Advanced TMS approaches to probe corticospinal excitability during action preparation

The motor system displays strong changes in neural activity during action preparation. In the past decades, several techniques, including transcranial magnetic stimulation (TMS), electroencephalography (EEG) and functional magnetic resonance imaging (fMRI), have allowed us to gain insights into the functional role of such preparatory activity in humans. More recently, new TMS tools have been proposed to study the mechanistic principles underlying the changes in corticospinal excitability during action preparation. The aim of the present review is to provide a comprehensive description of these advanced methods and to discuss the new knowledge they give access to, relative to other existing approaches. We start with a brief synthesis of the work that has been achieved so far using classic TMS protocols during action preparation, such as the so-called single-pulse and paired-pulse techniques. We then highlight three new approaches that recently arose in the field of action preparation, including (1) the exploitation of TMS current direction, known as directional TMS, which enables investigating different subsets of neurons in the primary motor cortex, (2) the use of paired-pulse TMS to study the suppressive influence of the cerebellum on corticospinal excitability and (3) the development of a double-coil TMS approach, which facilitates the study of bilateral changes in corticospinal excitability. The aim of the present article is twofold: we seek to provide a comprehensive description of these advanced TMS tools and to discuss their bearings for the field of action preparation with respect to more traditional TMS approaches, as well as to neuroimaging techniques such as EEG or fMRI. Finally, we point out perspectives for fundamental and clinical research that arise from the combination of these methods, widening the horizon of possibilities for the investigation of the human motor system, both in health and disease.