On the focal nature of inhibition and facilitation in the human motor cortex
Introduction
A weak transcranial magnetic stimulus over the human motor cortex can modulate the amplitude of the MEP produced by a stronger magnetic stimulus delivered a few milliseconds later through the same coil (Kujirai et al., 1993). The MEP is inhibited at condition-test intervals of 1–5 ms, facilitated from about 10 to 20 ms and then, particularly when stronger conditioning stimuli are used, inhibited again for a prolonged period (Kujirai et al., 1993, Nakamura et al., 1997). These modulations are believed to occur in the cortex, as they are accompanied by parallel changes in the descending corticospinal volleys recorded over the cervical cord (Nakamura et al., 1997, Di Lazzaro et al., 1998.). The early inhibition and the later facilitation are thought to arise from separate pools of interneurons (Ziemann et al., 1996b).
The early inhibition is only obtained by conditioning stimulation over the motor cortex (Kujirai et al., 1993) but it is not known how focal this process is. We report observations on a patient with a subdural grid of 64 electrodes placed over the motor and premotor cortex for the investigation of seizures. Stimulation through various electrodes allowed the modulations of corticospinal neuron excitability, produced by paired electrical stimulation to be observed with finer spatial resolution.
Section snippets
Methods
The studies were carried out with informed consent and with the approval of the institution's Human Experimentation Committee. The subject was a 32 year old woman with seizures resistant to medical therapy. She had had a febrile convulsion at 13 months of age followed by a transient right hemiparesis. At the age of 26, she developed complex partial seizures which began with facial tonicity, gagging and hypersalivation and then became generalized. The neurological examination and magnetic
Mapping the motor cortex
The approximate position of the cm grid of 64 electrodes is shown in Fig. 1. Single, 1 ms, anodal stimuli given at 1 Hz were used to map the motor cortex. The indifferent electrode was on the arm. The contour lines in Fig. 1 represent zones where visible muscle contractions of the shoulder, arm, face or tongue were obtained with single stimuli, at thresholds below 3 mA (central contours), between 3 and 5 mA (intermediate contours) and between 5 and 10 mA (most peripheral contours).
Modulation of the MEP by conditioning stimuli applied through neighbouring electrodes
The MEP
Discussion
We found that the MEP elicited by stimulating the motor cortex through one pair of subdural electrodes could be modulated by subthreshold stimuli, given through a neighbouring pair of electrodes. The early inhibition (∼1–5 ms), the facilitation (∼10–20 ms) and the late inhibition were similar to those described with paired transcranial magnetic stimuli (Kujirai et al., 1993). These 3 phases likely represent inhibition and facilitation of corticospinal neurons, as the changes in MEP amplitude
Acknowledgements
We thank the Medical Research Council of Canada for financial support (# 6727, P.A.).
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