Deciphering the dynamics of neuronal activity evoked by transcranial magnetic stimulation.
Transcranial magnetic stimulation (TMS), a non-invasive method for stimulating the brain, has been used for more than 35 years. Since then, there have been many human studies using sophisticated methods to infer how TMS interacts with the brain. However, these methods have their limitations, e.g. recording of EEG potentials, which are summation potentials from many cells and generated across many cortical layers, make it very difficult to localize the origin of the potentials and relate it to TMS induced effects. However, this is necessary to build accurate models that predict TMS action in the human brain. In recent years, we have developed a method that allows us to demonstrate nearly the direct effect of a TMS pulse at the cellular level. We transferred a TMS stimulation protocol from humans to a rat model. In this way, we were able to gain direct access to neurons activated by TMS, thereby reducing the parameter space by many factors. Our data show that a single TMS pulse affects cortical neurons for more than 300 ms. In addition to temporal dynamics, there are also spatial effects. These effects arise at both local and global scale after a single TMS pulse. The local effect occurs in the motor cortex and is very short-lived. It is characterized by a high-frequency neuronal discharge and is reminiscent of the I-wave patterns described in humans at the level of the spinal cord. The global effect occurs in many cortical and subcortical areas in both hemispheres and is characterized by an alternation of excitation and inhibition. Both effects either occur together or only the global effect is present. Next, we are planning to correlate these neurometric data with induced electric field modeling to create detailed TMS-triggered neuronal excitation models that could help us better understand cortical TMS interference.