Introduction - Robotized Transcranial Magnetic Stimulation - page 3

The strength of the electromagnetic field decreases quasi-exponentially with
increasing coil distance [ 7 ]. Therefore, the depth penetration of TMS in the tissue
is limited and stimulation of deeper brain areas is not possible in practice [ 25 ]. The
cortical target region is thus located approximately 10-60 mm beneath the TMS
coil [ 79 ] which is due to the individual scalp-to-cortex range [ 36 ]. In contrast to
other cortical regions, effects of TMS on the the Primary Motor Cortex (M1) can
be directly observed or measured. Stimulation of M1 in general leads to muscle
twitches of the associated muscle which can be detected by visual inspection or by
Electromyography (EMG) recordings using surface electrodes. In particular, the
Primary Motor Hand Area (M1-HAND) is easy to stimulate with low intensities as
it is relatively large and located at the surface of the precentral cortex. For the
Primary Motor Leg Area (M1-LEG), on the contrary, higher intensities are
required because it is located at the medial wall of the precentral gyrus. Responses
to stimulation of other brain regions are indirectly detectable as evoked neural
activity with Electroencephalography (EEG) [ 51 ] or changes in blood flow with
functional Magnetic Resonance Imaging (fMRI) [ 70 ], Single Photon Computed
Tomography (SPECT) [ 10 ] or Positron Emission Tomography (PET) [ 20 ]. See
[ 57 ]or[ 79 ] for a general overview.

1.1.2 Applications of TMS: Single-Pulse Versus Repetitive
Stimulation

At the very beginning of TMS, the stimulators were only able to produce single
pulses. Currently, repetitive stimulators with repetition frequencies of up to 50 Hz
are available [ 85 ]. The main difference between single pulse and repetitive stim-
ulation is that rTMS can change neuronal behavior whereas single pulse TMS
leads to an immediate reaction, e.g., muscle twitching.
Hence, clinical diagnosis is the main application of single pulse TMS. A single
TMS pulse is applied to the motor cortex and the corresponding muscle response is
recorded. For clinical routine mainly the central motor conduction time, the motor
threshold, the Motor Evoked Potential (MEP) amplitude or the silent period are of
importance [ 74 ]. This way, e.g., spinal cord injuries can be diagnosed and/or
investigated [ 39 ].
Another interesting and promising application for single pulse TMS is motor
cortex mapping. Muscle responses are recorded for different coil positions. The
representation of this muscle in the cortex can now be calculated based on the set of
recordings [ 48 ]. For neurosurgery, brain tumor removal can be planned and sup-
ported by motor cortex mapping [ 33 ]. Figure 1.2 displays a motor cortex mapping
for the Abductor digiti minimi (ADM) muscle before and after tumor removal. It
clearly shows that tumor removal (including a safety margin) is possible without
damage of the cortical control of this muscle. In this way, also the cortical plasticity
due to tumor growth can be investigated [ 18 ]. In brain research, motor cortex
mapping helps localizing specific brain areas and motor pathways [ 56 ].