Engine threshold
The motor threshold ( Engl. Motor threshold , hence MT abbreviated), and individual (cortical) threshold or motor threshold , is a measure, in the various forms of stimulation of the brain oriented. A distinction must be made between the idle motor threshold and the active motor threshold . Since stimulation of the precentral gyrus and thus the motor cortex produces a directly measurable reaction in the muscles, it is the simplest way to quantify the effect of a selected stimulation intensity. If other brain regions are stimulated, the effect is e.g. B. only recognizable in reaction time experiments or other psychological tests.
A similar concept is the “phosphene threshold”, i.e. the stimulation intensity at which phosphene becomes visible through stimulation of the visual cortex .
Motor threshold for magnetic stimulation
determination
For the motor threshold at rest, electrodes are connected to the examined person, mostly to the muscles of the fingers or thumb (e.g. interossei dorsales manus 1 , the muscle that emerges when the thumb and forefinger are pressed together at the base of the index finger) and the motor evoked potentials (MEP) measured.
In the case of transcranial magnetic stimulation , the intensity of the stimulation is determined that is necessary to an MEP of size 50 μ V to obtain, in half the tests.
The active motor threshold is used for magnetic stimulation with the theta burst sequence. Compared to the resting motor threshold, the muscle is kept under slight tension.
Sometimes MEP is not measured and instead an attempt is made to detect a visible twitching e.g. B. to generate the targeted hand. This method, which is easier to implement, is used primarily in the therapeutic use of transcranial magnetic stimulation. However, Westin et al. to the sometimes strong deviations of the measured values compared to the motor threshold determined by means of MEP and the resulting safety risk.
Dependency of the measured value
Once the motor threshold has been determined, the intensity for stimulating other areas of the brain is adapted to this measured value. The transfer of the excitability of the motor cortex to other brain regions is a. a. criticized due to the different distances between the neocortex and the head surface, as the field strength depends on the distance from the coil.
Various studies have also shown that the motor threshold fluctuates depending on the activity of the test person. So the engine threshold decreases z. B. easily with imagined movements of the examined hand.
The motor threshold varies individually due to the depth of the hand area under the skull and the thickness of the fatty tissue that is between the skull and the brain. When determining, the direction of flow of the current in the coil, the orientation of the three angles (coil opposite the head) and the direct placement of the coil on the head must be taken into account.
literature
- Adriana Bastos Conforto, Werner J. Z'Graggen, Alexandra S. Kohl, Kai M. Rösler, Alain Kaelin-Lang: Impact of coil position and electrophysiological monitoring on determination of motor thresholds to transcranial magnetic stimulation . In: Clinical Neurophysiology . tape 115 , no. 4 , April 2004, ISSN 1388-2457 , p. 812-819 , doi : 10.1016 / j.clinph.2003.11.010 , PMID 15003761 .
Individual evidence
- ↑ GG Westin, BD Bassi, SH Lisanby, B. Luber: Determination of motor threshold using visual observation overestimates transcranial magnetic stimulation dosage: safety implications. In: Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology. Volume 125, number 1, January 2014, pp. 142-147, doi : 10.1016 / j.clinph.2013.06.187 , PMID 23993680 , PMC 3954153 (free full text).
- ^ KR Mills, SJ Boniface, M. Schubert: Magnetic brain stimulation with a double coil: the importance of coil orientation . 1992.
- ^ Helge Topka: General procedure for deriving the MEP. In: The "TMS Book" . 2007, pp. 60-67.