Interhemispheric and Intrahemispheric Connectivity From the Left Pars Opercularis Within the Language Network Is Modulated by Transcranial Stimulation in Healthy Subjects

Abstract

Neural activity related to language can be modulated within widespread networks following learning or in response to disruption-including the experimental application of noninvasive brain stimulation. However, the spatiotemporal characteristics of such modulation remain insufficiently explored. The present study combined transcranial magnetic stimulation (TMS) and electroencephalography (EEG) to explore the modulation of activity across the language network following continuous theta-burst stimulation (cTBS) of the left pars opercularis. In 10 healthy subjects (21 ± 2 years old, four females), neuronavigated cTBS was delivered over the left pars opercularis of the frontal operculum (part of the traditional Broca's area) at 80% of active motor threshold (AMT) stimulation intensity. Real cTBS and sham cTBS were performed in two different visits separated by at least 48 h. Before, immediately, and 10 min after cTBS, 30 single pulses of TMS were delivered to the left pars opercularis at 80% of the resting motor threshold (RMT), whereas EEG was simultaneously recorded. We examined the cTBS-induced modulation of phase locking values (PLVs) between the TMS-evoked potentials (TEPs) recorded over the pars opercularis and those recorded over its right-hemispheric homolog area, the left supramarginal area, and the left superior temporal area in different EEG frequency bands and different time windows following cTBS. cTBS to the left pars opercularis induced within the gamma band: (1) a significant increase in TEP phase synchronization between the left and right pars opercularis at an early time window (250-350 ms) following cTBS; and (2) significantly increased PLV with the left supramarginal area and the left superior temporal area at a later time window (600-700 ms). In the theta and delta band, cTBS to the left pars opercularis induced significantly increased phase synchronization of TEPs between the left pars opercularis and the posterior left hemispheric language areas at a late time window. In sham condition, there was a significant decrease in TEP phase synchronization of the high beta band between left pars opercularis and left superior temporal area at 200-300 ms. These results contribute to characterize the dynamics of the language network and may have implications in the development of noninvasive stimulation protocols to promote the language rehabilitation in aphasia patients.

Keywords: TMS-evoked potentials; continuous theta-burst stimulation; gamma band; noninvasive brain stimulation; phase synchronization.

Figure 1

Post cTBS T0 TEP recorded at the FC5 electrode. cTBS, continuous theta-burst stimulation; TEP, transcranial magnetic stimulation (TMS)-evoked potential.

Figure 2

cTBS was applied on left pars opercularis area with three pulses at 50 Hz, with an interval of 200 ms between the last pulse of a triplet and the first pulse of a triplet, for a total number of 600 pulses. The figure indicates changes in phase synchronization of TEP by real cTBS on left pars opercularis area of the language network. (A) Top view. (B) Left lateral view. Black text: real cTBS condition. Blue text: sham condition. cTBS, continuous theta-burst stimulation; TEP, TMS-evoked potential.

Figure 3

Changes of phase locking value (PLV) by real cTBS on the left pars opercularis. cTBS, continuous theta-burst stimulation. FC5, FC6, CP3, and CP5 correspond to left pars opercularis, right pars opercularis, left supramarginal area, and left superior temporal area, respectively. *p < 0.05, **p < 0.01. (A) Gamma band changes. (B) Theta band changes. (C) Delta band chnages.

Figure 4

Changes of phase-locking value (PLV) by sham cTBS. FC5 and CP5 correspond to left pars opercularis and left superior temporal area, respectively. *p < 0.05.