in the brain stimulating cells of the pyramidal tract directly or via interneurons. Motor evoked potentials (MEPs) can be recorded with electromyographic registration (EMG) of limb muscle activity [l].

[2] 117w This technique enables one to measure the central motor conduction time (CMCT) and provides information regarding the stimulus threshold of the motor cortex, which is revealed by the intensity of the required stimulus and the form of the MEP But not only can a stimulus elicit a movement, it can interfere with a planned task as well. When such a stimulus is given immediately before a planned movement, a delay of the start ensues [2]. It is thought that before the performance of a movement planning of the firing sequence of the neurons forms a motor program. Stimulation of the brain can elicit simple movements but also suppress visual perception [3] or halt ongoing voluntary movements [2].

[3] 63w The first application of the magnetic stimulator has been to measure conduction times through the central nervous system to arm and leg muscles. It has been shown that when maximal intensity is used, the variability in CMCT and size of the MEP are minimal. Since there has been a growing interest in the effect of submaximal and of more focal modes of stimulation.

[4] 100w When stimulating the cortex magnetically, holding the coil and the center of the magnetic field over the vertex one hemisphere is preferentially stimulated. The threshold is different for each hemisphere with the same position of the coil. By turning over the coil, the other hemisphere is prcfercntially stimulated. It has been inferred that when the induced current flows in the anterior direction, the interncurons seem to be more easily activated. However, using this position of the coil over the vertex, both hemispheres are stimulated simultane~)usly and other events which cannot be seen or simply bc measured may take place 131.

[5] 66w When comparing persons with outspoken handedness to ambidextrous persons a lower threshold for the right hemisphere in ambidexter subjects was observed. In subjects with an outspoken handedness, there was no difference in threshold [4]. The corticospinal tracts are not asymmetric however, microscopically and cannot easily be held responsible. The conclusion must be that some other factor than simple sensitivity for cortical stimulation must play a role.

[6] 186w When a person is asked to perform a task simultaneously with both hands, information is processed in the left posterior cortex where the visual and somatosensory traces of the experience of movements are stored. From here, via the left premotor cortex, the left motor cortex activates the right hand. In order to move the left hand, an intact callosal pathway must exist and the right premotor cortex is activated in order to move the left hand [5]. When there is an impairment in the execution of a learned movement in response to a stimulus in a cooperative subject, the term apraxia is used. Ideomotor apraxia is defined as a failure to carry out a simple task, ideational apraxia denotes an impairment to perform a sequence of movements. Callosal lesions can therefore be responsible for ideomotor apraxia of the left hand in more complex movements, whole ideational apraxia might result from lesions of both hemispheres [6]. The question arises whether it would be possible to selectively stimulate one hemisphere and to test hypotheses on the function of the brain with the new technique of magnetic cortical stimulation.

[7] 84w Recent studies made clear that a cortical stimulus just prior to a movement is capable of postponing it without changing the EMG pattern [;?I, This would imply that a program is stored in a kind of short term memory. The execution or running of the program is thought to continue as soon as the inhibited neurons have recovered. The delay is longer when the stimulus is delivered closer to the intended onset of intended movement. No attention was given to the influence of dominance.