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Abstract

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.
Keywords: Transcranial magnetic stimulation (TMS), Animal models, Iwave, Electrophysiology.