The latest advancements in TMS/EEG: the field is changing

Current advances in EEG systems allow to extract cortical responses evoked by TMS pulses, i.e., TMS-evoked potentials, very early after the TMS pulse. Here, I will show evidence that these responses contain unique information on the network in which the stimulated area is embedded. Several studies suggest that TEPs represent secondary responses of distant areas connected to the initial target. Features of very early-latency TEPs are associated with structural and functional connectivity measures of specific white matter tracks and are modulated during task execution. Moreover, given the high temporal and spatial resolution of TMS, it is possible to exploit TEPs to track changes in cortico-cortical connectivity during task execution and gain a better understanding of the physiological mechanisms that subtend behaviour.  

In recent decades, systems neuroscience has provided evidence for the formation of brain networks transiently linking distributed brain regions, both during tasks and at rest. Concurrently, studies using Transcranial Magnetic Stimulation (TMS) have supported the notion that considering the brain's state just before stimulation—measured, for instance, through simultaneous ElectroEncephaloGraphy (EEG)—can significantly enhance the reliability of stimulation effects. These studies have mostly focused on information gathered from specific brain regions or EEG channels. In this paper, we propose potential avenues for defining and utilizing broad-scale, or network-level, brain states to guide TMS procedures. Initially, we outline a conceptual definition of these brain states, then illustrate examples from actual TMS–EEG data, demonstrating the feasibility of identifying such states. Finally, we contemplate how this approach could be used in real-time to deliver stimulation tailored to the brain's current state. 

The transcranial evoked potentials (TEPs), which are the EEG recorded responses to a TMS pulse, have been mainly used to study the late (>30 ms) cortical responses, due to the artifacts induced by the TMS. Specifically, the first 5 ms are covered by the magnetically induced ringing artifact, and the first 10 to 20 ms are affected by the twitch of the scalp muscles. Here, I will show how to minimize these artifacts in order to unveil the immediate brain response of the primary motor hand area (M1-HAND) to TMS (ca. 2 milliseconds after the pulse), which we called immediate TEPs (i-TEPs). Thereafter, I will talk about the properties of i-TEPs and about how they are affected by stimulation intensities and current directions. Finally, I will discuss their properties in relation to previously observed physiological brain activity with the aim of understanding their origin. 

Knowing the neural mechanisms of transcranial magnetic stimulation (TMS) from the cortex would allow to understand its effects in the stimulated region and in connected areas, and give an interpretation to the responses observed in EEG. Here, I will talk about TMS effects on intracranial electrocorticography (iEEG) in 22 neurosurgical patients. First, I will introduce the safety evaluations in a gel-based phantom, then I will show the intracranial responses to single TMS pulses targeting the dorsolateral prefrontal cortex (dlPFC). Our results show that TMS consistently induces responses in the dlPFC and connected regions, including the anterior cingulate and insular cortex.
The aim of my presentation is to show that TMS can be recorded with intracranial electrodes in humans, and that we observed no adverse effects of TMS-iEEG experiments in twenty-two participants to date. While encouraging, caution must be taken to ensure continued patient safety.
Finally, I will discuss on how TMS-iEEG can be an informative novel methodology in the ongoing efforts to understand the underlying mechanisms of TMS, and consequently the evoked activity observable in TMS/EEG .