The ups and downs of analyzing cortico-cortical connectivity with TMS-EEG

Authors

• JA Reeves
• C Altany
• MV Freedberg
• EM Wassermann

Abstract

Brain connectivity is integral to mental processes, such as memory and attention. Connectivity has traditionally been studied using fMRI. This technique has revealed important connectivity findings, but is relatively insensitive to temporal dynamics and only captures correlations between areas. Thus it cannot show the direction of connectivity or detect local changes in network nodes. A new and potentially revolutionary technique which addresses these issues is the combination of transcranial magnetic stimulation and electroencephalography (TMS-EEG). TMS-EEG works by perturbing underlying cortex with TMS and then measuring the TMS evoked potentials (TEPs) with scalp EEG. TEPs may reflect directional connectedness between stimulation and recording sites and/or the local state of either, thus providing a rich data source. However, caution must be taken when interpreting results, because late-phase (>50 ms) stereotyped noise can contaminate the signal. In this exploratory study, we used TMS-EEG to analyze connections between the right dorsolateral prefrontal cortex, left motor cortex, and left parietal cortex. We chose these areas due to their involvement in working memory, attention, and other critical cognitive processes. We applied sham stimulation as a control, by inserting a spacer between TMS coil and scalp. In agreement with a recent study, later TEP components (in particular the N100) were present after both real and sham stimulation. This finding is important because it indicates TEPs might be impossible to interpret after ~50 ms. Conversely, early components (

Scientific Focus Area: Neuroscience

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