Transcranial magnetic stimulation (TMS) is a non-invasive procedure that uses magnetic fields to modulate brain activity. This technique targets nerve cells in specific brain regions to address symptoms associated with certain neurological and mental health conditions. By precisely altering the activity of localized neural circuits, TMS influences brain function without the need for surgery or systemic medication. The therapy is often used when standard treatments have not provided sufficient relief.
The Mechanism of Magnetic Stimulation
The fundamental principle behind TMS is electromagnetic induction. The procedure involves placing a specialized coil, often shaped like a figure-eight, near the patient’s scalp over the targeted area. When activated, a rapid pulse of electric current passes through the coil, creating a powerful, transient magnetic field. This magnetic field penetrates the skull and underlying brain tissue without causing pain or significant attenuation. The rapidly changing magnetic field induces a localized electrical current within the neurons themselves. This induced current causes the nerve cells to depolarize, or “fire,” directly affecting their excitability. Depending on the frequency and pattern of the pulses, TMS can either increase activity (facilitation) or decrease activity (inhibition) in the targeted brain region.
Primary Cortical Targets
Dorsolateral Prefrontal Cortex (DLPFC)
The area of the brain most frequently targeted for clinical applications is the Dorsolateral Prefrontal Cortex (DLPFC). Located in the frontal lobe, the DLPFC plays a role in executive functions, mood regulation, and working memory. In Major Depressive Disorder (MDD), the left DLPFC is often underactive, associated with symptoms like low mood. Stimulating the left DLPFC is the standard, FDA-approved protocol for treating depression, as activating this area helps restore functional connectivity within mood-regulating networks.
The effects of TMS extend beyond the immediate stimulation site, influencing deeper, functionally connected regions such as the subgenual cingulate cortex, which is implicated in emotional processing. The DLPFC acts as a control hub, and modulating its activity can indirectly correct abnormal activity in these related limbic structures.
Primary Motor Cortex (M1)
A second significant target is the Primary Motor Cortex (M1), responsible for planning and executing voluntary movements. Stimulation of M1 is commonly used in research to measure cortical excitability, as a pulse delivered here causes an involuntary muscle twitch, known as a Motor Evoked Potential (MEP). M1 is also targeted clinically for conditions involving movement dysfunction or chronic pain. Other emerging targets include areas in the parietal and temporal lobes, which are being explored for specialized applications like treating auditory hallucinations or anxiety disorders.
Precise Targeting and Localization
Achieving therapeutic results depends heavily on the precise placement of the TMS coil over the intended cortical target. Since the brain is unique to every person, generalized placement techniques are unreliable. Clinicians use a process called “motor threshold mapping” to calibrate treatment intensity and locate the exact stimulation site.
During mapping, the TMS coil is placed over the Primary Motor Cortex, and intensity is gradually increased until it causes a minimal, observable twitch in a small muscle, typically in the hand. This establishes the patient’s individual motor threshold (MT), which is used as the benchmark for setting the therapeutic dose for the DLPFC.
For increased accuracy, many centers utilize “neuronavigation,” which co-registers the patient’s structural Magnetic Resonance Imaging (MRI) scan with the real-time position of the TMS coil. Neuronavigation creates a detailed, three-dimensional map of the patient’s brain, allowing the provider to visually confirm that the magnetic pulse is aimed at the exact anatomical coordinates within the DLPFC. This image-guided approach enhances the focality of the stimulation and optimizes treatment outcomes.
Therapeutic Applications Based on Target Site
Mood and Anxiety Disorders
The choice of the target site directly determines the therapeutic application of TMS. Targeting the left DLPFC with high-frequency (excitatory) stimulation is the primary method for treating Major Depressive Disorder, aiming to increase neural firing in that area. Conversely, disorders like Obsessive-Compulsive Disorder (OCD) involve hyperactivity in related circuits, which may require inhibitory stimulation of the right DLPFC.
Neurological Conditions
The Primary Motor Cortex (M1) is targeted for specific neurological conditions. For instance, low-frequency (inhibitory) stimulation over M1 can manage certain types of chronic neuropathic pain. M1 stimulation is also being explored in stroke rehabilitation to promote plasticity and functional recovery in motor pathways.
Emerging Targets
In emerging applications, targeting the visual cortex is sometimes used to treat migraine with aura. Stimulating areas in the temporal or parietal lobes is being researched for conditions like tinnitus or post-traumatic stress disorder. The precise location and frequency of the pulses are adjusted to either suppress abnormal activity or enhance weakened connections, depending on the condition being treated.