Intrinsic and Extrinsic Measures of Excitability: a synthesis

COEX 

Cortical excitability has been defined as the reactivity of a cortical area to external stimulation (usually TMS). Commonly it is measured via motor-evoked potentials (MEPs) that are elicited due to TMS over the primary motor cortex. It is however debated how this measure translates to the excitability of other cortical areas, specifically association cortices. Different approaches have been suggested to evaluate the excitability of brain areas beyond M1, among these, the combination of TMS with: behavioural outcomes, M/EEG-responses, BOLD-response as well as stimulation-free measures using fMRI or M/EEG signals. In this talk an overview about these so-called ‘extrinsic’ and ‘intrinsic’ excitability measures is given. This is followed by an overview of our ongoing research investigating different approaches to measure cortical excitability including combinations of TMS with behavioural outcomes and M/EEG as well as pure ‘intrinsic’ excitability measures based on M/EEG. Finally, a conclusion is given on how these measures relate to each other and how they can be applied in a clinical context, e.g. diagnostics and treatment of epilepsy, migraine or stroke.