Disentangling EEG responses to TMS due to cortical and peripheral activations

Abstract

Background: the use of combined transcranial magnetic stimulation (TMS) and electroencephalography (EEG) for the functional evaluation of the cerebral cortex in health and disease is becoming increasingly common. However, there is still some ambiguity regarding the extent to which brain responses to auditory and somatosensory stimulation contribute to the TMS-evoked potential (TEP). Objective/Hypothesis: to measure separately the contribution of auditory and somatosensory stimulation caused by TMS, and to assess their contribution to the TEP waveform, when stimulating the motor cortex (M1). Methods: 19 healthy volunteers underwent 7 blocks of EEG recording. To assess the impact of auditory stimulation on the TEP waveform, we used a standard figure of eight coil, with or without masking with a continuous noise reproducing the specific time-varying frequencies of the TMS click, stimulating at 90% of resting motor threshold. To further characterise auditory responses due to the TMS click, we used either a standard or a sham figure of eight coil placed on a pasteboard cylinder that rested on the scalp, with or without masking. Lastly, we used electrical stimulation of the scalp to investigate the possible contribution of somatosensory activation. Results: auditory stimulation induced a known pattern of responses in electrodes located around the vertex, which could be suppressed by appropriate noise masking. Electrical stimulation of the scalp alone only induced similar, non-specific scalp responses in the in the central electrodes. TMS, coupled with appropriate masking of sensory input, resulted in specific, lateralized responses at the stimulation site, lasting around 300 ms. Conclusions: if careful control of confounding sources is applied, TMS over M1 can generate genuine, lateralized EEG activity. By contrast, sensory evoked responses, if present, are represented by nonspecific, late (100e200 ms) components, located at the vertex, possibly due to saliency of the stimuli. Notably, the latter can confound the TEP if masking procedures are not properly used.

An analytical approach to identify indirect multisensory cortical activations elicited by TMS?

“…Here, we complement those advances by providing a novel statistical approach. Our results converge with the pattern of indirect activation identified by recent experimental work [3], which provides encouraging support for the potential utility of this novel approach.…”

“…One approach is to suppress the sources of multisensory stimulation and consequently minimize indirect cortical activations. This is powerfully illustrated in a recent study, in which the sound of the coil was very efficiently masked by playing noise-masking via earplugs as well as using ear defenders, and the vibrations of the coil were attenuated by using a foam layer beneath the coil [3]. Whether this suppression approach is completely effective is debated (e.g.…”

“…An alternative experimental approach is to use sham stimulation to mimic, and consequently isolate multisensory sources of indirect cortical activation [3e5]. Yet, this requires techniques to accurately reproduce the multisensory response elicited by TMS [3,4], which is challenging to achieve. Both of these approaches deal with indirect responses to stimulation using experimental manipulations within-subjects.…”

Preconditioning Stimulus Intensity Alters Paired-Pulse TMS Evoked Potentials

“…TEPs have been considered to include sensory feedback from muscle activation, which may contaminate TEP measurements [ 5 ]. Whilst this is a caveat to the interpretation of TEPs, the latency of sensory feedback (auditory) affects later portions of the TEP beyond 100 ms [ 40 , 41 ] and are largely confined to the auditory cortex. In fact, selective modulation of early TEP components and corticospinal excitability has been shown with transcranial direct current stimulation, showing that early TEP components are more likely to be reflective of M1 TMS and less likely to be influenced by sensory contamination [ 41 ].…”

A single- and paired-pulse TMS-EEG investigation of the N100 and long interval cortical inhibition in autism spectrum disorder

“…Recent research indicates that somatosensory artefacts might have effects across the TEP (99), and can specifically impact later TEP peaks (100,101), including the N100. This, therefore, must be considered when interpreting these results.…”