Biomedical Engineering Reference
the MEP amplitude exceeds 50 lv in base-to-negative peak amplitude or failure
otherwise. The algorithm calculates an estimate for the threshold (stimulator
output that evokes an MEP with a probability of 50 %) with a maximum likelihood
fit based on BestPEST ( Sect. 184.108.40.206 )[ 21 ]. The estimated MT is reported as a
percentage of the Maximum Stimulator Output (MSO).
The recordings are performed on 8 healthy male subjects with no history of
neurological disease aged 24 to 31 years after informed consent has been obtained.
Prior to recording, a structural MRI scan is obtained for navigation and evaluation.
This study has been approved by the local ethics committee.
220.127.116.11 Transcranial Magnetic Stimulation
We record MEPs on the Abductor hallucis muscle (AHM) of the right foot. The
AHM is located at the foot's inner border. For each subject two stimulation ses-
sions are performed on different days. We split the investigation into two sessions
to limit effects of varying vigilance and stress. The sessions are designed such that
coil orientation in session 1 is opposite to session 2. As we are using biphasic
stimulation, we expect two threshold minima (at slightly different stimulation
intensities) occurring at coil orientations differing by 180 . In this way, we can
further verify the optimal coil orientation in terms of stability within the subjects.
For session 1 we use a left-to-right coil orientation as reference (Fig. 3.1 b), and for
session 2 we use a right-to-left coil orientation as reference (Fig. 3.1 a) which is the
current standard orientation. For each session we first perform a hot-spot search.
We use the median in MEP amplitude of 5 subsequent stimulations, in standard
orientation and opposite orientation, respectively. A grid of positions with a dis-
tance of 1 cm is used and stimuli are applied with fixed stimulation intensity
(usually 70 % of MSO).
The hot-spot is defined as the stimulation point that is surrounded by four other
stimulation points with smaller MEP amplitudes. Subsequently, we place the coil
Fig. 3.1 Definition of coil orientation angles for stimulation of the right foot. a As standard
reference for stimulation of the right foot a lateral right-to-left coil orientation is used. We
indicate this coil orientation with 180 and use it in session 2 as reference; b The opposite coil
orientation (indicated with 0 ) to the standard coil orientation. The arrows denote the rotational