Modulation of cortical oscillatory activity during transcranial magnetic stimulation

Abstract

Transcranial magnetic stimulation (TMS) can transiently modulate cortical excitability, with a net effect depending on the stimulation frequency (< or =1 Hz inhibition vs. > or =5 Hz facilitation, at least for the motor cortex). This possibility has generated interest in experiments aiming to improve deficits in clinical settings, as well as deficits in the cognitive domain. The aim of the present study was to investigate the on-line effects of low frequency (1 Hz) TMS on the EEG oscillatory activity in the healthy human brain, focusing particularly on the outcome of these modulatory effects in relation to the duration of the TMS stimulation. To this end, we used the event-related desynchronization/synchronization (ERD/ERS) approach to determine the patterns of oscillatory activity during two consecutive trains of sham and real TMS. Each train of stimulation was delivered to the left primary motor cortex (MI) of healthy subjects over a period of 10 min, while EEG rhythms were simultaneously recorded. Results indicated that TMS induced an increase in the power of brain rhythms that was related to the period of the stimulation, i.e. the synchronization of the alpha band increased with the duration of the stimulation, and this increase was inversely correlated with motor-evoked potentials (MEPs) amplitude. In conclusion, low frequency TMS over primary motor cortex induces a synchronization of the background oscillatory activity on the stimulated region. This induced modulation in brain oscillations seems to increase coherently with the duration of stimulation, suggesting that TMS effects may involve short-term modification of the neural circuitry sustaining MEPs characteristics.

Figure 1

Average data of the event‐related power modulations induced by 1 Hz rTMS for the α frequency band (Panel A) and for the β frequency band (Panel B) using the sham TMS reference. The data are shown as a function of three successive stimulation blocks: first in white—from trial 1 to 200; second in gray—from trial 201 to 400; third in black—from trial 401 to 600. On the x‐axis the analyzed recording electrodes are reported. Bars correspond to the standard error of mean.

Figure 2

Scalp distribution maps of the average ERD/ERS induced by the real rTMS for the α frequency band, with sham as reference, represented separately for the three stimulation blocks. Red color represents maximum relative synchronization.

Figure 3

Panel A: Motor‐evoked potential recorded from the right APB; grand averaged data elicited during real TMS of the left MI in three successive stimulation blocks: the first: 1–200 magnetic stimuli (solid line), the second: 201–400 magnetic stimuli (thin line), and the third: 401–600 magnetic stimuli (dashed line). Panel B: Mean amplitude of the motor‐evoked potential (on the left) and of the power synchronization recorded over the electrode C3 (on the right) elicited during the first and the third blocks of stimulation. Bars correspond to the standard error of mean. Panel C: The scatterplot shows the significant correlation between the changes in the amplitude of the MEPs, i.e. a decrease, on the y‐axis and the changes in the power synchronization recorded over the electrode C3, i.e. an increase, on the x‐axis, between the first and the third stimulation blocks. The dotted lines represent 95% confidence limits.