IS 24. TMS-EEG to measure excitability and connectivity of human cortical circuits

Abstract

Recording TMS-evoked EEG potentials (TEPs) that are unconfounded by electromagnetic and biological artifacts is challenging. Neuronavigation guided TMS-EEG recordings performed in brain-injured patients showed that, when confounding factors are appropriately controlled, TMS elicits a significant EEG response only when is targeted on a scalp region overlying healthy cortical tissue and no response when is targeted over scalp areas overlying dead portions of the cortex while, in healthy subjects TEPs are highly reproducible over time. Thus, TEPs seem to purely reflect the response of the cortical tissue to TMS and can be used reliably to detect changes of excitability and connectivity in physiological, psychiatric and neurological conditions. For instance, in awake healthy subjects each cortical region tends to generate TEPs characterized by a dominant frequency (natural frequency) either when directly or indirectly activated by a TMS pulse. On the contrary, the tendency of different cortical circuits to oscillate at a specific frequency when directly perturbed is lost in schizophrenic patients. Most importantly, TMS-EEG measurements showed that effective connectivity is reduced during loss of consciousness in deep sleep, anesthesia and vegetative state. Interestingly, effective cortical connectivity recovered during REM sleep, and in acute brain injured patients before they could reestablish a functional communication with the environment. Overall, these observations suggest that TMS-EEG may represent a valuable tool to probe directly and non-invasively cortical excitability and connectivity in humans.