Transcranial magnetic stimulation combined with electroencephalography (TMS/EEG) has always been considered a promising tool to study local excitability and causal connectivity. Now, for the first time, there is the possibility to transform the promise into reality.
Since the field was perturbed by the discovery that the TEPs are affected by undesired sensory inputs more than previously suspected, many questioned the technique itself. On the other hand, this provided new fuel to the researchers to look for new approaches. So, in the very last years, the field became extremely dynamic proposing new approaches. In this symposium, we will present some of the most promising of them that have been used to unveil the brain response to TMS with EEG. Finally, our aim is to convince the audience that the field is changing and that it is not only promising, it is becoming a reality.
1) TMS/EEG, what it is and what are the limitations
2) Using brain states and connectivity allows to study EEG responses to TMS
3) Combining TMS with intracranial EEG is feasible and can show brain response to TMS
4) With proper methodological approaches, it is possible to study the very early and the immediate evoked responses (EEG) to TMS
The target audience are all neuroscientists and clinicians that are interested in probing local excitability of specific brain regions, and test causal brain connectivity. Indirectly, the method can also be used for evaluating inhibition/excitation balance in healthy and disease.
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.
, IRCCS Centro San Giovanni di Dio, Fatebenefratelli Brescia, Lombardia
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.
, University "G. d'Annunzio" Chieti/Pescara, Abbruzzo
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.
, Hvidovre hospital Hvidovre, Region Hovedstaten
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 .
, Stanford University Stanford, CA