The side of the coil with which stimulation is applied also will affect the outcome. With a monophasic pulse, the current travels clockwise with respect to one face of the coil and counterclockwise with respect to the other. This disposition can be used to bias stimulation in one or other direction and has been used to stimulate selectively one or other hemisphere while apparently stimulating in the midline Amassian et al. Stimulation with a figure-of-eight coil increases the focality of stimulation Ueno, Tashiro, and Harada, This configuration is of two circular coils that carry current in opposite directions, and, where the coils meet, there is a summation of the electric field.
However, in experiments where the center of the figure eight is placed over the region of interest, the outer parts of the coil are usually several centimeters away from the scalp and thus unlikely to induce effective fields, therefore increasing the probability that stimulation will be relatively focal: But how focal figure 3. The circular coil has The figure-of-eight coil has 56 mm inside-turn diameter, 90 mm outside-turn diameter mean 73 mm , and nine turns of copper wire on each wing.
The outline of each coil is depicted with dashed white lines on the representation of the induced fields. Figure created by Anthony Barker, used with permission. See plate 1 for color version.
See plate 2 for color version. Given the differences in induced fields, the results of an experiment done with the figure-ofeight—shaped coil may not be reproducible with a circular coil centered over the same brain region because different brain areas would be affected. From the foregoing discussion, one might be forgiven for thinking it impossible to target specific cortical areas with TMS. Several converging lines of evidence now show that there is good reason for confidence in the anatomical focality and, more important, in the functional focality of TMS.
There are many other examples of surface validity: Phosphenes are more likely if the coil is placed over the visual cortex Marg, ; Meyer et al. Mapping of the motor cortex with EMGrecorded responses also shows discrete representations of the fingers, hand, arm, face, trunk, and legs in a pattern that matches the gross organization of the motor homunculus Wasserman et al.
There are more direct measures of the specificity of TMS. Wassermann et al. In all subjects, the estimated fields induced by TMS met the surface of the brain at the anterior lip of the central sulcus and extended along the precentral gyrus for a few millimeters anterior to the central sulcus. When compared with the PET activations, the MRI locations were all within 5 to 22 mm—an impressive correspondence across three techniques.
There are reasons for caution in interpreting these data see Wasserman et al. The evidence for transsynaptic actvation comes from a comparison of the EMG latencies elicited by electrical or magnetic stimulation Day et al.
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Magnetically evoked latencies are approximately 1—2 msec longer than electrically evoked ones, which can be explained on the basis of which neurons are most likely to be stimulated by each technique Rothwell, TMS is more likely to stimulate neurons that run parallel to the cortical surface, whereas electrical stimulation can directly stimulate pyramidal output neurons that run orthogonal to the cortical surface.
Thus, the 1—2 msec delay between electrical and magnetic cortical stimulation may be accounted for by the time taken for the stimulation to be transmitted from the interneurons to the pyramidal cells.
Knowledge of which kinds of cells are stimulated based on temporal information can inform the interpretation of functional specificity. A clear example comes from the work of Heinen and colleagues , who measured central motor-conduction time CCT by recording TMS-evoked EMG responses from the first dorsal interosseous muscle of children mean age seven years and adults mean age twenty-nine years in relaxed and facilitated i.
This relative difference between relaxed and facilitated CCT is attributed to the temporal summation of descending corticospinal volleys at the alpha motoneuron Hess, Mills, and Murray, ; Chiappa et al. Heinen et al. Other evidence strengthens the correlation between targeted and activated cortical regions.
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Paus and colleagues , , have carried out a number of studies in which TMS has been combined with analysis of PET activations using a method of frameless stereotaxy that aligns MRI landmarks and the center of the stimulating coil with an accuracy within 0. The first critical finding of these experiments is that TMS has a major effect approximately under the center of a figure-of-eight coil and secondary effects at sites that are known to be anatomically connected figure 3. The bottom figure shows one of the cortical regions that most likely was activated through spread of stimulation effects, namely the parieto-occipital PO cortex of the ipsilateral hemisphere—a region similar to that known to be connected with the FEF in monkeys.
From Paus et al. From Siebner et al. Further evidence of the accuracy of TMS is seen in figure 3. Siebner and colleagues compared the changes in regional cerebral blood flow caused by 2 Hz rTMS over the motor cortex, at an intensity sufficient to elicit an arm movement, with blood flow changes caused by the actual movement of the arm.
The correspondence was striking. TMS-induced movements and voluntary movements both activated primary sensorimotor cortex SM1; area 4 ipsilateral to the site of stimulation. Voluntary movement also activated the ipsilateral supplementary motor area SMA area 6 , and the motor activity associated with the voluntary movement was more extensive than that elicited by rTMS. This difference could be because the voluntary arm movement was slightly greater than the TMS movement or because voluntary activity involves more muscles than TMS activity.
Whatever the difference, it is a clear example of the specificity of TMS and of the physiological validity of TMS effects. The TMS was given at 1 Hz for 8 sec. The spatial and temporal resolution of the measurements are approximately 2 mm and 3 sec. From Bohning et al. These studies are important examples of the spatial specificity of TMS. They do not mean that the induced electric field is limited to the functional units stimulated, nor do they suggest that activation of neurons is limited to the areas seen in PET and fMRI, but they show unequivocally that the theoretical spread of the induced field is not the determinant of the area of effective stimulation and that the functional localization of TMS is to a significant degree under experimenter control.
