Introduction - Robotized Transcranial Magnetic Stimulation - page 6

1.1.4 Motor Evoked Potentials and Motor Threshold
An easy way to detect macroscopic responses is observation of muscle contraction
or twitching after motor cortex stimulation. A Motor Evoked Potential (MEP) can
be measured using Electromyography (EMG) with surface electrodes over an
associated muscle. The MEP represents the electrical potential at this muscle,
which is a correlate of muscle contraction. In general, the stronger the muscle
contraction the higher the MEP amplitude.
For the use of TMS, determination of the Motor Threshold (MT) for the target
muscle is often the first step. In general, the MT is a measure of (corticomotor)
excitability. It is defined as the stimulation strength at which a muscle contraction
occurs with a probability of 50 %. In this case, a muscle contraction is recorded if
the base-to-peak MEP amplitude exceeds 50 lV (for the resting muscle). The MT,
beside its routine application in diagnosis, plays a key role in rTMS treatments:
The stimulation strength for treatment is calculated based on the individual MT
[ 57 , 79 , 85 ]. However, the MT highly depends on the used equipment and setup,
i.e., stimulator, coil and pulse waveform. The MT is therefore traditionally
expressed in percentage of Maximum Stimulator Output (MSO) which makes it
almost impossible to directly compare the MT between different studies. However,
as the MEP amplitude has a very high variance [ 88 ], often the MT is used in brain
research as a more stable quantitative measure for cortical excitability. However,
also the MT can change due to vigilance, stress or muscle pretension—even within
subjects [ 34 ].
As enhancement to the MT, recently the computed electric field on the cortex at
MT strength was introduced as a more stable and comparable measure of cortical
excitability. By using navigated TMS (see below) and the field properties of the
TMS coil used, an estimate of the electric field strength on the cortex can be
computed at MT strength which should theoretically accurately reflect the anatomy
and the used system [ 14 ]. Even though first studies have shown promising results,
due to the complexity of its computation, the computed electric field is far away
from becoming a standard technique [ 31 ].
From a mathematical point of view, the MT is explicitly defined. However, an
accurate determination is rather complex, which is mainly due to natural excit-
ability changes [ 1 ]. Recently, different methods have been proposed to determine
the MT. The three most common methods are presented in some more detail:
1.1.4.1 Rossini Criterion
The first method to estimate the MT was presented in the general guidelines on TMS,
published by the International Federation of Clinical Neurophysiology (IFCN) in
1994 [ 66 ]. A standardized algorithm can be derived from this method where the MT
is defined as the stimulation intensity at which 5 MEPs are evoked in 10 trials [ 24 ].
After placement of the coil at the optimal stimulation site (the hot-spot), the