Assessing SICI Cortical Inhibition With DLPFC TMS-EEG Using Different ISI Values

The evaluation of cortical inhibition (CI) in dorsolateral prefrontal cortex (DLPFC) using transcranial magnetic stimulation combined with electroencephalography (TMS-EEG) has focused great attention in recent years. One of the most common procedures to assess such inhibition is the short-interval cortical inhibition (SICI), a paired-pulse paradigm defined by the interstimulus interval (ISI). While SICI was initially defined and extensively used in studies targeting the motor cortex, its application to the DLPFC is relatively recent. However, little is known about how ISI values affect the DLFPC, especially in terms of the TMS-evoked potentials (TEP). This study aims to quantitatively compare the effects of different ISI values in the SICI protocol on CI in the DLPFC by assessing TEPs. Eighteen healthy subjects underwent the SICI protocol using two different ISIs: 2 ms and 4 ms. TMS-EEG responses for both ISIs were characterized in terms of amplitude, latency, peak-to-peak amplitudes, and the area under the rectified curve of the TEPs. The results indicate that TEPs differed depending on the ISI used, finding a significantly greater inhibition with 2 ms ISI, as reflected by a more pronounced reduction in TEP amplitude. These findings are consistent with previous literature on motor cortex stimulation, suggesting that a greater reduction in TEP, and hence greater inhibition, is likely achieved with a 2 ms ISI. Therefore, the study helps in the standardization of SICI protocol in DLFPC.