Topography of the inhibitory and excitatory responses to transcranial magnetic stimulation in a hand muscle

doi:10.1016/0168-5597(93)90116-7

Electroencephalography and Clinical Neurophysiology/Evoked Potentials Section

Volume 89, Issue 6, December 1993, Pages 424-433

https://doi.org/10.1016/0168-5597(93)90116-7 Get rights and content

Abstract

We studied the excitatory motor evoked potentials (MEPs) and the inhibitory (silent period) responses to focal transcranial magnetic stimulation (TMS) in the abductor pollicis brevis (APB) of 5 normal subjects to learn whether the scalp topography of the two responses differed. At the scalp location where stimulation produced the highest-amplitude MEP in the voluntarily activated APB, stimulus intensities below the MEP threshold produced silent periods with little or no preceding facilitation. The silent periods had a mean duration of 26.8±6.8 msec and a mean onset latency of 27.6±3.6 msec, which was 7.2±2.3 msec longer than the latency of MEPs produced in the by higher stimulus intensities. A period of excitation, with an onset latency of 50–80 msec, often followed the silent period. On averaged trials, a stimulus intensity just above the threshold of the MEP at its optimal position produced MEPs followed by silent periods at a cluster of scalp locations 1 cm apart on the central scalp (medial area) and silent periods with very slight or no preceding facilitation in 3–9 locations lateral to the MEP area (lateral area). This finding was confirmed in 3 subjects with maps constructed from statistical analysis of multiple trials. These maps also showed that MEPs produced from the medial area occurred 4–6 msec earlier than those produced from the lateral area. The integral of the silent period tended to be larger in the lateral area. The motor representation of APB, as defined by TMS, is not homogeneous but rather contains at least two components that differ physiologically and topographically.

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Electroencephalogram (EEG) is a well-known painless and noninvasive method, measuring the real time brain electrical activity. Transcranial Magnetic Stimulation (TMS) is another noninvasive, painless and safe method, to modulate nerve cells activity. TMS has been increasingly used as a tool in neurosciences. However, its basic mechanisms are not yet fully understood. Simultaneous TMS/EEG recordings are technically challenging, but could provide some more useful information. Eleven normal subjects were submitted to TMS/EEG to investigate its effect on the primary motor cortex area, describing its properties both in time and frequency domains. TMS was also applied to a non-human head model (muskmelon - Cucumis sp.), to evaluate the effect of stimuli artifacts in the electrodes. Time domain techniques revealed the P60, N100, P190 and N280 components. In frequency domain analysis, oscillations between 0.5–70 Hz were observed. After adjusting the TMS capacitor recharge time in order to avoid artifacts during the acquired signal stretch, oscillations were reduced to the 0.5−20 Hz range. Results from other publications were replicated in our population. This information may be useful for motor pathways future studies. TMS capacitors recharge time must be setup to avoid undesired artifacts that could put results in doubt. It is also advisable that the recharge time must also be clearly informed in other works involving TME and EEG.
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The relationship between fMRI and nTMS mapped excitatory (MEP) and inhibitory (SP) responses, and whether the choice of motor task affects this relationship, have not been studied before.
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Motor mapping can also demonstrate the presence of weak and probably polysynaptic ipsilateral corticospinal pathways to upper extremity muscles (Wassermann et al., 1994; Ziemann et al., 1999). Additionally, CSPs can be mapped (Wassermann et al., 1993). Introduced some years ago, navigation systems, integrating individual brain imaging data and dedicated to TMS practice, serve several objectives (Lefaucheur, 2010): (i) to determine the exact cortical location of a TMS target; (ii) to ensure the reproducibility of TMS targeting during repeated sessions or follow-up studies; (iii) to improve the accuracy of TMS motor mapping methods; (iv) to determine the functional involvement of a cortical region (e.g., motor ability or speech), especially in the context of presurgical mapping.
These guidelines provide an up-date of previous IFCN report on “Non-invasive electrical and magnetic stimulation of the brain, spinal cord and roots: basic principles and procedures for routine clinical application” (Rossini et al., 1994). A new Committee, composed of international experts, some of whom were in the panel of the 1994 “Report”, was selected to produce a current state-of-the-art review of non-invasive stimulation both for clinical application and research in neuroscience.
Since 1994, the international scientific community has seen a rapid increase in non-invasive brain stimulation in studying cognition, brain–behavior relationship and pathophysiology of various neurologic and psychiatric disorders. New paradigms of stimulation and new techniques have been developed. Furthermore, a large number of studies and clinical trials have demonstrated potential therapeutic applications of non-invasive brain stimulation, especially for TMS. Recent guidelines can be found in the literature covering specific aspects of non-invasive brain stimulation, such as safety (Rossi et al., 2009), methodology (Groppa et al., 2012) and therapeutic applications (Lefaucheur et al., 2014).
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Moreover, the SPs and the MEPs do not have similar dependence on stimulation intensity (Inghilleri et al., 1993). There is also some evidence that SPs and MEPs might have a different cortical topography (Wassermann et al., 1993). However, there are also studies where similar topographies have been revealed (Wilson et al., 1993).
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Topography of the inhibitory and excitatory responses to transcranial magnetic stimulation in a hand muscle

Abstract

Brain Res.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Brain Res.

Neurosci. Lett.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Magnetic coil versus electrical stimulation of monkey motor cortex

J. Physiol. (Lond.)

Basic mechanisms of magnetic coil excitation of the nervous system in humans and monkeys: application of focal stimulation of different cortical areas in humans

Topographic organization of cortical efferent zones projecting to distal forelimb muscles

Exp. Brain Res.

Topographic mapping of the human motor cortex with magnetic stimulation: factors affecting accuracy and reproducibility

Electroenceph. clin. Neurophysiol.

Motor unit responses in human wrist flexor and extensor muscles to transcranial cortical stimuli

J. Neurophysiol.

Patterns of facilitation and suppression of antagonist forelimb muscles from motor cortex sites in the awake monkey

J. Neurophysiol.

Cortical outflow to proximal arm muscles in man

Brain

Inhibition of voluntary contraction by transcranial magnetic stimulation of the brain subthreshold for excitation in man

J. Physiol. (Lond.)

Electrical and magnetic stimulation of human motor cortex: surface EMG and single motor unit responses

J. Physiol. (Lond.)