Introduction - Robotized Transcranial Magnetic Stimulation - page 7

stimulation intensity is increased in steps of 5 % of MSO until MEPs larger than
50 lV are consistently produced. Subsequently, the intensity is decreased in steps of
1 % of MSO until less than 5 MEPs larger than 50 lV are produced in 10 trials. This
intensity plus 1 is then used as the MT.
Even though this method is easy applicable and estimates the MT quite accu-
rately, it requires a relatively high number of stimulation pulses (about 75) and is
therefore relatively time-consuming [ 83 ].
1.1.4.2 Two-Threshold Method
Another approach estimates an upper and a lower threshold [ 50 ]. The MT is then
defined as the arithmetic mean between these two thresholds. The lower threshold
is therefore defined as the highest stimulation intensity at which no positive MEP
is measured in 10 consecutive stimuli. Accordingly, the upper threshold is defined
as the lowest intensity at which no MEP smaller than 50 lV is recorded in 10
consecutive stimuli.
In contrast to the Rossini methods, the two-threshold method requires about 45
stimuli to estimate the two thresholds. However, the MT is only approximated with
this method.
1.1.4.3 Threshold Hunting
This adaptive method is based on the assumption that the relation between the
likelihood of an evoked MEP and the stimulus intensity can be modeled as a
sigmoidal function. The threshold hunting method therefore calculates the prob-
ability to evoke an MEP for a given stimulus based on the already performed trials.
Using a maximum likelihood estimation, based on best PEST (parameter esti-
mation by sequential testing) [ 59 ], the most likely MT intensity is calculated [ 2 ].
For the next stimulation pulse this strength is used for measuring the MEP. It was
reported that on average 17 stimulation pulses are required to calculate a reliable
MT with this method [ 3 ]. As a computer is essential to use this method, Awiszus
and Borckardt developed the TMS Motor Threshold Assessment Tool ,a
freeware program to perform this threshold hunting [ 4 ].
1.1.4.4 Brief Comparison
The method by Rossini is very easy to utilize as it is straight-forward. It is
therefore commonly used in clinical practice. With some experience a physician is
able to reduce the number of required pulses to estimate a reliable MT. In research,
the two-threshold method is at present replaced by the threshold hunting method.
The advantage of this method is that it is an adaptive method that takes the
previous recordings into account. Therefore, it requires a smaller number of pulses