Given a coil placed on the scalp with the intention of stimulating the motor cortex, we can say with complete confidence that we will be able to stimulate an unknown number of different kinds of neurons in the vicinity of the motor cortex. With an electrode placed stereotactically in the brain of a rat, we can do much better, but there are still severe constraints. The question is how to interpret the meaning of the stimulation results. If the hypothesis and the knowledge of the system under investigation are sufficient, one can think cellular cf. In most neurocognitive experiments and in the experiments discussed throughout this book, thinking cellular is not an option other than to compare effects with the known properties of neurons from intra- and extracellular recording studies or to combine TMS with pharmacological manipulations.
Studies of EEG responses by Ilmoniemi and colleagues provide another demonstration of the relative primary and secondary specificity of TMS. Within 20—30 msec, this activity is mirrored by a secondary area of activity in the homotopic regions of the contralateral hemisphere. These delays in homotopic areas are a rich source of hypotheses regarding the timing of effects in interhemispheric interactions see chapter 5. Four milliseconds after TMS over the occipital lobe, most of the electrical activity recorded with high-resolution EEG is around the area directly under the TMS stimulation site marked by the X.
By 7 msec, this activity has spread to the midline, and by 28 msec there is clearly contralateral activation. From Ilmoniemi et al. When the intact hemisphere was stimulated, EEG responses were seen in the motor cortex of both hemispheres.
When the motor cortex ipsilateral to the affected basal ganglia was stimulated, some EEG response was seen ipsilaterally, but none was transmitted interhemispherically to the intact hemisphere. Models of the electric field at different depths from the coil suggest that relatively wide areas are stimulated close to the coil, decreasing in surface area as the field is measured at distances farther from the coil. The image offered by these models is of an egg-shaped cone, with the apex, which marks the point of the smallest area of stimulation, farthest from the coil.
This is a result of an interaction between the decrease in magnetic field strength and a progressive loss of focality. For a standard figure-eight coil, one estimate is that stimulation 5 mm below the coil will cover an area of approximately 7 by 6 cm. This area decreases to 4 by 3 cm at 20 mm below the coil—that is, in the region of the cortical surface figure 3. Calculations of induced electric fields as a function of depth also can be used as a guide to specificity because stimulation at points where the fields overlap allows subtraction of the effects.
If the coil positioned at the central site in figure 3. From models of TMS-induced electrical fields, one can infer the region of stimulation. By stimulating at neighboring regions on the scalp, one can refine the these inferences and, notwithstanding the uncertainty of any one field, can make reasonable functional anatomical attributions. So our notion of the effective resolution of TMS can be refined: whereas a single pulse of TMS cannot be said to have a small, volumetric resolution in the cortex, from a functional point of view it can be shown to have a small scalp resolution and an inferred or subtracted volumetric resolution when multiple sites are compared.
A comparison might be made here with fMRI and, say, a cortical area such as visual area V5 Watson et al. Rather, the specificity of this area is inferred, quite properly, by subtracting the activations caused by stationary or colored stimuli or different kinds of visual motion.
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The figure-of-eight coil has five 6 mm inside-turn diameter, 90 mm outside-turn diameter mean 73 mm , and nine turns of copper wire on each wing. Figure created by Anthony Barker; used with permission. See chapter 3. Plate 2 Rate of change of the electric field calculated in the direction of the nerve along the axis AB, measured in the same plane as coils shown in plate 1.
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The duration of the effect in the cortex is difficult to determine because the neurons stimulated by the field may take time to recover their normal functional state and normal interactions with other cells. Several TMS studies have applied single-pulse TMS at intervals of 10 msec and obtained effects that suggest that TMS can distinguish processes within such a small time window, but the time window is probabilistic rather than fixed.
First, the effect of TMS is likely to be an ON step function or at least a steep ramp function because many fibers will be stimulated simultaneously. However, the offset of the effects are likely to be a shallow ramp function because fibers of different sizes and at different orientations will be affected to different degrees and will recover at different rates. As the population of neurons recovers, the neural noise added to the system diminishes. If one also assumes a finite period during which the area stimulated is critical to the task and that this criticality is also probabilistic, then the degree to which TMS will interfere with processing is a function of the noise induced at any time T and the probability that the neurons in that area are involved in the task.
Jalinous, Guide to Magnetic Stimulation The use of TMS is rightly subject to approval by local ethical committees, and some precautions must be taken in all studies using the technique. The upper portion of the figure shows that the intensity of the TMS pulse is greatest close to the time of onset and then declines within 1 msec. The effect this pattern has on behavior is a function of the intensity of the physiological effects of TMS and the probability that the neurons stimulated are critical to the task. The pulse at a would not have a behavioral effect because it is applied too early.
The pulse at b would interfere with behavior because an early i.
Thus, the product of neural noise times neural necessity would be higher at e than at c. The appropriate application of TMS may well be able to have effects at times well before b and c or well after e , the reported peak. The use of ear plugs is recommended in all experiments. Some subjects may experience headaches or nausea or may simply find the face twitches and other peripheral effects of TMS too uncomfortable. Such subjects obviously should be released from any obligation to continue the experiments.
In a number of cases, rTMS did induce epileptic fits, and caution is necessary. As a guide in experimentation, any subject with any personal or family history of epilepsy or other neurological condition should be precluded from taking part in an experiment that does not involve investigation of that condition. Pascual-Leone et al. The paper presents some guidelines for the use of rTMS, so familiarity with this paper should be a prerequisite of using rTMS. However, the paper is not exhaustive; it is based on only three sites of stimulation and expresses pulse intensity as a percentage of motor threshold.
It recently has been argued that studies that apply rTMS to areas other than the motor cortex cannot simply lift stimulation parameters and criteria based on motor cortex excitability and assume they transfer to other conditions. There is no necessary relationship between motor cortex excitability and that of other cortical regions Stewart,Walsh, and Rothwell, The TMS community is constantly reviewing safety procedures, and this Web site is a starting point for access to sound information although much of it is directed to a clinical psychiatry audience.