Motor threshold (MT) refers to the threshold of intensity of transcranial magnetic stimulation necessary to trigger activation of the skeletal muscles when the coil is placed above the motor cortex. The motor cortex is a region of the brain central to movement and general motor function.  At the motor threshold, the stimulation should effectively activate the motor cortex and thus skeletal muscles when TMS is used over the motor cortex,. Motor thresholds vary across individuals. Administering TMS in relation to an individuals motor threshold allows for determination of appropriate stimulation levels to ensure safety and maximize success of treatment. Motor thresholds have been shown to be correlated to the distance between an individuals scalp and their motor cortex.

Motor Threshold (MT) in Transcranial Magnetic Stimulation

The coil used to deliver TMS in depression treatment is placed over the Dorsolateral Prefrontal Cortex, or the DLPFC. The coil is most frequently placed over the left DLPFC, and less frequently over the right DLPFC.

Coil Location in Transcranial Magnetic Stimulation

Adjustable Parameters and Aspects of TMS include:
- Location on the scalp of the coil used to deliver stimulation
- Motor Threshold (MT)
- Intensity, Frequency, and duration of stimulus pulses
- Number of pulses

Northwestern University

A non-invasive method of brain stimulation that relies on electromagnetic induction using an insulated coil placed over the scalp which creates a magnetic field to induce a small electric current in a specific part of the brain.

Transcranial Magnetic Stimulation 

Janicack, P.G., Dokucu, M.E. (2015). Transcranial Magnetic Stimulation for the treatment of major depression. Neuropsychiatr Dis Treat. 11, 1549-1560.

Transcranial magnetic stimulation for the treatment of major depression

Empirical evidence has shown statistically significant results supporting the efficacy of TMS in treatment of depression and improving depressive symptoms. Additionally, following completion of a course of TMS, effects have been shown to persist through 12 months after treatment. 

Efficacy of TMS in treatment of depression.

In TMS treatment of depression, continual, rapid electromagnetic stimulation given to the brain can lead to increased neuron activity as well as increased blood flow to the targeted region. 

Mechanics of rapid, continual Transcranial Magnetic Stimulation treatment for depression

Potential Side Effects of Transcranial Magnetic Stimulation:
- Discomfort at the site of stimulation on scalp.
- Constricting of muscles surrounding eyes during stimulation.
- Headaches accompanied by a tension sensation. 
- Rarely, seizures. 

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