Abstract
In this study, we investigated the effects of repetitive paired-pulse transcranial magnetic stimulation (TMS) on the motor cortex excitability. The interstimulus intervals (ISIs) between the conditioning (first) stimulus and test (second) stimulus were 1,000, 200 and 100 ms. A total of 20 stimuli were delivered to the primary motor cortex using paired-pulse TMS at 10 s intervals, and the intensity of the magnetic stimulus was 110% of the resting motor threshold for each subject. For all ISIs, there was no significant correlation between the number of stimuli and the motor evoked potential (MEP) amplitude. However, there was slight correlation between the number of stimuli and the MEP amplitude in the 200 and 100 ms ISIs (200 and 100 ms ISIs: approximately r = −0.40; 1,000 ms ISI: r = −0.17). In addition, MEP amplitude increased during the second paired pulse stimulation when an ISI of 200 ms was used, but it decreased dramatically when an ISI of 100 ms was used. In contrast, MEP amplitude did not change significantly when a 1,000 ms ISI was employed. Therefore, 10 consecutive paired-pulse TMS stimuli with a 1,000 ms ISI may not have a cumulative effect on cortical excitability. Previous studies used a long duration (e.g., 6 minutes or longer by 0.1 Hz magnetic stimulation) of magnetic stimulation to evaluate cortical excitability. In evaluating cortical excitability by magnetic stimulation, it is important to shorten the period to reduce the subject’s burden and to prevent body movement. Thus, our present findings suggest that repetitive paired-pulse TMS with an ISI of 1,000 ms is suitable for the rapid evaluation of cortical excitability.
Highlights
Transcranial magnetic stimulation (TMS) is a non-invasive method for directly stimulating the human brain
The amplitude of the motor evoked potential (MEP) induced by the second stimulation at an interstimulus intervals (ISIs) of 200 ms increased significantly compared with the amplitude of the MEP evoked by the first stimulation (p < 0.05, paired t-test), and the rate of increase of the MEP amplitude was approximately 150% compared with the first stimulus (1.47 ± 1.29, mean ± SD)
The amplitude of the MEP elicited by the second stimulation at an ISI of 100 ms decreased significantly (p < 0.01), and the rate of decrease in MEP amplitude was approximately 60% compared with the first stimulus (0.42 ± 1.33)
Summary
Transcranial magnetic stimulation (TMS) is a non-invasive method for directly stimulating the human brain. The magnetic field pulse generated by a magnetic coil over the scalp induces eddy currents in the brain.. The development of the figure-eight flat coil permits stimulation of brain tissues at 5 mm resolution.. The development of the figure-eight flat coil permits stimulation of brain tissues at 5 mm resolution.2 These techniques have advanced the treatment of neurological disease and studies of brain function. Using successive magnetic stimulation at supra-motor threshold, magnetic stimulation below 1 Hz suppresses cortical excitability, while stimulation above 5 Hz facilitates cortical excitability.. Previous studies in which successive magnetic stimulation was given to the primary motor cortex suggest that peripheral muscle activity may affect cortical excitability. Using successive magnetic stimulation at supra-motor threshold, magnetic stimulation below 1 Hz suppresses cortical excitability, while stimulation above 5 Hz facilitates cortical excitability. Previous studies in which successive magnetic stimulation was given to the primary motor cortex suggest that peripheral muscle activity may affect cortical excitability.
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